CHAPTER III.

DEFENSIVE TORPEDO WARFARE--_continued_.

BY electrical submarine mines is meant those whose charges are ignited by the agency of electricity.

_Submarine Mines during the Crimean and American Wars._--It was during the Crimean war (1854-6) that this description of defensive torpedoes was for the first time employed on actual service. Several of the principal Russian harbours were protected by this form of submarine mine, but owing to the smallness of their charges, and to the want of electrical knowledge on the part of the Russian officers and men in charge of them, none of the ships of the Allies were sunk, or even rendered _hors de combat_ by this mode of harbour defence, though in several instances ground known to be covered with submarine mines was passed over by both English and French vessels of war.

Subsequently the Confederates, during the American civil war, employed electrical submarine mines in considerable numbers for the defence of their numerous harbours, rivers, &c.; but though in so far as the size of the torpedo charges was concerned, they did not make the same mistake as the Russians, yet, owing to the absence of proper electrical apparatus, and the want of any practical knowledge of the manipulation of electrical sea mines, on the part of the Confederate torpedoists, they were almost entirely unsuccessful in destroying the Federal warships; the _Commodore Jones_ being the sole instance, out of the large number of vessels belonging to the Northerners which were sunk and severely injured by torpedoes, of a war steamer being sunk by means of electrical submarine mines.

In the Franco-German and Russo-Turkish wars which have lately occurred, electrical sea mines were very extensively used in coast defence, but with the exception of the loss of the gunboat _Suna_ to the Turks, during the latter struggle, by this form of defensive torpedo, no other damage to vessels resulted from their use, yet owing to the vast moral power possessed by these submarine weapons, they were enabled to most effectually carry out the work of defence entrusted to their care.

Of late years many important discoveries have been made in the science of electricity, and vast improvements have been effected in electrical apparatus, to which causes may be traced the vastly improved system of electrical submarine mines as adopted by the English, American, and principal European governments at the present day, as compared with those that have hitherto been employed.

The certainty of action when required of electrical submarine mines, which is of course the desideratum of all torpedoists, has, by the improved mode and manner of ascertaining the exact electrical condition of each particular mine, and of the system as a whole, which is at present in vogue, been made almost absolute.

_Advantages of Electrical Submarine Mines._--This form of defensive torpedo possesses numerous important advantages, the principal of which are as follows:--

1.--They are always absolutely under control.

NOTE.--By detaching or connecting the firing battery, which is effected by means of a plug, key, &c., they may be respectively rendered harmless, or dangerous. Thus friendly ships may pass over them in safety, whilst those of the enemy are debarred from so doing. On this account harbours, &c., protected by such mines are termed "Harbours of refuge."

2.--Fresh mines may be added to a system of such defensive torpedoes, thereby allowing an exploded mine to be replaced.

NOTE.--This is a very important point in connection with a system of defence by submarine mines, as in the case of a deep water channel, a hostile vessel being sunk by one of them, would not become an obstruction, as, were the channel a comparatively shallow one would most probably be the result, and therefore it would be necessary to put a fresh mine in the place of the exploded one; this would also apply were a mine to be prematurely ignited, or if any portion of its firing apparatus were injured.

3.--At night, or in a fog, no vessel can pass through a channel, &c., so protected without affording a means of ascertaining her presence.

NOTE.--This is also a very important advantage of a system of defence by electrical sea mines, affording as it does a complete safeguard against surprise.

4.--The power of obtaining proof, without going near it, by a system of testing that the electrical condition of the mine, &c., is perfect.

NOTE.--This again is an extremely important point. For were a charge to become wet, one of the electric cables of the mine broken, or damaged, &c., it would instantly be made apparent at the firing station, and could be at once remedied.

5.--They can be raised for examination, or removed when no longer required, with ease and safety.

Such are some of the chief advantages of employing the agency of electricity to effect the ignition of the charge in a system of defence by submarine mines.

_Defects of Electrical Submarine Mines._--The following are the chief defects connected with the use of electrical mines:--

1.--The number of wires that are required to be used with them.

2.--The necessity of employing specially trained men in their manipulation.

In time there seems little doubt but that the former obstacle will be to a considerable extent overcome, but the latter must always be a flaw in an otherwise perfect system of coast defence by submarine mines.

_Rules to be observed in using Electrical Submarine Mines._--In connection with a system of electrical submarine mines the following rules should be carefully observed:--

1.--They should be moored in deep channels, that is to say, where the larger class of vessels would in attempting to force a passage be obliged to go.

NOTE.--Mechanical submarine mines should never be used under these circumstances, as the difficulties of mooring them and keeping them in position would be very considerable, also a vessel being sunk in a very deep channel would not necessarily block it, and as a mechanical mine cannot be replaced, a gap would be left in the defence.

2.--They should be placed in the narrowest parts of the channel.

NOTE.--The object of this rule is evident, fewer mines being required, and consequently in the case of electrical ones, a far less number of wires are needed, which gives an increase of simplicity, and consequently more effectiveness. This point should be observed in connection with mechanical, as well as electrical submarine mines.

3.--They should where practicable be moored on the ground.

NOTE.--The advantages attendant on an observance of this rule are:--

_a._--Increased vertical effect.

_b._--Avoidance of mooring difficulties.

_c._--Less liability of shifting from its original position.

_d._--Less chance of its being discovered and rendered useless by an enemy.

_e._--By far heavier charges may be conveniently employed.

4.--Where possible, no indication whatever should be given of the position of the mines by their circuit closers, or in the case of small buoyant ones, by the mines themselves.

NOTE.--In some instances this will be almost impracticable, as for example, where there is a very great rise and fall of tide. For instance, at Noel Bay in the Bay of Fundy, the rise is over fifty feet. Here, when circuit closers, or small buoyant mines are used, both of which ought never to be more than twenty feet below the surface, long before low water they would be found floating on the surface in full view. Many attempts have been made to overcome this difficulty, but as yet no really practicable means have been devised.

5.--The stations where the firing batteries, &c., are placed, should be in the defensive work likely to be held the longest, thus enabling the mines to be commanded up to the last moment.

6.--The electric cables should be laid in positions such that their discovery by the enemy would be extremely difficult, and almost impossible.

NOTE.--This may be to a certain extent effected by leading them from the mines to the firing and observing stations by circuitous routes, and by burying them in trenches.

7.--They should not be thrown away on boats.

NOTES.--As they can in all cases be fired by will, even when circuit closers are used, this rule is easily observed. But to prevent an enemy's boats from rendering the mines useless, a line of small torpedoes might be placed in advance of the large ones, or the circuit closers themselves might be charged.

At night, or in foggy weather it will be necessary to employ guard-boats, electric lights, &c., to protect them against damage by an enemy's boats, &c.

In the foregoing pages of this chapter will be found the requirements and conditions essential to a perfect system of electrical submarine mines for the defence of a harbour, river, &c.; in the following pages a general description of the component parts of such defensive torpedoes, under the following heads--Form and Construction of Case; Electrical Fuzes; Electric Cables; Watertight Joints; Junction Boxes; and Mode of Mooring, will be considered.

_Form and Construction of Torpedo Case._--The case of a submarine mine should be capable of fulfilling the following conditions:--

1. It must be able at great depths to withstand a great pressure of water, and remain perfectly watertight.

NOTE.--This in the case of a charge of gunpowder being an imperative necessity.

2. As a buoyant mine, it must be capable of affording a considerable excess of buoyancy, by which it may be rendered stationary when moored.

NOTE.--This is generally obtained by having an air space within the torpedo, thus requiring a much larger case in which the charge is enclosed than would otherwise be necessary, causing increased difficulties in transportation, mooring, and raising them for examination, &c.

3. When explosive agents which require a certain time for thorough combustion are used as the charge, such as gunpowder, picric powder, gun-cotton (not fired by detonation), &c., a much stronger case is necessary to obtain the full explosive effect than would be the case were detonated charges, under the same conditions, employed.

NOTE.--This is an extremely important point, for if a weak case is employed with a charge of gunpowder, &c., fired by a fuze primed with powder only, a portion of it on being fired would generate a sufficient quantity of gas to burst the case, thus blowing out the remainder of the charge before its ignition had been effected.

4. It should be of such a form that the complete ignition of the charge is obtained by the employment of the least number of fuzes possible to effect this result.

NOTE.--This point is especially to be observed when gunpowder is the explosive agent.

The various forms of defensive torpedo cases may be classed under the following heads:--

1.--Spherical shape. 2.--Cylindrical shape. 3.--Conical shape.

_Spherical Shape._--This form of case is theoretically the very best one possible to devise, but on account of the difficulty of constructing it, and its comparative costliness, such a form may be put aside as being impracticable.

_Cylindrical Shape._--Torpedoists in general have hitherto adopted the cylindrical form of case as being the best adaptable for both ground and buoyant mines containing a heavy charge.

The Confederates employed exclusively this shape for their electrical submarine mines, which were ground ones, and the Austrians in the war of "66" approved of this form of case for their electrical submarine mines, which were buoyant ones. Figs. 19 and 20 represent respectively the American and Austrian mines.

In England the cylindrical shape has up to quite lately found most favour with her torpedoists for both buoyant and ground mines. At Fig. 21 is represented a 100-lb. buoyant electrical mine, surrounded by a wooden jacket, _e_, and having its circuit closer, _C_, enclosed within it; and at Fig. 22 is shown a 250-lb. electrical mine, which may be used either as a buoyant or ground one.

For large ground mines, the best form of torpedo case seems to be that of the turtle mine, which is shown at Fig. 9. A heavy charge may be contained in it; it forms its own anchor; and it would withstand an explosion of an adjacent mine without sustaining any injury. At present the cylindrical shape is the form generally used, though as far as retaining its position on the ground in a strong tide, it cannot be compared to the turtle form.

_The Conical Shape._--Hitherto this shape of submarine mine case was only used in connection with mechanical mines, but now it is the form considered most suitable for all buoyant mines, electrical or mechanical. At Fig. 23 is shown the conical shaped mechanical mine, employed by the Confederates for use with sensitive fuzes. The conical form of torpedo case lately approved of by the English torpedo authorities is somewhat similar to that one, the charge being contained in a kind of box hung from the top of the case, and the circuit closer is screwed into the bottom of the case; surrounding the upper part of the case is a thick buffer of wood, by which damage to the mine is prevented by the passage of friendly ships. This is altogether a very neat and serviceable form of torpedo case. This form of case is also more difficult to discover by dragging, and easier to retain in position.

_Electrical Fuzes._--The fuzes employed in connection with electrical submarine mines may be divided into two classes:--

1. Platinum wire bridge fuzes.

NOTE.--That is where the evolution of heat is caused by a large _quantity_ of the electric force flowing through a good conductor of large section, such as the copper core of electric cables, being suddenly checked by a very thin wire composed of a metal which compared with the conductor offers a very great resistance, such as _platinum_.

2. High tension fuzes.

NOTE.--That is where the evolution of heat is caused by the electric spark, or by the electric discharge taking place through a substance which offers very great resistance to the passage of the electric force.

_Platinum Wire Fuze._--This is the form of electrical fuze most commonly used, and which will most certainly supersede altogether the high tension fuze.

There are numerous advantages accruing from the use of platinum wire fuzes, the chief of which are here enumerated:--

_a._--Great facilities for, and entire safety whilst testing the circuit.

_b._--Extreme simplicity of manufacture.

_c._--Non-liability to deteriorate.

_d._--Perfect insulation of the electric cables used in connection with submarine mines not necessary.

_English Service Platinum Wire Fuze._--The following is a description of the platinum wire fuze of the form adopted in the English service, a section of which is shown at Fig. 24. It consists of a head of ebonite _a_, hollowed out, in which a metal mould is fixed, the wires which have been previously bared are inserted into holes in this mould, and firmly fixed thereto by means of a composition poured into the mould, whilst hot; this is shown at _b_. The two bared ends of the wires which project beyond the metal mould, as _c_, _c_, are connected by a bridge of platinum-silver wire ·0014" in diameter and weighing ·21 grs. per yard. This is effected as follows:--

A very fine shallow groove is made in the flat ends of the bare wires _c_, _c_, and the platinum-silver wire is laid across in the incisions, and fixed there by means of solder. The length of the bridge _d_ is ·25."

A tube _e_, made of tin, and soldered to a brass socket _f_, is fixed by means of cement to the ebonite head _a_; in this tube is placed the fulminate of mercury, the open end of the tube _g_ being closed with a pellet of red lead and shellac varnish; around the bridge of the fuze is placed some loose gun-cotton.

_McEvoy's Platinum Wire Fuze._--Another form of platinum wire fuze, which has been devised by Captain McEvoy, formerly of the Confederate Service, is shown at Fig. 25. It consists of the head _a_, formed of a mixture of ground glass, or Portland cement, worked up with sulphur as a base: this mixture when hot is poured into a mould, in which the two insulated copper wires, _b_, _b_, have been previously placed; when cold, the mixture with the wires affixed is removed from the mould, and the platinum wire bridge _c_ being secured to the bare ends of the copper wires, the whole is firmly fixed in a brass socket _d_, by means of cement; the space _e_ is filled with loose dry gun-cotton, so as to surround the bridge _c_; a copper tube _f_, closed at one end, is partly filled with fulminate of mercury, and when the fuze is required for service, this tube is secured to the brass socket _d_ by means of cement.

In this form of low tension fuze there is no liability whatever of any injury being caused to the bridge by the working of the wires in the head, or by damp even after lying in the water for a month or more. One peculiarity of this fuze is that the composition is run over the insulated wires without materially softening the dielectric, or affecting in the slightest degree the insulation of the wires.

_High Tension Fuzes._--The high tension fuze was devised for use with electrical submarine mines, in the place of the platinum wire fuze, on account of the little knowledge possessed, in the early days of submarine warfare, in regard to the manipulation of Voltaic batteries.

Platinum wire requires a temperature of some 500° F. to heat it to incandescence, and therefore necessitates the use of a powerful Voltaic battery, both in intensity and power, to effect the ignition of gunpowder by this means at considerable distances.

The Grove and Bunsen pile were the only suitable form of Voltaic battery known at the period of the introduction of high tension fuzes, both of which possessed the defects of uncertainty and inconstancy, and also were by far too cumbersome and too difficult to keep in effective working order to be of any real practicable value.

High tension fuzes may be ignited by means of either an electro-magneto machine, an electro-dynamo machine, a frictional machine, or by a Voltaic battery, generating an electric current of high intensity. Various kinds of this form of electrical fuze have been designed, the principal of which are as follows:--

1.--Statham's fuze. 2.--Beardslee's fuze. 3.--Von Ebner's fuze. 4.--Abel's fuze. 5.--Extempore fuze.

_Statham's Fuze._--A section and elevation of this electric fuze are shown at Fig. 26; _a_, _b_ is a gutta percha tube, with an opening cut in it, as shown in figure. The interior of this vulcanised gutta percha tube is coated with a thin layer of sulphide of copper, which coating is obtained by leaving a bare copper wire for some time in connection with the above-mentioned tube. The extremities of two insulated copper wires _c_, _c_, considerably smaller than the conducting wires, are uncovered, scraped, and then inserted into the tube _a_, _b_, with an interval of ·15 inch between them. The wires are then bent as shown in the figure, and the priming placed between the terminals. The whole is covered with a gutta percha bag, which is filled with fine grained gunpowder. The priming substance is composed of fulminate of mercury worked up with gum water. The objection to this fuze, which was used by the Allies in their destruction of the Russian fortifications at Sebastopol, is the want of sensitiveness of sulphide of copper, and the consequent necessity of a very powerful firing battery.

_Beardslee's Fuze._--This high tension fuze is shown at Fig. 27. It consists of a cylindrical piece of soft wood a, which is about three-quarters of an inch in length and in diameter; two copper nails, _b_, _b_, are driven through this piece of wood _a_, in such a way that while the two heads come together as close as possible without absolutely touching, the pointed ends are some distance apart from each other, and project through the wood _a_; two insulated copper wires, _c_, _c_, are firmly soldered to these projecting ends, and a piece of soft wax, _d_, is pressed around the junction points. In a groove, across the heads of the copper nails, is placed a little black lead, to which is added a minute quantity of some substance, the nature of which is known only to Mr. Beardslee. Several folds of paper are wrapped round the wooden cylinder, forming a cylinder about 2-1/2 inches long, one end of which is tightly fastened round the insulated wires as at _e_. The other end of the cylinder is then filled with powder, _f_, and closed by a piece of twine. The whole fuze is then coated with black varnish. Though not highly sensitive, Beardslee's fuze is exceedingly efficient, and extremely simple.

_Von Ebner's Fuze._--This form of fuze was devised by Colonel Von Ebner of the Austrian Engineers. A section and elevation of it is shown at Fig. 28. It consists of an outer cylinder, _a_, of gutta percha, and an inner one of copper, _b_, which latter encloses a core formed of ground glass and sulphur, _c_, which core is cast round the two conducting wires _d_, _d_ in such a way that they are completely insulated from one another. In the first instance the wire is in one continuous length, the opening _e_ being subsequently made, and carefully gauged, so as to ensure a uniform break, or interval in the conductor of each fuze. The priming composition, which consists of equal parts of sulphide of antimony and chlorate of potash, is placed in the hollow _f_, to which is added some powdered plumbago, for the purpose of increasing the conducting power of the composition. This mixture is put into the hollow, _f_, of the fuze under considerable pressure, the terminals being connected with a sensitive galvanometer, in circuit with a test battery, and the pressure applied so as to obtain, as far as possible, uniformity in the electrical resistance of each fuze.

The Austrians employed this form of high tension fuze in connection with a frictional machine for the electrical mines used in their defence of Venice, &c. during the war of 1866.

_Abel's Fuze._--Mr. Abel devised a high tension fuze, which in 1858 was extensively experimented with; the Beardslee and Von Ebner fuze being based upon the principles applied for the first time in Abel's fuze.

Many modifications of it have been from time to time devised by Mr. Abel; a section and elevation of the more recent form of his fuze is shown at Fig. 29. It consists of _b_, _b_, a body of beech wood, hollowed for half its length, in which space the priming charge is placed; it is also perforated by three holes, one vertical for the reception of the capsule of sensitive mixture, the other two horizontal, in which the conducting wires are placed; _a_, _a_ are two insulated copper wires, passing into the vertical hole, and resting on the sensitive mixture; in a cavity, _d_, of the body of the fuze is placed some mealed powder, which is fired by the ignition of the sensitive mixture on the passage of the electrical current.

The insulated wires used in connection with this fuze consist of two copper wires, about 2 inches long, and ·022 inch in diameter, enclosed in a covering of gutta percha ·13 inch in diameter, and separated about ·06 inch from each other.

At one end the wires are bared to 1·25 inch, at the other they are merely cut across by a very sharp pair of scissors. This end of the double covered wire is inserted into a paper cylinder _c_, _c_, which holds a small quantity of the priming mixture. This capped end of the wires is inserted into the wooden body of the fuze through the vertical hole _i_, and projects ·15 inch into the cavity _d_. The bare ends of the double covered wires are pressed into small grooves in the head of the cylinder _e e_, and each extremity is bent into one of the small channels _d' d'_, which are at right angles to the vertical perforation. _d' d'_ are two small copper tubes driven into these channels over the wire ends, to keep the wires in position, and to form the opening into which the conducting wires _f_ are inserted and bent round, as at _e'_.

The priming mixture of Abel's original fuze, which was the one used by the Confederates, was composed of 10 parts of subphosphide of copper, 45 parts of subsulphide of copper, and 15 parts of chlorate of potash. These ingredients reduced to a very fine state of division, and intimately mixed, in a mortar, with the addition of a little alcohol, are dried at a low temperature and preserved in bottles until required for use. The sensitive mixture used by Mr. Abel more recently for his submarine electrical high tension fuzes, is composed of an intimate mixture of graphite and fulminate of mercury. By the process of ramming, the electrical resistance of the fuze is regulated.

_Extempore Fuzes._--It may be necessary in some cases, when a specially manufactured fuze is not attainable, to make a fuze on the spot. The following is a neat and simple method of constructing an extempore high tension fuze.

_Fisher's Extempore Fuze._--This form of fuze was devised by Lieutenant now Captain Fisher, R.N. It consists of a small disc of gutta percha, through which the ends of two wires are inserted about 1/4 inch apart, their ends terminating in small copper plates formed by hammering down the wire. These flat ends should be fixed parallel, and in the first place in contact with one another, also should be level with the surface of the gutta percha. The other two extremities of the wires are then placed in circuit with a sensitive galvanometer and a test battery; the needle of the former deflects violently, there being a complete metallic circuit; the flat ends of the wires or poles of the fuze are then separated very carefully, until the needle just ceases to deflect. In the space thus formed, a little scraped charcoal is placed, and rammed in by a piece of wood. By the application of pressure, any degree of sensitiveness may be attained, merely observing the deflection of the galvanometer needle. Over the charcoal a little powdered resin is shaken, and pressed down, by which means the charcoal is fixed in position, and owing to the inflammability of the resin, the ignition of the gunpowder priming is ensured. The disc of gutta percha is then placed in an empty Snider ball cartridge, &c., and by the application of a little warm gutta percha applied externally, the holes where the projecting ends of the wires pass are closed, and the disc is fixed and insulated. The case is then filled with some mealed powder and fine grained powder, on the top of which is placed a little cotton wool, and the whole pressed down tightly with the finger, the open end of the case being then choked, as in Beardslee's fuze and Abel's extempore one. The apex is then covered with some warm gutta percha, and the whole of the fuze coated over with red sealing-wax dissolved in methylated spirits.

_Insulated Electric Cables._--For the work of defence by electrical submarine mines, the wires along which the electric current flows have, on account of their being led underground and through the water, to be covered with some substance which shall prevent the current during its passage from escaping to earth, or in other words, they (the wires) must be insulated.

The substances in general use for such purposes are as follows:--

1.--Gutta percha. 2.--Ordinary india rubber. 3.--Hooper's material.

_Gutta Percha._--This substance was used by Messrs. Siemens in the cables manufactured by them for the Austrian government in 1866, and is to some extent still employed, though Hooper's material or vulcanised india rubber, has been found to be more suitable. The dielectric, gutta percha, possesses the following advantages:--

_a._--It can be put on the conducting wire, as an unbroken tube.

_b._--It only absorbs 1 per cent. of water.

_c._--It has the property of clinging to the metallic conductor, by which is meant, that should it (conductor) be cut through, and any strain be brought on the cable, there is a tendency on the part of the gutta percha to cling to the conducting wire, thereby not increasing the fault.

The defects of such an insulator are:--

_a._--Its liability to become hard and brittle when exposed to dry heat, and consequently it requires to be stored under water.

_b._--It becomes comparatively a bad dielectric at 100° F.

_c._--It becomes plastic at high temperatures, which causes the conducting wire to alter its position.

In some particulars ordinary india rubber is a better insulator than gutta percha, but this substance is equally inferior to Hooper's material, &c. The advantages possessed by this substance are:--

_a._--It is not easily affected by a dry heat.

_b._--It is a very excellent dielectric.

The defects of this mode of insulation are:--

_a._--It must be put on the conducting wires in a series of jointed pieces.

_b._--It does not cling to the conducting wire, so that if the electric cable be cut, and any strain be brought on it (cable), the previous fault is increased.

_c._--It absorbs 25 per cent. of water.

_Hooper's Material._--This insulating material consists of an inside coating of pure india rubber, then another similar coating in conjunction with oxide of zinc, which is termed the separator, and an outside coating of india rubber combined with sulphur. The use of the separator is to prevent any damage to the conducting wires by the action of the sulphur. The three coatings are then baked for some hours at a very high temperature, which fuses the whole into a solid mass, and vulcanises the outer coating. The properties of the pure india rubber which is in contact with the metallic conductor are thus preserved, while any decay of the outer covering is prevented by the vulcanising process.

The advantages claimed by Mr. Hooper for this mode of insulating electric submarine cables, are:--

_a._--High insulation.

_b._--Flexibility.

_c._--Capability of withstanding the bad effects of dry heat.

The qualifications essential to a perfect insulated electrical cable for use with submarine mines are as follows:--

1.--Capacity to bear a certain amount of strain without breaking.

2.--Perfect insulation, or at least as nearly so as it is possible to obtain, and composed of a substance capable of being readily stored, and kept for a considerable length of time without being injured.

3.--Pliability so that it may be wound on, or paid out from, a moderately sized drum without injury.

4.--Provided with an external covering capable of protecting the dielectric from injury when used in situations where there is a rocky or shingly bottom, &c.

The insulated wire of a submarine cable is technically spoken of as its _core_.

By a _cable_ is meant to be understood any piece of covered wire.

Several forms of submarine electrical cables have been devised, all of which more or less possess the qualifications enumerated above. The following are some of the most effective:--

1.--Siemens's cable. 2.--Hooper's cable. 3.--Gray's cable. 4.--Service cable.

_Siemens's Cable._--This form of cable is represented at Fig. 30. It consists of a strand _a_, which is composed of three or more copper wires formed by laying up the several single copper wires spirally, several layers of gutta percha, or india rubber, _b_, two coverings of hemp, saturated with Stockholm tar, _c_ and _d_, and several plies of copper tape _e_, wound on, so that each strip overlaps the preceding one, as shown at Fig. 30. The conductivity of the copper employed for the strand is equal to at least 90 per cent. of that of pure copper.

This exterior covering of copper tape is a patent of Messrs. Siemens Brothers, and when once laid down, the cable so covered is very efficiently protected, and of course it is little affected by the action of the sea water. This mode of protection has one great defect, viz., that in the event of a kink occurring in paying out the line, and at the same time a sharp strain being applied, the copper tape at that point is extremely likely to destroy the insulation by being drawn in such a way as to cut through the dielectric. On this account great care must be observed in handling this form of cable.

In practice precautions must be taken to prevent the copper tape covering from being brought into contact with any iron, for were such to happen, electrical action would at once ensue, causing the iron to corrode with enormous rapidity.

In some of Siemens's cables, vulcanised india rubber replaces the gutta percha insulation. Iron covered cables, either galvanised or plain, are manufactured as well as the copper tape covered ones by that firm.

_Hooper's Cable._--This form of cable is represented at Fig. 31. It consists of a metal conducting wire, generally copper, _a_, covered with an alloy to protect it from chemical action, the insulating substance _b_, known as Hooper's material, previously described at page 39, a covering of tarred hemp _c_, and an outer covering of iron wires (No. 11 B. W. G.), each of which is separately covered with tarred hemp and wound on spirally, _d_.

Gray's cable is very similar to the one just described, the chief difference in it as compared with Hooper's being the absence of the separator.

_Silvertown Cables._--The following is a description of the core of an electrical submarine cable, which is used by the English government, and is supposed to contain all the advantages of the foregoing, and none of their defects. It consists of a strand conductor of four copper wires (No. 20 B. W. G.) of quality not less than 92 per cent. of pure copper, and possessing an electrical resistance of not more than 14 ohms per nautical mile. This strand is tinned and insulated with vulcanised india rubber to a diameter of ·24 inch, and then covered with a layer of felt, and the whole subjected to a temperature of 300° F. under steam pressure. This forms the core of the various kinds of cables employed in connection with a system of defence by electrical submarine mines, which are enumerated as follows:--

1.--Single core armoured cable.

2.--Multiple cable.

3.--Circuit closer cable.

4.--Single core unarmoured cable.

5.--Special cables for firing by cross bearings.

_Single Core Armoured Cable._--This form of cable is used in connection with each mine of a group or system, and also to connect forts, &c. across an arm of the sea. Over the core, which has been fully described, is laid a spiral covering of tanned, picked Russian hemp, over this are laid ten galvanised iron wires (No. 13 B. W. G.), each one of which is covered with a similar hemp, which is laid in an opposite spiral to the former similar covering, with a twist of one revolution in about thirteen inches; in order to prevent these wires from gaping when the cable is kinked, a further covering of two servings of hemp passed spirally in opposite directions is laid, and the whole passed through a hot composition of a tar and pitch mixture. Exterior diameter of this cable is 7/8 inch. Its weight in air is 27-50/112 cwt., and in water 14-40/112 cwt. per nautical mile. The breaking strain of a cable thus manufactured is 62-1/2 cwt., and its cost about £47 per nautical mile. A diagram of this cable is shown at Fig. 32.

_Multiple Cable._--This form of cable is employed in cases where it is necessary to carry a large number of cables into the firing station, &c. It consists of seven single cores formed into a strand, over which a padding of hemp fibres is laid longitudinally, and over this again is laid an armouring of sixteen (No. 9 B. W. G.) galvanised iron wires, each one of which is covered with a layer of tarred tape put on spirally with a twist of one revolution in 15 inches. The exterior covering consists of two layers of hemp and composition, which is laid on with a short twist, and in opposite directions. The external diameter of this cable is 1-1/4 inch. Its weight in air and water is 78-25/112 cwt., and 45-32/112 cwt. respectively per nautical mile. Its breaking strain is 135 cwt., and cost about £357 per nautical mile. This form of cable is used in connection with a junction box, from which the single armoured cables leading to the different mines radiate, and is shown at Fig. 33.

_Circuit Closer Cable._--This cable, which connects the mine and circuit closer, has been found to be subjected to exceptional wear and tear, and therefore requires a special form of exterior protection. The core of this cable is the same as the one described at page 41, also it is covered with a similar padding of hemp, but instead of the iron wires as in the case of the multiple cable, &c., nine strands, each of which is composed of fourteen No. 22 Bessemer Steel Wires, are wound on, each such strand being covered with hemp, which is put on with a twist of one revolution in every 7-1/2 inches, the external covering being the same as in other cables.

This form of armouring for an electric cable possesses the qualifications of pliability, lightness, and great tensile strength. Its weight in air is 52-106/112 cwt., and in water 28-4/112 cwt. per nautical mile. Its breaking strain 65 cwt., and cost about £127 per nautical mile.

_Single Core Unarmoured Cable._--This form of cable is used in a system of defence by submarine mines to connect the detached works of a maritime fortress, &c., for the purpose of telegraphing.

It consists of the ordinary service core, over which are laid two servings of tarred hemp, put on spirally. The weight of this cable in air is 4-13/112 cwt., and in water 1-36/112 cwt. per nautical mile; its breaking strain is 7-1/2 cwt., and its cost per nautical mile is about £35.

_Special Cables._--In firing electrical submarine mines by means of cross bearings, a special cable is employed. As a general rule there would be three lines of mines placed to converge on one of the stations.

Each of these lines would be provided with a conducting wire in connection with the firing arrangements, while one line of wire in connection with the firing station would be required for telegraphing. For the purpose in question a four cored cable is used.

_Land Service Cable._--The cable employed for this service consists of a core formed similar to that of the multiple cable, described at page 41; over which is laid a padding of hemp, and finally two servings of tarred hemp laid spirally in opposite directions are wound on. Its weight in air is 16 cwt., and in water 4-50/112 cwt. per nautical mile. Its breaking strain 17-1/2 cwt., and cost per nautical mile about £137.

_Sea Service Cable._--This consists of a similar core to the land service cable, and padding of hemp, over which is laid an armouring of fifteen No. 13 galvanised iron wires, each one being covered with tarred tape, and finally the ordinary servings of tarred hemp. Its weight in air is 49-101/112 cwt., and in water 25-109/112 cwt. per nautical mile. Its breaking strain 65-100/112 cwt., and cost per nautical mile about £202.

When frictional electricity is used to fire high tension fuzes, it has been found by experiment that if several lines of insulated cables are laid in the same trench for a few hundred yards, the inductive effect of the electrical charge generated by a frictional machine is so great that its discharge through one cable is sufficient not only to fire the fuze in immediate connection with it, but by induction every other fuze in connection with the remaining wires laid in the trench. And this effect equally occurs when the electric cables are some feet apart, provided they run parallel for a few hundred yards, and whether the shore ends of the cables, the fuzes in connection with which are not intended to be fired, are insulated, or put directly to earth, the connections beyond the fuzes being to earth, or even insulated, provided a very few yards of conductor exist beyond the fuze.

The length of wire which it is necessary to use between the mine and its circuit closer would be quite sufficient for the purpose of effecting ignition by induction. With platinum wire fuses there is no danger whatever of the above happening, nor in the case of high tension fuzes is there so much danger of ignition by induction, when a constant instead of a frictional electric battery is used to generate the current.

Another mode of protecting an insulated cable is to place it, as it were, in the core of a hempen cable. In forming the rope on the cable, great care is necessary to prevent any serious amount of torsion, or tension coming on the insulated wire, either of which would most assuredly result in injury to the cable. This form of cable might in connection with obstructions, &c., be of great use, as on account of its closely resembling an ordinary rope, it would be very unlikely to excite suspicion, and so would most probably be cut, the result of which, by previous arrangement, would be an explosion of a mine, or by means of a galvanometer, &c., an indication that the obstructions, &c., were being interfered with.

_Jointing Electrical Cables._--This is a very important point in connection with a system of defence or offence by electrical torpedoes. In many instances it will be found necessary to join either two lengths of cable, or an insulated wire and a cable, together, in both of which cases, great care must be used in making the joints, so that the insulation and the continuity of the circuit may be perfect.

Many species of junctions have been from time to time devised, the most practical and generally employed of which are:--

1.--India rubber tube joint. 2.--Mathieson's joint. 3.--Beardslee's joint. 4.--McEvoy's joint. 5.--Permanent junction.

_India rubber Tube Joint._--This form of joint is a very useful one for extempore purposes, being easily and quickly made, and being very effective. At Fig. 34 is shown a sketch of such a junction. About 1·5 inches of the copper conductor of the two insulated cables are laid bare and connected together by means of Nicoll's metallic joint, as shown at Fig. 36, or by turning one of the conductors round the other, their ends being carefully pressed down by means of pliers, to prevent any chance of the india rubber tube being pierced; over the splice thus formed serve some twine, and over the whole put a coating of india rubber cement, grease, &c., then draw the vulcanised india rubber tube, which has been previously placed on one of the insulated cables, over the splice _a_, as shown at _b_, and secure it firmly by means of twine, _c_, _c_, and then to prevent any strain being brought on the joint, form a half-crown as shown in Fig. 35 at _A_.

In forming the splice, it is very important that the metallic ends should be perfectly clean. The danger to this mode of jointing of the piercing of the tube by the ends of the conductors is entirely removed by employing the Nicoll metallic joint, which is formed as follows:--

_Nicoll Metallic Joint._--One of the conducting wires, as _a_, Fig. 36, is formed into a spiral twist by means of a very simple instrument, and the other wire _b_, which is left straight, is inserted into the spiral, the whole being placed on an anvil, and pressed closely and securely together by a single blow of a hammer.

_Mathieson's Joint._--This somewhat complicated, though very effective mode of jointing, which is adopted in the English torpedo service, is shown at Fig. 37, in elevation and section. It consists of two ebonite cylinders _a_, _a_, through which the cables to be connected are passed. Within these cylinders an ebonite tube _b_, _b_ is placed, the ends of which are wedge-shaped, and which press against two vulcanite rings _c_, _c_; in the interior of this tube _b_, _b_ is the metallic joint _d_ of the two cables. The centre of the tube _b_, _b_ is of square section, and fits into a hollow of similar form in the cylinders _a_, _a_, the object of this being to prevent any twisting of the wires during the process of screwing up, which would be liable to injure the metallic joint _d_.

The manner of making this joint will be easily understood from the figure. With this, as with all other temporary joints, it is advisable to form a half-crown in the cable, including the joint.

_Beardslee's Joint._--This form of temporary joint when used with strand conductors, which are composed of a number of small wires, has been found to be exceedingly useful and effective, the only defect of such a joint being the liability of straightening the wires of the conductors should a direct strain be brought upon the wire extremities. Fig. 38 represents a section of this joint; it consists of an ebonite cylinder _a_, one end of which is solid, and the other open and fitted with a screw thread, into which is screwed a plug _b_; through both the plug _b_, and the solid end of the cylinder _a_, perforations are made just large enough to admit the insulated wires _c_, _c_; about half an inch of the extremities of these wires are bared and cleaned, and then passed, the one through the plug _b_, a disc of vulcanised india rubber _d_, and a metal disc _e_, and the end of the strand conductor turned back on the face of this metal disc, the other through the perforation in the solid end of the cylinder _a_, then through similar discs _d_ and _e_, and the end of the strand conductor treated in the same manner as the former one; then by means of the screw plug _b_, the two metallic discs _b_, _b_, and consequently the bare extremities of the strand conductors are brought into close metallic contact.

_McEvoy's Joint for Iron Wire covered Cable_.--This form of joint is shown in section at Fig. 39. Two brass caps _a_, _a_ are slipped over the ends of the cables required to be joined, then the iron wire and other coverings of the cables down to the insulating substance are removed, the former being bent back close against the bottom of the caps _a_, _a_, as shown in Fig. 39 at _b_, _b_; the cores of the cables are then joined by an india rubber temporary joint _c_, which has been described at page 45: the whole is then placed in the body of the joint, and the brass caps _a_, _a_ screwed up, jamming the bent back iron wires against a solid piece of brass _d_, _d_, by which means a firm and perfect joint is made in the cables.

Fig. 40 represents a section of a McEvoy temporary joint for single cored unarmoured cables, which seems to fulfil all the conditions necessary to a perfect joint of that description. This joint is, with the exception of there being two screw plugs instead of one, very similar to Beardslee's joint described at page 46; this alteration is a great improvement, remedying as it does the one defect of Beardslee's joint, viz., the liability of the cables to be drawn apart due to any great tension being brought on them.

A permanent joint in electrical submarine cables, which from its nature requires to be an exceptionally good one, is a somewhat difficult and troublesome operation, and also requires a considerable time to form a thoroughly reliable one.

_Siemens's Methods of Jointing._--The following methods, and instructions for forming such joints, are those adopted by Messrs. Siemens Brothers in connection with their telegraph cables, and which will be found generally applicable to all insulated cables.

_The Formation of a Joint in the Conductor of an Insulated Cable._--The conductor is either covered with a gutta percha or an india rubber dielectric. In both cases cut off the dielectric so as to bare the conductor-wire for a length of about three inches, taking care never to cut at right angles to the conductor-wire, for fear of injuring it with the cutting-knife or scissors.

Then clean the wires forming the strand with file-card and emery-paper, and solder them into a solid bar for a length of about one inch.

Having soldered the wires, forming the ends of the two lengths of conductors to be joined, into two solid rods, file each of them off in a slanting manner, so that they will form a scarf-joint when put together.

Place the two ends of strand in the two small vices on a stand which is supplied for the purpose, so that the two scarfed ends overlap each other, and bind them round with a piece of fine black iron wire, in the shape of a spiral, so as to keep the ends close together, then solder the two ends together by applying a hot soldering iron.

Then remove the iron binding wire and clean up the joint, filing off all unnecessary solder.

And make a band of four fine tinned copper wires, and bind them tightly side by side round the joint, covering the whole length of the scarf, and then solder the band and joint solidly together.

Then make another band of four fine tinned copper wires and bind them round the joint in the same manner as before, but extending about a quarter of an inch beyond each end of the other binding wire, the parts only of this second binding which project beyond the end of the first binding are to be soldered, so that the centre part remains loose and may keep up a connection between the two ends by forming a spiral between them in the event of the scarf giving way and the two ends of the conductor separating slightly.

This form of joint is called the "spring" joint.

The finished joint should be washed with spirit of wine and brushed, so as to take away all particles of soldering flux, and to avoid oxidation of the wire. The washed joint should then be dried with a piece of cloth and exposed to the flame of a spirit lamp to dry it thoroughly. A cable conductor ought never to be jointed with the help of soldering acid, but with that of resin, sal ammoniac, or borax only, so that any chance oxidation, and consequently destruction, of the conducting wire may be avoided.

There are other modes of jointing conductors, such as the twisting and scale joint, but the foregoing method will sufficiently explain this part of electric cable work.

_The Formation of a Joint in an India rubber Insulated Cable._--In making a joint in any insulated cable, the very greatest care must be taken to keep the hands, tools, and materials clean and dry.

Remove the felt for about twelve inches from each end of the core by soaking it with mineral naphtha and then rubbing it off clean with the file-card. The cleaned surface sear with a red-hot iron, to burn off all remaining fibres of the felt. Wash these seared ends clean with naphtha.

Then cut off about four inches of the insulating material (taking care never to cut at right angles to the conducting wire for fear of injuring it) so as to leave enough of the conductor bare to join and solder in the manner described at page 47.

After the conductor is jointed and soldered, clean again the seared parts of the insulator with the glazed side of the squares of cloth moistened with mineral naphtha, so as to leave a clean adhesiveness only; taper again the insulating material down to the conductor for about two inches on each side of the conductor-joint with a pair of curved and very clean scissors.

The tapering must be completed in such slanting way that the different layers of the dielectric are so far exposed as to enable a secure laying on of the new jointing material.

India rubber core consists chiefly of three layers of insulating material: the first layer next to the strand is called the pure or brown; the second layer is the white or separating; the third layer is the light red or jacket rubber.

Coat the conductor with a pure (brown) rubber tape tightly laid on in a spiral form, commencing at the spot where the separator (white) ends, across the corresponding place on the opposite side of the joint and back again in a contrary direction. The ends are fastened down by pressing a clean, heated searing-iron or a heated knife on them. By doing so the band will stick; the remaining portions of the band to be cut off with the scissors.

Lay on tightly the separating india rubber tape in the same manner, but beginning where the jacket or outer layer of rubber ends. One lap will be sufficient.

Complete the insulation by lapping on tightly two layers of red india rubber tape: the last lap must cover each end of the core to four inches on each side of the conductor-joint, or extend to the searing or tackiness, but not beyond it.

Lay on three tight bindings of the cloth tapes, all in the same direction, care being taken to avoid wrinkles. The ends of the cloth tapes are cemented down with a thin coating of india rubber cement.

Immerse the joint in the jointing-bath at 150° to 200° F. and gradually raise the heat so that in half an hour the temperature will be 320° F., at which temperature keep the joint for twenty minutes: then take it out and let it cool in the open air.

_The Formation of a Joint in a Gutta percha Insulated Cable._--Having jointed the conducting wires in the manner described at page 47, clean and dry the joint well and cover the bare conductor with a thin layer of compound. This is best done by heating a small stick of compound to nearly its melting point, and rubbing it over the bare conductor, which has been previously heated with the flame of a spirit-lamp.

Heat the gutta percha covering of both ends gently until it is quite soft, without, however, causing it to bubble or burn. Draw, then, with the fingers, the gutta percha coverings of both ends down, tapering them off until they meet in the middle of the joint; heat them sufficiently to make them adhere together.

Apply a layer of compound on the tapered-off gutta percha in the same manner as described for coating the bare conductor, and cover it with a first coating of gutta percha sheet to about half the thickness necessary to finish the joint. This is done by heating a small sheet of gutta percha, of about one-eighth of an inch in thickness, until it is quite soft, and by pressing it in that state round the joint to the required size; the greatest care to be taken to expel all the air.

The projecting lips are then cut off with a pair of curved scissors. The seam thus produced is to be rubbed with a hot iron until it is completely closed and the joint well rounded off.

Apply another layer of compound and a second layer of gutta percha in exactly the same manner as described for the first layer; care, however, is to be taken to get the seam in this second layer of gutta percha not over, but as nearly as possible right opposite to, the seam in the layer underneath.

The whole to be worked as cylindrical as possible, and to a size not exceeding the original core. The joint, so far finished, is then to be cooled with water until the gutta percha is quite consolidated.

Another, the overlapping gutta percha joint, is made in the following manner:--

Cut off the two ends of the core, so that the gutta percha and the conductor-wire are flush. Warm the gutta percha for a distance of about three inches from each of the ends with the flame of a spirit lamp, and, when sufficiently soft, push it back until it forms an enlargement. The two ends of the conductor are then to be soldered according to instructions for making joint in conductors.

To have a perfectly clean surface of the two gutta percha enlargements, remove all impurities by the way of peeling them with a sharp knife. Warm gently both knobs and the copper joint, and cover the whole length of the bare wire with compound, planing it with a warm smoothing-iron.

Draw then with the fingers one of the warmed and softened knobs carefully up to the other knob or enlargement, leaving on its way a perfect tube of gutta percha upon the wire, decreasing gradually to the thickness of the copper strand towards the other knob. Any superfluous gutta percha is removed. This scarf is finished with a warm smoothing-iron, so as to unite it to the compound on the wire strand, and a thin layer of compound is also put over the scarf in the same manner as before.

The other knob is then warmed and drawn in the same way over the tube already formed, which is at the same time heated sufficiently to make the two adhere.

Apply a layer of compound on the second scarf of gutta percha, covering it in the same manner as described for coating the bare conductor, and cover it with a small sheet of gutta percha in the same manner as described above, so as to make the finished joint to the size of the core as manufactured.

_Rules to be observed in forming Joints._--The following rules must be carefully observed in forming either a temporary or permanent joint:--

1.--In laying bare the conductor, the dielectric should be warmed and then pulled off, so preventing any chance of it being damaged, which might be the case were the dielectric to be cut off.

2.--For a perfect junction, soldering is necessary.

3.--The wires before connection should be carefully cleaned, and the hands of those performing the work must be dry.

4.--Gutta percha should not be given too much heat, for it then becomes oily and will not, in that state, properly adhere.

5.--Grease and dirt must be scrupulously avoided.

Great care is absolutely necessary in making junctions, as they are the principal sources of defect in the insulation of electrical submarine cables.

_Junction Boxes._--When it is necessary to employ a multiple cable, a junction box is used to facilitate the connection of the several separate wires diverging from the extremities of such a cable. In one angle of such a box the multiple cable is introduced, while the separate cables make their exit on the opposite sides and pass to the different mines. Different views of a junction box are shown at Fig. 41, where _A_ is a plan of the top or lid, _B_ a plan of the bottom, with the lid off, _C_ an elevation, and _D_ a section of the box.

The manner of using the junction box is as follows:--

The multiple cable is put in at _a_, and secured there by means of a nipping hook, shown at Fig. 42, which hook passes through the bottom of the junction and is made secure by means of a nut. The single core cables radiating from the junction box pass through the openings _b_, _b_, _b_ on the sides, and angle opposite to where the multiple cable a enters. Each multiple cable is composed of seven cores, and each of these is connected by means of joints with the mine cables within the junction box, and each of these seven cables is secured by means of a nipper similar to, but smaller than, the one shown at Fig. 42, which are also secured by means of nuts, as in the case of the multiple cable nipping hook. When all the connections are made, the lid _A_ is placed so as to rest on the studs _c_, _c_, _c_, and firmly secured by a bolt _d_, which is made water-tight by means of a washer and nut.

By means of the nipping hooks, which take any strain that may be brought on the cables, the connections within the box are ensured against injury by such a cause.

To enable the whole to be lifted together for the purposes of examination of the cables, &c., a buoyed rope is connected to the eye-bolt _e_. For this service a dummy circuit closer is the best form of buoy, it having great buoyancy and resembling in appearance an active circuit closer.

A junction box should be placed in such a position as to be easily attained, even in the presence of an enemy, and its buoy should, if possible, not be seen. It is also very essential that it should be in a safe and guarded position, for any injury to the junction box or multiple cable would be fatal to the group of mines in connection.

In the following cases, special junction boxes are used:--

1.--A seven cored armoured cable to be connected direct to another length of the same.

2.--A single armoured cable to be connected as in foregoing instance.

3.--A T junction box for the branch system of electrical contact mines.

_Junction Box for Multiple Cables._--At Fig. 43 is represented a plan of lower half of this form of junction box. It consists of a pair of cast iron plates of precisely similar form to the one shown at Fig. 43, and so made as to be capable of being fastened tightly together by means of four bolts and nuts passing through the holes _a_, _a_. The grooves _b_, _b_ at the two extremities are just large enough to grip the armoured cable firmly, when the upper and lower parts are screwed together. A larger space is provided in the hollow for the joint.

_Junction Box for Single Cored Cables._--For this purpose a junction box similar to, but smaller than the one above described is employed.

_T Junction Box._--This form of junction box is employed when the system of electrical contact mines on branches from a single cable is used. This system is dependent on the use of a platinum wire fuze in connection with a platinum wire bridge in each branch close to its junction with the main cable.

This form of junction box, which is shown at Fig. 44 is very similar to the one used for the connection of two multiple cables, only differing in its shape, which is that of a T. _a_ is a disconnector, which will be described further on; _b_, _b_, _b'_ are the armoured electric cables, _b_, _b_ being the main, and _b'_ the branch cable in connection with the forked joint formed within the T junction box; _c_, _c_, _c_ are Turk's heads formed to prevent any strain being brought on the forked joint. This form of Turk's head is made by turning back the wires of the cable armouring, and frapping them round with spun yarn until the necessary size and shape is attained.

_McEvoy's Turk's Head._--Another form of Turk's head, devised by Captain McEvoy, is shown at Fig. 45. It consists of two separate pieces of brass, _a_ and _b_, the former screwing over the latter. The mode of using it is as follows:--

Slip the piece of brass _b_ over the cable _c_, and turn back the wires of the cable _d_, _d_, &c., so that they lie against the shoulder of the brass piece _b_, then slip the other piece of brass _a_ over the cable and screw it on the piece _b_, firmly jamming the turned back wires _d_, _d_, &c. This is a very neat and quick method of forming a Turk's head, and it should be invariably used in preference to the foregoing method, which is clumsy, and which takes some time to form.

The section of a disconnector is shown at Fig. 46. It consists of an iron cover, or dome _a_, which is provided with a screw fitting on to another screw on the ebonite body _b_ of the apparatus. When the dome _a_ is screwed tightly down on the washer _i_, the whole is made perfectly watertight. _c_, _c_ are insulated terminals for connecting the cores of the branch and main cables after their armouring has been removed, as shown at Fig. 44. _d_, _d_ are two copper conducting wires (No. 16 B. W. G.) passing through the centre of the ebonite body _b_, and projecting into the interior of the apparatus. These wires are held in position and insulated by means of a composition formed of a mixture of pitch, tallow, beeswax and gutta percha. This composition is put on whilst hot and allowed to cool gradually, when it becomes hard and durable. Great care is necessary to ensure the cavity within the ebonite body _b_ being completely filled, as otherwise a leakage might occur, owing to the great pressure of water at depths where the disconnection would be generally used. _f_ is a boxwood cover which is slipped on, and fits fairly tight to the ebonite body _b_; _g_ is a piece of thin platinum wire, weighing 1·6 grains to the yard, and being 4/10 inch in length; _h_ is an ebonite pin, which passes through two small holes in the boxwood cover _f_, into which it fits tightly, and in such a position as to be directly beneath the platinum wire bridge _g_, when the boxwood cover _f_ is fixed on. The pin _h_ is pushed through the holes in the cover _f_ from the outside, so as to pass beneath the bridge _g_ after the priming has been inserted, and the cover has been placed on.

When prepared for use, the platinum wire bridge _g_ is surrounded by some loose gun-cotton priming, sufficient in quantity to blow off the boxwood cover _f_, without destroying the dome _a_; the cover _f_ being blown off, carries the ebonite pin _h_ with it, and through the platinum wire bridge _g_, thereby rupturing it, and breaking the continuity of the circuit. The object of so doing is to cut off the connection of an exploded mine, so that the full amount of the firing current is available for the other mines, and not suffered to be wasted by passing through the exposed wire of the broken circuit, which, were the disconnector not employed, would be the case.

When any particular mine of a system is struck, the current passes through the main cable _b_, the disconnector _a_ (which is in connection with that mine), and branch cable _b'_ to the fuze, and so explodes the mine, and destroys the platinum wire bridge _g_ of the disconnector at practically the same instant. The effect of the latter operation would be to cut off and insulate the branch cable of the exploded mine, and so prevent any loss of the electrical current, when another mine of that system is required to be fired.

The platinum wire bridge _g_ is 4/10 inch long, while that of the fuze is 3/10 inch, the object of this difference in length of the bridges being to ensure the former one _g_ being fired, and thus the insulation made doubly sure. Many other forms of disconnectors have been devised, but none have proved in practice so effective as the one just described.

_Mooring Electrical Submarine Mines._--This is one of the most difficult problems to be solved in connection with a system of submarine mines. The objects to be attained in mooring are as follows:--

1.--The mines should preserve the exact positions in which they are laid down.

NOTE.--From the comparatively small radius of destructive effect, of even heavily charged submarine mines, it will be understood how absolutely essential, in the case of mines fired by judgment, it is that this object should be attained.

2.--The mooring chains, or ropes, must be so arranged that no twisting whatever should occur, as otherwise fracture of the insulated wire would be likely to happen.

3.--In the case of buoyant mines, their distance from the bottom must be so adjusted, that at no time shall a vessel passing over them be out of their vertical range of destruction, nor shall they be visible.

The difficulties attendant upon the efficient mooring of submarine mines are immense, as will be understood when the action of gales of wind, and strong tides, which latter vary continually in their direction and in their rise and fall, are taken into consideration.

The foregoing remarks apply more particularly to a system of buoyant submarine mines, as those placed on the ground are comparatively easy to moor.

Several modes of mooring buoyant submarine mines have been suggested, the most practicable of which are as follows:--

1.--Ladder moorings. 2.--Fore and aft moorings. 3.--Austrian method of mooring. 4.--Single rope mooring.

_Ladder Mooring._--This is a method of mooring, which in places where it may be necessary to place the anchors far apart will be found useful.

The circuit closer is connected to the mine by two ropes which lead thence to two anchors, the ropes being separated by wooden rounds, or spreaders, 1 to 3 feet long, by which the tendency to twisting is prevented.

The anchors are placed some 12 feet apart.

The only defect of the ladder mooring is the quantity of sea-weed, &c., that is liable to be lodged on the rounds, thus causing the circuit closer to be drawn out of its proper position.

_Fore and aft Mooring._--This mode may be advantageously employed in a tideway where the current runs very strong, that is to say, five knots per hour, or more. It consists simply of two anchors, one of which is moored up, and the other down the stream.

_Austrian Method of Mooring._--This method of mooring, adopted by the Austrians during the war of 1866, is shown at Fig. 47. It consists of a wooden triangular platform on which several heavy weights _a_, _a_, _a_ are placed; the mine is attached to this platform by means of three wire ropes _b_, _b_, _b_, connected to the angles of the latter, and fastened to three chains, which by means of a catch holds the mine at the position required.

This catch consists of a pulley attached to the extremity of the wire rope of the platform, through which the mooring chain of the mine is passed, and fastened by a key at the required depth by means of a self-acting arrangement.

This key, which is of considerable weight, slips down as the mine is being hauled into position, but the moment the chain is slacked, two arms catch into a link of the chain, and so hold the mine in position. The weight of such a key is about 60 lbs. It is fitted with nuts, &c., to enable it to be taken to pieces.

This plan of mooring proved very effective in the harbours of the Adriatic, where there is hardly any tide or current to twist the mooring ropes, or otherwise disturb the mines. The Austrians have lately adopted the mushroom sinker in place of the wooden platform and weights, for their anchor.

_Single Rope Mooring._--This simple method of mooring has after numerous exhaustive experiments been adopted as the most practicable and effective of all others. Whenever possible, a wire instead of hempen cable should be used to connect the mine and its circuit closer to the mooring anchor, as the former is less liable to twist, kink, or wear from friction than the latter.

A ground mine with circuit closer attached is represented at Fig. 48, where _a_ is the wire mooring rope, _b_ the electric cable leading from the mine to the circuit closer, _C_, and _c_ the cable leading from the firing station to the mine; _d_ is the oblong sinker attached to the mine, and _e_ the tripping chain leading to the shore, to which the cable _c_ is attached at intervals, so that by underrunning the electric cable, the tripping chain may be easily picked up, and the mine raised.

At Fig. 49 is shown a buoyant mine. The only difference in the mooring of this and the one before described, is that instead of resting on its anchor on the ground, it is moored at a certain distance above its anchor _d_, to which it is secured by a chain _e_.

Fig. 50 represents an electro contact mine. _M_ is the mine with circuit closer enclosed, _a_ the wire mooring rope, _d_ the mushroom anchor, and _b_ the electric cable leading from the mine to the disconnector _D_.

The mushroom sinker or anchor, which is undoubtedly the most effective of all other forms of mooring anchors used for the purposes of anchoring submarine mines, is shown at _e_, Fig. 49; the legs are added for use on rocky or hard bottoms, under which circumstances the weight of the anchor should also be increased.

For ground mines the form of sinker shown at _d_, Fig. 48 is employed; it is of an oblong shape, and hollowed out in the centre to allow of its being lashed close up to the mine.

Large blocks of stones with their bases slightly hollowed are useful as extempore moorings, so also is the one shown at Fig. 51, which consists of a strong heavy wooden shaft _a_, with a number of wooden arms _b_, _b_ attached to its base; this form of extempore sinker was considered very efficient by the American authorities.

The wooden weighted platform, which was described at page 56, is also a very useful form of extempore sinker.

For dead weight moorings, pigs of ballast, heavy stones, &c., may be used.

The weight of the anchor or sinker for mooring submarine mines is a very important consideration. It will depend on the amount of buoyancy of the mine, on the strength of current, and on the nature of the bottom, also whether the mines are to be hauled down to, or moored with the anchor.

Stotherd uses the following formula:

W = [2rt](B^{2} + P^{2})

where B is the excess of the flotation over the weight of the charge of a given submarine mine;

P is the pressure exerted by any given current on the same buoyant mine;

W the weight of sinker necessary to overcome the tendency of the mine to move. In still water P becomes nothing, and therefore W equal to 2 B, that is, in still water double the buoyancy of a mine is a sufficient weight for its anchor.

The value of P may be found from the formula P = 4·085 × V^{2}, where V is the velocity of the current in miles per hour.

From this equation P will be found in terms of pressure in pounds per square foot of flat surface, which is nearly double that on the curved surface of a cylinder.

In regard to the amount of buoyancy of a submarine mine, it has been found by actual practice that in the case of a mine moored in still water it should certainly be not less than the weight of the charge, whilst if subjected to the lateral pressure due to a current, it should be not less than three times the pressure exerted by the current.

It is always necessary to allow an excess of buoyancy over the calculated amount to counteract any leakage, or other disturbing cause which might otherwise materially affect the efficiency of the mine.

There are two modes of placing a mine in position; either by attaching the anchor, with the cable necessary for the depth of water, to the mine, and lowering both together, or by placing the anchor first, and then hauling the mine down to it, and by means of a catch, fastening it at the required depth.

The first mode is exceedingly simple, but except under very favourable circumstances cannot be relied on when firing by observation is the means adopted to explode a system of submarine mines. The second plan is practically easy to carry out, and by it a mine may be placed more accurately. To enable either of the above methods to be properly carried out, specially fitted steamboats, &c., are requisite.

At Fig. 52 is represented a 42 feet launch fitted for laying down a submarine mine by the first of the two modes enumerated above.

_a_ is the mine; _b_ is the electric cable carried from the drum _c_ to the charge, and connected for use; _d_ is the circuit closer, which is attached to the mine by its electric cable and mooring rope; _f_ is the mushroom sinker attached by means of its mooring chain to the mine, it is suspended by a slip rope _g_, which passes over a small crutch fitted with a sheave _h_; _i_ is a hollow iron derrick, and _k_ the tackle and fall for lifting mine into boat; this derrick is formed of an iron tube about 3 inches diameter, 3/8 inch thick, and 10 feet 6 inches long; it is attached to an iron tube mast of similar diameter and thickness to the derrick, but 12 feet 3 inches long, an iron chain 6 feet 6 inches long and 5/8 inch diameter, connects the derrick to the mast; _m_ is a leading sheave to keep the cable clear whilst it is being paid out; _l_ is a crab, for working the tackle _k_, &c., and _c_ is the drum on which the electric cable is wound.

In connection with the defence of a harbour by a system of electrical submarine mines of large size, it will be necessary to employ a service of steamtugs, steamboats, mooring-barges, &c., specially fitted for such work. One of the great advantages of the hauling down method of placing mines in position, is, that the anchors, with the cables connected thereto, may be carefully and accurately got into position during the time of peace, and the mines themselves, which should be kept in store ready fitted for immediate use, need not be placed in position until they are actually required. The drums used for reeling a multiple cable on, are capable of holding half a nautical mile in length. That used for a single core armoured cable is similar to but smaller than the aforesaid drum, and is capable of stowing one nautical mile of such a cable. For transportation wooden drums are ordinarily used.