Part 27

In the experiments made before the Atlantic Telegraph was finally decided on, 2000 miles of subterranean and submarine telegraphic wires, ramifying through England and Ireland and under the waters of the Irish Sea, were specially connected for the purpose; and through this distance of 2000 miles 250 distinct signals were recorded and printed in one minute.

First, as to the _Cable_. In the ordinary wires by the side of a railway the electric current travels on with the speed of lightning--uninterrupted by the speed of lightning; but when a wire is encased in gutta-percha, or any similar covering, for submersion in the sea, new forces come into play. The electric excitement of the wire acts by induction, through the envelope, upon the particles of water in contact with that envelope, and calls up an electric force of an opposite kind. There are two forces, in fact, pulling against each other through the gutta-percha as a neutral medium,--that is, the electricity in the wire, and the opposite electricity in the film of water immediately surrounding the cable; and to that extent the power of the current in the enclosed wire is weakened. A submarine cable, when in the water, is virtually _a lengthened-out Leyden jar_; it transmits signals while being charged and discharged, instead of merely allowing a stream to flow evenly along it: it is a _bottle_ for holding electricity rather than a _pipe_ for carrying it; and this has to be filled for every time of using. The wire being carried underground, or through the water, the speed becomes quite measurable, say a thousand miles in a second, instead of two hundred thousand, owing to the retardation by induced or retrograde currents. The energy of the currents and the quality of the wire also affect the speed. Until lately it was supposed that the wire acts only as a _conductor_ of electricity, and that a long wire must produce a weaker effect than a short one, on account of the consequent attenuation of the electrical influence; but it is now known that, the cable being a _reservoir_ as well as a conductor, its electrical supply is increased in proportion to its length.

The electro-magnetic current is employed, since it possesses a treble velocity of transmission, and realises consequently _a threefold working speed_ as compared with simple voltaic electricity. Mr. Wildman Whitehouse has determined by his ingenious apparatus that the speed of the voltaic current might be raised under special circumstances to 1800 miles per second; but that of the induced current, or the electro-magnetic, might be augmented to 6000 miles per second.

Next as to a _Quantity Battery_ employed in these investigations. To effect a charge, and transmit a current through some thousand miles of the Atlantic Cable, Mr. Whitehouse had a piece of apparatus prepared consisting of twenty-five pairs of zinc and silver plates about the 20th part of a square inch large, and the pairs so arranged that they would hold a drop of acidulated water or brine between them. On charging this Lilliputian battery by dipping the plates in salt and water, messages were sent from it through a thousand miles of cable with the utmost ease; and not only so,--pair after pair was dropped out from the series, the messages being still sent on with equal facility, until at last only a single pair, charged by one single drop of liquid, was used. Strange to say, with this single pair and single drop distinct signals were effected through the thousand miles of the cable! Each signal was registered at the end of the cable in less than three seconds of time.

The entire length of wire, iron and copper, spun into the cable amounts to 332,500 miles, a length sufficient to engirdle the earth thirteen times. The cable weighs from 19 cwt. to a ton per mile, and will bear a strain of 5 tons.

The _Perpetual Maintenance Battery_, for working the cable at the bottom of the sea, consists of large plates of platinated silver and amalgamated zinc, mounted in cells of gutta-percha. The zinc plates in each cell rest upon a longitudinal bar at the bottom, and the silver plates hang upon a similar bar at the top of the cell; so that there is virtually but a single stretch of silver and a single stretch of zinc in operation. Each of the ten cells contains 2000 square inches of acting surface; and the combination is so powerful, that when the broad strips of copper-plate which form the polar extensions are brought into contact or separated, brilliant flashes are produced, accompanied by a loud crackling sound. The points of large pliers are made red-hot in five seconds when placed between them, and even screws burn with vivid scintillation. The cost of maintaining this magnificent ten-celled Titan battery at work does not exceed a shilling per hour. The voltaic current generated in this battery is not, however, the electric stream to be sent across the Atlantic, but is only the primary power used to call up and stimulate the energy of a more speedy traveller by a complicated apparatus of “Double Induction Coils.” Nor is the transmission-current generated in the inner wire of the double induction coil,--and which becomes weakened when it has passed through 1800 or 1900 miles,--set to work to print or record the signals transmitted. This weakened current merely opens and closes the outlet of a fresh battery, which is to do the printing labour. This relay-instrument (as it is called), which consists of a temporary and permanent magnet, is so sensitive an apparatus, that it may be put in action by a fragment of zinc and a sixpence pressed against the tongue.

The attempts to lay the cable in August 1857 failed through stretching it so tightly that it snapped and went to the bottom, at a depth of 12,000 feet, forty times the height of St. Paul’s.

This great work was resumed in August 1858; and on the 5th the first signals were received through _two thousand and fifty miles_ of the Atlantic Cable. And it is worthy of remark, that just 111 years previously, on the 5th of August 1747, Dr. Watson astonished the scientific world by practically proving that the electric current could be transmitted through a _wire hardly two miles and a half long_.[55]

Miscellanea.

HOW MARINE CHRONOMETERS ARE RATED AT THE ROYAL OBSERVATORY, GREENWICH.

The determination of the Longitude at Sea requires simply accurate instruments for the measurement of the positions of the heavenly bodies, and one or other of the two following,--either perfectly correct watches--or chronometers, as they are now called--or perfectly accurate tables of the lunar motions.

So early as 1696 a report was spread among the members of the Royal Society that Sir Isaac Newton was occupied with the problem of finding the longitude at sea; but the rumour having no foundation, he requested Halley to acquaint the members “that he was not about it.”[56] (_Sir David Brewster’s Life of Newton._)

In 1714 the legislature of Queen Anne passed an Act offering a reward of 20,000_l._ for the discovery of the longitude, the problem being then very inaccurately solved for want of good watches or lunar tables. About the year 1749, the attention of the Royal Society was directed to the improvements effected in the construction of watches by John Harrison, who received for his inventions the Copley Medal. Thus encouraged, Harrison continued his labours with unwearied diligence, and produced in 1758 a timekeeper which was sent for trial on a voyage to Jamaica. After 161 days the error of the instrument was only 1m 5s, and the maker received from the nation 5000_l._ The Commissioners of the Board of Longitude subsequently required Harrison to construct under their inspection chronometers of a similar nature, which were subjected to trial in a voyage to Barbadoes, and performed with such accuracy, that, after having fully explained the principle of their construction to the commissioners, they awarded him 10,000_l._ more; at the same time Euler of Berlin and the heirs of Mayer of Göttingen received each 3000_l._ for their lunar tables.

The account of the trial of Harrison’s watch is very interesting. In April 1766, by desire of the Commissioners of the Board, the Lords of the Admiralty delivered the watch into the custody of the Astronomer-Royal, the Rev. Dr. Nevil Maskelyne. It was then placed at the Royal Observatory at Greenwich, in a box having two different locks, fixed to the floor or wainscot, with a plate of glass in the lid of the box, so that it might be compared as often as convenient with the regulator and the variation set down. The form observed by Mr. Harrison in winding up the watch was exactly followed; and an officer of Greenwich Hospital attended every day, at a stated hour, to see the watch wound up, and its comparison with the regulator entered. A key to one of the locks was kept at the Hospital for the use of the officer, and the other remained at the Observatory for the use of the Astronomer-Royal or his assistant.

The watch was then tried in various positions till the beginning of July; and from thence to the end of February following in a horizontal position with its face upwards.

The variation of the watch was then noted down, and a register was kept of the barometer and thermometer; and the time of comparing the same with the regulator was regularly kept, and attested by the Astronomer-Royal or his assistant and such of the officers as witnessed the winding-up and comparison of the watch.

Under these conditions Harrison’s watch was received by the Astronomer-Royal at the Admiralty on May 5, 1766, in the presence of Philip Stephens, Esq., Secretary of the Admiralty; Captain Baillie, of the Royal Hospital, Greenwich; and Mr. Kendal the watchmaker, who accompanied the Astronomer-Royal to Greenwich, and saw the watch started and locked up in the box provided for it. The watch was then compared with the transit clock daily, and wound up in the presence of the officer of Greenwich Hospital. From May 5 to May 17 the watch was kept in a horizontal position with its face upwards; from May 18 to July 6 it was tried--first inclined at an angle of 20° to the horizon, with the face upwards, and the hours 12, 6, 3, and 9, highest successively; then in a vertical position, with the same hours highest in order; lastly, in a horizontal position with the face downwards. From July 16, 1766, to March 4, 1767, it was always kept in a horizontal position with its face upwards, lying upon the same cushion, and in the same box in which Mr. Harrison had kept it in the voyage to Barbadoes.

From the observed transits of the sun over the meridian, according to the time of the regulator of the Observatory, together with the attested comparisons of Mr. Harrison’s watch with the transit clock, the watch was found too fast on several days as follows:

h. m. s. 1766. May 6 too fast 0 0 16·2 May 17 ” 0 3 51·8 July 6 ” 0 14 14·0 Aug. 6 ” 0 23 58·4 Sept. 17 ” 0 32 15·6 Oct. 29 ” 0 42 20·9 Dec. 10 ” 0 54 46·8 1767. Jan. 21 ” 1 0 28·6 March 4 ” 1 11 23·0

From May 6, which was the day after the watch arrived at the Royal Observatory, to March 4, 1767, there were six periods of six weeks each in which the watch was tried in a horizontal position; when the gaining in these several periods was as follows:

During the first 6 weeks it gained 13m 20s, answering to 3° 20′ of longitude.

In the 2d period of 6 weeks (from Aug. 6 to ” 8 17 ” 2 4 Sept. 17)

In the 3d period (from ” 10 5 ” 2 31 Sept. 17 to Oct. 29)

In the 4th period (from ” 12 26 ” 3 6 Oct. 29 to Dec. 20)

In the 5th period (from ” 5 42 ” 1 25 Dec. 20 to Jan. 21)

In the 6th period (from ” 10 54 ” 2 43 Jan. 21 to Mar. 4)

It was thence concluded that Mr. Harrison’s watch could not be depended upon to keep the longitude within a West-India voyage of six weeks, nor to keep the longitude within half a degree for more than a fortnight; and that it must be kept in a place where the temperature was always some degrees above freezing.[57] (However, Harrison’s watch, which was made by Mr. Kendal subsequently, succeeded so completely, that after it had been round the world with Captain Cook, in the years 1772-1775, the second 10,000_l._ was given to Harrison.)

In the Act of 12th Queen Anne, the comparison of chronometers was not mentioned in reference to the Observatory duties; but after this time they became a serious charge upon the Observatory, which, it must be admitted, is by far the best place to try chronometers: the excellence of the instruments, and the frequent observations of the heavenly bodies over the meridian, will always render the rate of going of the Observatory clock better known than can be expected of the clock in most other places.

After Mr. Harrison’s watch was tried, some watches by Earnshaw, Mudge, and others, were rated and examined by the Astronomer-Royal.

At the Royal Observatory, Greenwich, there are frequently above 100 chronometers being rated, and there have been as many as 170 at one time. They are rated daily by two observers, the process being as follows. At a certain time every day two assistants in charge repair to the chronometer-room, where is a time-piece set to true time; one winds up each with its own key, and the second follows after some little time and verifies the fact that each is wound. One assistant then looks at each watch in succession, counting the beats of the clock whilst he compares the chronometer by the eye; and in the course of a few seconds he calls out the second shown by the chronometer when the clock is at a whole minute. This number is entered in a book by the other assistant, and so on till all the chronometers are compared. Then the assistants change places, the second comparing and the first writing down. From these daily comparisons the daily rates are deduced, by which the goodness of the watch is determined. The errors are of two classes--that of general bad workmanship, and that of over or under correction for temperature. In the room is an apparatus in which the watch may be continually kept at temperatures exceeding 100° by artificial heat; and outside the window of the room is an iron cage, in which they are subjected to low temperatures. The very great care taken with all chronometers sent to the Royal Observatory, as well as the perfect impartiality of the examination which each receives, afford encouragement to their manufacture, and are of the utmost importance to the safety and perfection of navigation.

We have before us now the Report of the Astronomer-Royal on the Rates of Chronometers in the year 1854, in which the following are the successive weekly sums of the daily rates of the first there mentioned:

Week ending secs.

Jan. 21, loss in the week 2·2 ” 28 ” 4·0 Feb. 4 ” 1·1 ” 11 ” 5·0 ” 18 ” 4·9 ” 25 ” 5·5 Mar. 4 ” 6·0 ” 11 ” 6·0 ” 18 ” 1·5 ” 25 ” 4·5 Apr. 1 ” 4·0 ” 8 ” 1·5 ” 15, gain in the week 0·4 Apr. 22, ” 2·6 ” 29, loss in the week 1·4 May 6 ” 2·1 ” 13 ” 3·0 ” 20 ” 5·1 ” 27 ” 3·3 June 3 ” 2·8 ” 10 ” 1·8 ” 17 ” 2·0 ” 24 ” 3·0 July 1 ” 2·5 ” 8 ” 1·2

Till February 4 the watch was exposed to the external air outside a north window; from February 5 to March 4 it was placed in the chamber of a stove heated by gas to a moderate temperature; and from April 29 to May 20 it was placed in the chamber when heated to a high temperature.

The advance in making chronometers since Harrison’s celebrated watch was tried at the Royal Observatory, more than ninety years since, may be judged by comparing its rates with those above.

GEOMETRY OF SHELLS.

There is a mechanical uniformity observable in the description of shells of the same species which at once suggests the probability that the generating figure of each increases, and that the spiral chamber of each expands itself, according to some simple geometrical law common to all. To the determination of this law the operculum lends itself, in certain classes of shells, with remarkable facility. Continually enlarged by the animal, as the construction of its shell advances so as to fill up its mouth, the operculum measures the progressive widening of the spiral chamber by the progressive stages of its growth.

* * * * *

The animal, as he advances in the construction of his shell, increases continually his operculum, so as to adjust it to his mouth. He increases it, however, not by additions made at the same time all round its margin, but by additions made only on one side of it at once. One edge of the operculum thus remains unaltered as it is advanced into each new position, and placed in a newly-formed section of the chamber similar to the last but greater than it.

That the same edge which fitted a portion of the first less section should be capable of adjustment so as to fit a portion of the next similar but greater section, supposes a geometrical provision in the curved form of the chamber of great complication and difficulty. But God hath bestowed upon this humble architect the practical skill of the learned geometrician; and he makes this provision with admirable precision in that curvature of the logarithmic spiral which he gives to the section of the shell. This curvature obtaining, he has only to turn his operculum slightly round in its own place, as he advances it into each newly-formed portion of his chamber, to adapt one margin of it to a new and larger surface and a different curvature, leaving the space to be filled up by increasing the operculum wholly on the outer margin.

* * * * *

Why the Mollusks, who inhabit turbinated and discoid shells, should, in the progressive increase of their spiral dwellings, affect the peculiar law of the logarithmic spiral, is easily to be understood. Providence has subjected the instinct which shapes out each to a rigid uniformity of operation.--_Professor Mosely_: _Philos. Trans._ 1838.

HYDRAULIC THEORY OF SHELLS.

How beautifully is the wisdom of God developed in shaping out and moulding shells! and especially in the particular value of the constant angle which the spiral of each species of shell affects,--a value connected by a necessary relation with the economy of the material of each, and with its stability and the conditions of its buoyancy. Thus the shell of the _Nautilus Pompilius_ has, hydrostatically, an A-statical surface. If placed with any portion of its surface upon the water, it will immediately turn over towards its smaller end, and rest only on its mouth. Those conversant with the theory of floating bodies will recognise in this an interesting property.--_Ibid._

SERVICES OF SEA-SHELLS AND ANIMALCULES.

Dr. Maury is disposed to regard these beings as having much to do in maintaining the harmonies of creation, and the principles of the most admirable compensation in the system of oceanic circulation. “We may even regard them as regulators, to some extent, of climates in parts of the earth far removed from their presence. There is something suggestive both of the grand and the beautiful in the idea that while the insects of the sea are building up their coral islands in the perpetual summer of the tropics, they are also engaged in dispensing warmth to distant parts of the earth, and in mitigating the severe cold of the polar winter.”

DEPTH OF THE PRIMEVAL SEAS.

Professor Forbes, in a communication to the Royal Society, states that not only the colour of the shells of existing mollusks ceases to be strongly marked at considerable depths, but also that well-defined patterns are, with very few and slight exceptions, presented only by testacea inhabiting the littoral, circumlittoral, and median zones. In the Mediterranean, only one in eighteen of the shells taken from below 100 fathoms exhibit any markings of colour, and even the few that do so are questionable inhabitants of those depths. Between 30 and 35 fathoms, the proportion of marked to plain shells is rather less than one in three; and between the margin and two fathoms the striped or mottled species exceed one-half of the total number. In our own seas, Professor Forbes observes that testacea taken from below 100 fathoms, even when they are individuals of species vividly striped or banded in shallower zones, are quite white or colourless. At between 60 and 80 fathoms, striping and banding are rarely presented by our shells, especially in the northern provinces; from 50 fathoms, shallow bands, colours, and patterns, are well marked. _The relation of these arrangements of colour to the degree of light penetrating the different zones of depth_ is a subject well worthy of minute inquiry.

NATURAL WATER-PURIFIERS.

Mr. Warrington kept for a whole year twelve gallons of water in a state of admirably balanced purity by the following beautiful action:

In the tank, or aquarium, were two gold fish, six water-snails, and two or three specimens of that elegant aquatic plant _Valisperia sporalis_, which, before the introduction of the water-snails, by its decayed leaves caused a growth of slimy mucus, and made the water turbid and likely to destroy both plants and fish. But under the improved arrangement the slime, as fast as it was engendered, was consumed by the water-snails, which reproduced it in the shape of young snails, which furnished a succulent food to the fish. Meanwhile the _Valisperia_ plants absorbed the carbonic acid exhaled by the respiration of their companions, fixing the carbon in their growing stems and luxuriant blossoms, and refreshing the oxygen (during sunshine in visible little streams) for the respiration of the snails and the fish. The spectacle of perfect equilibrium thus simply maintained between animal, vegetable, and inorganic activity, was strikingly beautiful; and such means might possibly hereafter be made available on a large scale for keeping tanked water sweet and clean.--_Quarterly Review_, 1850.

HOW TO IMITATE SEA-WATER.