Chapter 3

The _Gitana_ was tried with speeds varying between 0 and 4 kilometers. The corresponding tractive stresses have been reduced to the same transverse section as in the Pictet model in order to render the observations comparable. At slight speeds, and up to 19.5 kilometers per hour, the _Gitana_, which is the sharper, runs easier and requires a slighter tractive stress. At such a speed there is an equality; but, beyond this, the Pictet boat presents the greater advantages, and, at a speed of 27 kilometers, requires a stress about half less than does the _Gitana_. Such results explain themselves when we reflect that at these great speeds the _Gitana_ sinks to such a degree that the afterside planks are at the level of the water, while the Pictet model rises simultaneously fore and aft, thus considerably diminishing the submerged section.

With low or moderate speeds there is a perceptible equality between the theoretic curve and the curve of the fast boat; but, starting from 16 kilometers, the stress diminishes. The greater does the speed become, the more considerable is the diminution in stress; and, starting from a certain speed, the rise of the boat is such as to diminish its absolute tractive stress--a fact of prime importance established by theory and confirmed by experiment.

The curves in Fig. 4 show the power in horses necessary to effect progression at different speeds. The curve, A, has reference to an ordinary boat that preserves its water lines constant, and the curve, B, to a swift boat of the same tonnage. Up to 16 kilometers, the swift vessel presents no advantage; but beyond that speed, the advantage becomes marked, and, at a speed of 27 kilometers, the power to be expended is no more than half that which corresponds to the same speed for an ordinary boat.

The water escapes in a thin and even sheet as soon as the tractive stress exceeds 2,000 kilogrammes; and the intensity and size of the eddies from the boat sensibly diminish in measure as the speed increases.

The interesting experiments made by Mr. Pictet seem, then to clearly establish the fact that the forms deduced by calculation are favorable to high speeds, and will permit of realizing, in the future, important saving in the power expended, and, consequently, in the fuel (much less of which will need to be carried), in order to perform a given passage within a given length of time. Thus is explained the great interest that attaches to Mr. Pictet's labors, and the desire that we have to soon be able to make known the results obtained with such great speeds, not when the boat is towed, but when its propulsion is effected through its own helix actuated by its own engine, which, up to the present, unfortunately, has through its defects been powerless to furnish the necessary amount of power for the purpose.--_La Nature_.

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INITIAL STABILITY INDICATOR FOR SHIPS.

For a vessel with a given displacement, the metacenter and center of gravity being known, it is easy to lay off in the form of a diagram its stability or power of righting for any given angle of heel. Such a diagram is shown in Fig. 3, in which the abscissæ are the angles of the heel, and the ordinates the various lengths of the levers, at the end of which the whole weight of the vessel is acting to right itself. The curve may be constructed in the following manner: Having found by calculation the position of the transverse metacenter, M, for a given displacement--Figs. 1 and 2--the metacentric height, G M, is then determined either by calculations, or more correctly by experiment, by varying the position of weights of known magnitude, or by the stability indicator itself. Suppose, now, the vessel to be listed over to various angles of heel--say 20 deg., 40 deg., 60 deg., and 80 deg.--the water lines will then be A C, D E, F K, and H J respectively, and the centers of buoyancy, which must be found by calculation, will be B1, B2, B3, and B4. If lines are drawn from these points at right angles to the water levels at the respective heels, the righting power of the vessel in each position is found by taking the perpendicular distances between these lines and the center of gravity, G. This method of construction is shown to an enlarged scale in Fig. 2, where G is the center of gravity, B1 Z1, B2 Z2, B3 Z3, and B4 Z4 the lines from centers of buoyancy to water levels; and G N, G O, and G P the distances showing the righting power at the angles of 20 deg., 40 deg., and 60 deg. respectively, and which to any convenient scale are set off as the ordinates in the stability curve shown in Fig 3.

Having obtained the curve, A, in this manner for a given metacentric height, we will suppose that on the next voyage, with the same displacement, it is found that, owing to some difference in stowage, the center of gravity is 6 in. higher than before. The ordinates of the curve will then be G¹ N¹ and G¹ O¹--Fig.2--and the stability curve will be as at C--Fig. 3--showing that at about 47 deg. all righting power ceases. Similarly, if the center of gravity is lowered 6 in. on the same displacement, the curve, B, will be found, and in this manner comparative diagrams can be constructed giving at a glance the stability of a vessel for any given draught of water and metacentric height.

The object of Mr. Alexander Taylor's indicator is to measure and show by simple inspection the metacentric height under every condition of loading, and therefore to make known the stability of the vessel. It consists of a small reservoir, A, Fig. 4, placed at one side of the ship, in the cabin, or other convenient locality, communicating by a tube with the glass gauge, B, secured at the opposite side, the whole being half filled with glycerine, which is the fluid recommended by Mr. Wm. Denny, though water or any other liquid will answer the purpose. At one side of the gauge is the circular scale, C, capable of being revolved round its vertical axis, as well as adjusted up and down, so as to bring the zero pointer exactly to the top of the fluid when the vessel is without list. Round the top of the scale, at D, are engraved four different draughts, and under these are the metacentric heights. Test tanks of known capacity are placed at each side of the vessel, but in no way connected with the reservoir or gauge. The metacentric height is found as follows: The ship being freed from bilge water, the roller scale is turned round to bring to the front the mark corresponding with the mean draught of the vessel at the time, and the zero pointer is placed opposite the surface of the liquid in the gauge. One of the test tanks being filled with a known weight of water, the vessel is caused to list, and in consequence the liquid in the tube takes a new position corresponding with the degree of heel, the disturbance being greater according as the vessel has been more or less overbalanced. The scale having previously been properly graduated, the metacentric height for the draught and state of loading can be at once read off in inches, while as a check the water can be transferred from the one test tank to the other, and the metacentric height read off as before, but on the opposite side of the zero pointer. At the same time the angle of heel is shown on a second graduated scale, E. Having obtained the metacentric height, reference to a diagram will at once show the whole range of stability; and this being ascertained at each loading, the stowage of the cargo can be so adjusted as to avoid excessive stiffness in the one hand and dangerous tenderness on the other. It will thus be seen that Mr. Taylor's invention promises to be of great practical value both in the hands of the ship-builder and ship-owner, who have now an instrument placed before them, by the proper use of which all danger from unskillful loading can be entirely avoided.--_The Engineer_.

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SCRIVANOW'S CHLORIDE OF SILVER PILE.

Considerable attention has been attracted lately at Paris among those who are interested in electrical novelties to a chloride of silver pile invented by Mr. Scrivanow. The experiments to which it has been submitted are, in some respects, sufficiently extraordinary to cause us to make them known to our readers, along with the inventor's description of the apparatus.

Mr. Scrivanow's intention appears to be to apply this pile to the lighting of apartments, and even to the running of small motors, and, for the purpose of actuating sewing machines, he has already constructed a small model whose external dimensions are 160 x 100 x 90 millimeters.

"My invention," says the inventor, "is intended as an electric pile capable of regeneration. The annexed Fig. 1 shows a vertical arrangement of the apparatus, and Fig. 2 a horizontal one. In the latter, two elements are represented superposed.

"My pile consists of a prism of retort carbon (a) covered on every side with pure chloride of silver (b). The carbon thus prepared is immersed in a solution of hydrate of potassium (KHO) or of hydrate of sodium (NaHO), marking 1.30 to 1.45 by the Baumé areometer, the solvent being water.

"In the vicinity of the carbon is arranged the plate to be attacked--a plate of zinc (c) of good quality. The surface of the electrodes, and their distance apart, depends upon the effects that it is desired to obtain, and is determined in accordance with the well known principles of electro-kinetics.

"The chemical reactions that take place in this couple are multiple. In contact with a sufficiently concentrated solution of hydrate of potassium or sodium, the chloride of silver, especially if it has been recently prepared, passes partially into the state of brown or black oxide, so that the carbon becomes covered, after remaining sufficiently long in the exciting liquid, with a mixture of chloride and oxide of silver. When the circuit is closed, the chloride becomes reduced to a spongy metallic state and adheres to the surface of the carbon. At the same time the zinc passes, in the alkaline solution, into a state of chloride and of soluble combination of zinc oxide and of alkali.

"To avoid all loss of silver I cover the carbon with asbestos paper, or with cloth of the same material, d. My piles are arranged in ebonite vessels, A, which are flat, as in Fig. 1, or round, as in Fig. 2.

"In Fig 1 there is seen, at e, gutta-percha separating the zinc from the carbon at the base.

"Under such conditions, we obtain a powerful couple that possesses an electro-motive power of 1.5 to 1.8 volts, according to the concentration of the exciting liquid. The internal resistance is extremely feeble. I have obtained with piles arranged like those shown in the figures nearly 0.06 ohm, the measurements having been taken from a newly charged pile.

"When the element is used up, and, notably, when all the chloride of silver is reduced, it is only necessary to plunge the carbon with its asbestos covering (after washing it in water) into a chloridizing bath, in order to bring back the metallic silver that invests the carbon to a state of chloride, and thus restore the pile to its primitive energy. After this the carbon is washed and put back into the exciting liquid.

"These reductions of the chloride of silver during the operation of the pile can be reproduced _ad infinitum_, since they are accompanied by no loss of metal. The alkaline liquid is sufficient in quantity for two successive charges of the couple.

"The chloridizing bath consists of 100 parts of acetic acid, 5 to 6 parts, by weight, of hydrochloric acid, and about 30 parts of water.

"Other acids may be employed equally as well. A bath composed of chlorochromate of potassium and nitric or sulphuric acid makes an excellent regenerator.

"To sum up, I claim as the distinctive characters of my pile:

"1. The use of the potassic or sodic alkaline liquid conjointly with chloride of silver, and the oxide of the same, that forms through the immersion of the carbon in a chloridizing bath.

"2. The use of retort or other carbon covered with the salt of silver above specified.

"3. The arrangement and construction of my pile as I have described."

In the experiments recently tried with Mr. Scrivanow's pile, a large sized battery was made use of, whose dimensions were 300 x 145 x 125 millimeters, and whose weight was from 5 to 6 kilogrammes. The results were: intensity, 1 ampere; electro-motive power, 25 volts, corresponding to an energy of 25 volt-amperes, or about 2.5 kilogrammeters per second. The pile was covered with a copper jacket whose upper parts supported two Swan lamps. Upon putting on the cover a contact was formed with the electrodes, and it was possible by means of a commutator key with three eccentrics to light or extinguish one of the lamps or both at once. A single element would have sufficed to keep one Swan lamp of feeble resistance lighted for 20 hours. Accepting the data given above and the 20 hours' uninterrupted duration of the pile's operation the power furnished by this large model is equal to 2.5 x 20 x 3,600 = 180,000 kilogrammeters.

In our opinion, Mr. Scrivanow's pile is not adapted for industrial use because of the expense of the silver and the frequent manipulations it requires, but it has the advantage, however, of possessing, along with its small size and little weight, a disposable energy of from 150,000 to 200,000 kilogrammeters utilizable at the will of the consumer and securing to him a certain number of applications, either for lighting or the production of power. It appears to us to be specially destined to become a rival to the bichromate of potash pile for actuating electric motors applied to the directing of balloons.--_Revue Industrielle_.

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ON THE LUMINOSITY OF FLAME.

The light emitted from burning gases which burn with bright flame is known to be a secondary phenomenon. It is the solid, or even liquid, constituents separated out by the high temperature of combustion, and rendered incandescent, that emit the light rays. Gases, on the other hand, which produce no glowing solid or liquid particles during combustion burn throughout with a weakly luminous flame of bluish or other color, according to the kind of gas. Now, it is common to say, merely, in explanation of this luminosity, that the gas highly heated in combustion is self-incandescent. This explanation, however, has not been experimentally confirmed. Dr Werner Siemens was, therefore, led recently to investigate whether highly-heated pure gases really emit light.

The temperature employed in such experiments should, to be decisive, be higher than those produced by luminous combustion. The author had recourse to the regenerative furnace used by his brother, Friedrich, in Dresden, in manufacture of hard glass. This stands in a separate room which at night can be made perfectly dark. The furnace has, in the middle of its longer sides, two opposite apertures, allowing free vision through. It can be easily heated to the melting temperature of steel, which is between 1,500° and 2,000° C. Before the furnace apertures were placed a series of smoke blackened screens with central openings, which enabled one to look through without receiving, on the eye, rays from the furnace walls. If, now, all air exchange was prevented in the furnace, and all light excluded from the room, it was found that not the least light came to the eye from the highly-heated air in the furnace. For success of the experiment, it was necessary to avoid any combustion in the furnace, and to wait until the furnace-air was as free from dust as possible. Any flame in the furnace (even when it did not reach into the line of sight), and the least quantity of dust in it, illuminated the field of vision.

As a result of these experiments, Dr. Siemens considers that the view hitherto held, that highly-heated gases are self-luminous, is not correct. In the furnace were the products of the previous combustion and atmospheric air: consequently oxygen, nitrogen, carbonic acid, and aqueous vapor. If even one of these gases was self-luminous, the field of vision must have been always illuminated. The weak light given by the flame of burning gases that separate out no solid nor liquid constituents cannot, therefore, be explained as a phenomenon of glow of the gaseous products.

It appealed to the author probable, that heated gases did not, either, emit heat rays; and he set himself to test this idea, experimenting, in company with Herr Fröhlich, in Dresden. They first convinced themselves in this case that the light emission of pure heated gases sunk to zero, even when the field of vision was not always quite dark, and it was only possible to observe this a short time; but the repeatedly observed perfect darkness of the field of vision was demonstrative. On the other hand, experiments made with sensitive thermopiles, in order to settle the question of emission of heat-rays from highly-heated gases, failed.

Afterward, however, Dr. Siemens was convinced, by a quite simple experiment of a different kind, that his supposition was erroneous. An ordinary lamp, with circular wick, and short glass cylinder, was wholly screened with a board, and a thermopile was so placed that its axis lay somewhat higher than the edge of the board. As the room-walls had pretty much a uniform temperature, the deflection of the galvanometer was but slight, when the tube-axis of the thermopile was directed anywhere outside of the hot-air current rising from the flame. When, however, the axis was directed to this current, a deflection occurred, which was as great as that from the luminous flame itself. That the heat radiation from hot gases is but very small in comparison with that from equally hot solid bodies, was shown by the large deflection produced when a piece of fine wire was held in the hot-air current. On the other hand, however, it was far too considerable to admit of being attributed to dust particles suspended in the air current.

It must be conceded to be possible (the author says) that the light radiation of hot gases, as also the heat radiation, is only exceedingly weak, and therefore may escape observation. It is, therefore, much to be desired that the experiments should be repeated at still higher temperatures and with more exact instruments, in order to determine the limit of temperature at which heated gases undoubtedly become self-incandescent. The fact, however, that gases, at a temperature of more than 1,500° C, are not yet luminous, proves that the incandescence of the flame is not to be explained as a self-incandescence of the products of combustion. This is confirmed by the circumstance that, with rapid mixture of the burning gases, the flame becomes shorter because the combustion process goes on more quickly, and hotter because less cold air has access. Further, the flame also becomes shorter and hotter if the gases are strongly heated previous to combustion. As the rising products of combustion still retain for a time the temperature of the flame, the reverse must occur if the gases were self-luminous. The luminosity of the flame, however, ceases at a sharp line of demarkation, and evidently coincides with completion of the chemical action. The latter, itself, therefore, and not the heating of the combustion products, which is due to it, must be the cause of the luminosity. If we suppose that the gas-molecules are surrounded by an ether-envelope, then, in chemical combination of two or several such molecules, there must occur a changed position of the ether-envelopes. The motion of ether-particles thus caused may be represented by vibrations, which form the starting-point of light and heat-waves.

In quite a similar manner we may also, according to Dr. Siemens, represent the light-phenomenon occurring when an electric current is sent through gases, which always takes place when the maximum of polarization belonging to them is exceeded. As the passage of the current through the gas seems to be always connected with chemical action, the phenomenon of glow may be explained in the same way as in flame, by oscillating transposition of the ether envelopes, by which the passage of electricity is effected. In that case the light of flame may be called electric light by the same light as the light of the ozone tube or the Geissler tube, which is mainly to be distinguished from the former in that it contains a dielectric of an extremely small maximum of polarization. This correspondence in the causes of luminosity of flame, and of gases traversed by electric currents, is supported by the similarity of the flame-phenomena in strength and color of light.

[These researches were lately described by Dr. Werner Siemens to the Berlin Academy.]

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A QUICK WAY TO ASCERTAIN THE FOCUS OF A LENS.

It is well known that if the size of an object be ascertained, the distance of a lens from that object, and the size of the image depicted in a camera by that lens, a very simple calculation will give the focus of the lens. In compound lenses the matter is complicated by the relative foci of its constituents and their distance apart; but these items, in an ordinary photographic objective, would so slightly affect the result that for all practical purposes they may be ignored.

What we propose to do--what we have indeed done--is to make two of these terms constant in connection with a diagram, here given, so that a mere inspection may indicate, with its aid, the focus of a lens. All that is required in making use of it is to plant the camera perfectly upright, and place in front of it, at exactly fifteen feet from the center of the lens, a two foot rule, also perfectly upright and with its center the same height from the floor as the lens, and then, after focusing accurately with as large a diaphragm as will give sharpness, to note the size of the image and refer it to the diagram. The focus of the lens employed will be marked under the line corresponding to the size of the image of the rule on the ground glass.

As our object is to minimize time and trouble to the utmost, we may make a suggestion or two as to carrying out the measuring. It will be obvious that any object exactly two feet in length, rightly placed, will answer quite as well as a "two-foot," which we selected as being about as common a standard of length and as likely to be handy for use as any. The pattern in a wall paper, a mark in a brick wall, a studio background, or a couple of drawing pins pressed into a door, so long as two feet exactly are indicated, will answer equally well.

And, further, as to the actual mode of measuring the image on the ground glass (we may say that there is not the slightest need to take a negative), it will perhaps be found the readiest method to turn the glass the ground side outward, when two pencil marks may be made with complete accuracy to register the length of the image, which can then be compared with the diagram. Whatever plan is adopted, if the distance be measured exactly between lens and rule, the result will give the focus with exactitude sufficient for any practical purpose.--_Br. Jour. of Photo_.

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THE HISTORY OF THE PIANOFORTE.

[Footnote: A paper recently read before the Society of Arts, London.]

By A. J. HIPKINS.