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the centre as radii'; the extremities being equidistant from each other. A wire passes through the centre of each plate, and is attached as an axle; upon this the plates turn in the cavities at the bottom of the pendent arms of the crosspiece. Under the pendent arms two oblong troughs are made in the wooden basis, to contain mercury; metal cups to contain mercury are affixed to two wires, which proceed from each side of the basis, and penetrate into the mercury contained in the oblong troughs. When sufficient mercury is poured into these troughs, so that the points of the plates just touch it,-if two connecting wires, passing from the zinc and copper extremities of a battery, be placed in the cups, an electric current is established through both plates by the axle; but no motion will be produced until two horse-shoe magnets are placed with their poles extending underneath the axle, and resting on the basis of the apparatus. When the similar poles of the magnets are interior and exterior of the plates, then the plates will rotate rapidly; the course of rotation being reversed, by reversing the order of the connecting wires, or by changing the direction of the magnetic poles. To insure perfect contact, the tips of the radii should be amalgamated, as should, also, the points of the wire which support the metallic cups: the powerful magnet will be sufficient to produce the effect." -Page 82.

In experiments made with any of these three descriptions of apparatus, the aid of a separate Voltaic battery is necessary; but among the ingenious contrivances for which we are indebted to Am. père, is one by which that auxiliary may be entirely dispensed with:

"In his apparatus the electricity is produced by the instrument itself; it consists of a double

Mr. W. has followed a plan in his descriptions rather unusual with scientific works; he never quotes his authorities (book, chapter, or verse, we mean), and never makes any use of letters of reference, either in the engravings or letter-press (those which appear in the preceding extracts having been supplied by ourselves). Although he often speaks, too, in the body of the work, of inventions and contrivances, minute descriptions of which are afterwards given, there are no helps by which the reader can at once refer to these descriptions. As there can be no good reason for these omissions, and as they lessen considerably the present value of Mr. W.'s work, we make no doubt he will see the propriety of remedying them, to the utmost of his power, in future editions.

cylinder of copper, between the interior surfaces of which a cylinder of zinc is introduced, saspended by arched wires c c, on a point d attached another arched wire, by which the whole machine is sustained on the pole of a strong bar magnet inserted through the interior copper cylinder. When diluted acid is poured into the copper cylin drical cell, the zinc cylinder, being free to move upon its point of suspension, begins to revolve with a greater or less velocity, depending on the strength of the acid, and the power of the magnet."-Page 56.

Sir,-All those persons who have ever assisted to work a fire-engine (and who, at one time or another, has not?) will readily call to mind the unpleasant sensation in the joints of the arms and shoulders, caused by the violent jar that takes place every time the handles come down on the body of the engine,-which they do with very considerable force. -My object in now addressing you, is to lay before your readers a plan for the removal of the above inconvenience, and which I hope to render perfectly intelligible, by means of the above sketch, which represents the hinder part of a fire-engine, to which is firmly attached a strong bow-spring, by its centre A.

VOL. XI.

A similar spring is also placed at the opposite end of the engine. Upon the extremities of these springs B B, the connecting bar of the two cross levers (which is of iron, and parallel with the handles), strikes, a short time before it reaches the stop C, at the bottom of the engine. As the force pressing upon the handle is great, the spring will be carried down into the bent position shown in the drawing; but from the moment the handle first impinges upon the spring B, its motion will be progressively checked, and it will strike but lightly upon the stop C; and the greater part, if not the whole, of the jar at present experienced will be avoided. That springs would

E

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have this good effect, I am convinced, from having observed (and my observation is confirmed by others), that in those engines that are mounted on spring carriages,as the one in the above drawing is represented to be,-the concussion is infinitely smaller than in those which rest directly on the wheels; but here an inconvenience arises, because the body and works of the engine, taking the motion of the springs, flinch from their work. It would, therefore, be desirable to stop this action of the carriage springs, by means of wedges, that the engine might be kept up to its work. In the arrangement which I have suggested, a considerable advantage would arise from the quantity of power that would be gained; because it will be evident that the spring B will press the handles upward, with the same force that they resisted its downward motion, and thus communicate a considerable quantity of starting power, when the motion of the handles is changed. The power of the men, and, consequently, the force of downward pressure, increases greatly, just before the handles arrive at the stop; when, at present, a blow is given against the body of the engine, which shakes it violently on its carriage, while the men feel the blow up to their shoulders; they then have to change their motion, from pressing downward, to pulling upward: during this change in the direction of power, the men work at a considerable disadvantage; and it is at this moment that the springs would assist them, by restoring that accumulated momentum which is at present expended in knocking the body of the engine to pieces.

Although I may not have chosen the best arrangement of the mechanism, still I am convinced that the principle is worth the serious consideration of all those who either use or construct these useful machines. My suggestion has received the approbation of a foreman of one of the fire offices, to whom I submitted it some time since; and I hope, ere long, it will be submitted to the

ordeal of experiment-when I am
confident that all the advantages
which I have ascribed to it will be
found correct.
I remain, Sir,

Yours, &c.
WM. BADDELEY, Jun.

ON EXTINGUISHING FIRES. Sir,-Although the following observations may not be new, they are so fraught with sound sense and practical utility, that they cannot be too generally known; and this object cannot be better attained than through the medium of your excellent Magazine, if you think them worth a place in it. They are extracted and translated from an exceedingly clever German work upon the best means of safety in cases of fire.

"Fires only become dangerous from endeavouring to extinguish them too quickly. Paradoxical as this assertion may appear, it yet may be regarded as a fundamental principle.

"If it were possible to pour at one jet a torrent, or large mass of water, upon the fire, no doubt it would be extinguished instantaneously but this is impossible; therefore, let us examine the effects of water thrown gradually upon the fire. The flames are momentarily extinguished, it is true, and the water is soon decomposed, or evaporates, through the great heat, and the surrounding flames increase in intensity. Consequently, throwing water upon the centre, or focus of the fire, is the most improper way. of proceeding, and yet the one most generally pursued. If, on the contrary, that which is already the prey of the flames were abandoned, in order to save the surrounding parts, and prevent the fire from spreading, and the engines were directed to those points which the fire had not yet reached, the flames would not only be arrested in their progress, but repelled towards their centre of activity; and much time would be saved, by preventing the

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The four engravings BCDE illus trate a curve of a different character to A, but derived from the same principle of motion. When a surface is so put in motion as to describe ellipses, it is well known that, upon apply. ing a point to one part of it, a right line will be described; if to another, a circle will result; and if to other parts, various ellipses. Now, in the case of this curve, one position of the describing point will give a circle; another a curved line, as shown in C. In another position of the point, the curved line opens, and the figures in B, and those exterior to the circle and curved line in C, are produced. The describing point being adjusted in the contrary direction, the circle changes in the manner shown in D, and becomes a figure of eight, and then degenerates into the curves shown in E. The motion which gives this very curious result is a certain combination of pure circular motion; and this case is an instance where a surface is so moved by the action of compound circular motion, that, supposing a number of points to be presented to it at the same instant of time, they would simultaneously describe, each of them, a curve of a different character. I am Sir, Yours, &c.

JOHN HOLT IBBETSON. 12, Smith-street, Chelsea, Feb. 7, 1829.

Sir, I have been expecting to see in your pages some specimen from Mr. Child, illustrating the observa

tions he has made, through the me dium of the "Mechanics' Magazine, on the powers of his curve-delineating apparatus. In bis communication inserted in your publication of the 26th April, 1828, he says, that he "had sent you such specimens as prove that his model possesses the power of equalizing the motion of a point in the periphery of an ellipsis.". These specimens, 1 believe, have not yet appeared in your pages; and I shall be gratified by your giving them as early an insertion as you conveniently can; being anxious for an opportunity of deciding my opinion. We are all naturally inclined to give preference to our own productions; and I think Mr. Child and myself are not the most competent judges of the merit of our respective inventions. Whether he can "copy the productions of the geometric chuck, and produce figures much more intricate," or whether I can do so with respect to his apparatus, cannot, I conceive, be decided by opinion and assertion; the specimens which our machines can produce are the best, indeed the only, criterion of what they can do. Long experience has taught me that there is a very great difference between saying and doing; and I am, I confess, an admirer of plain, genuine, unsophisticated truth. Mr. Child's apparatus I know nothing, save from the specimens he has exhibited. When they first appeared in your Magazine, I instantly copied two of them with the geometric chuck; and I had exècuted figures similar to others of them, years ago, and presented specimens to the Society of Arts, where they are now to be seen. So far I then proved,-that I not only understood the nature of Mr. Child's curves, but that the geometric chuck could copy them. Mr. Child, in a subsequent communication, points out two other curves among his specimens, for me to copy, and says, that "if I cannot produce figures on that principle, and think it worth my attention, he will give me the best information he can." I certainly feel myself bound, in common courtesy, to thank Mr.

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