Imatges de pÓgina
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pear, it was adopted by Cavendish in his theory. BIOT, in the Traite de Physique, has not remarked this difficulty, and consequently, the explanation which he has given after Poisson, of the equilibrium of the electric fluids, is not sufficient. In a subsequent work, however, he has observed, that the absence of electricity in the interior of bodies leads to the important result, that the electric fluid is incompressible, and in other instances speaks of it as flowing after the manner of liquids; such comparisons are probably merely inaccurate modes of expression. If the phenomena of electricity be explained by the intervention of peculiar fluids, these must be assumed as elastic, but when compressed within certain limits, increasing the law of their repulsion to a high power of the inverse distance. The chief difference between fluids so constituted, and such elastic fluids as the gases, consists in the latter diffusing themselves uniformly, whilst the former everywhere press to their boundaries, and if not free, almost immediately acquire a uniform velocity of radiation.

A second hypothesis, which is required to explain the equilibrium of the electric fluids at the surface of conductors is, that the atmosphere immediately surrounding the conductor, becomes highly charged with the same species of electricity, which, by its repulsion, retains that on the surface for the air of itself cannot retain it there, having no repulsion, but rather an attraction for each of the electricities; as is shewn by many experiments: it is impossible to work an electric machine for any length of time without charging the air of the apartment with electricity; if a screen of tin foil connected with an electrometer (fig.4), be carried into the room, it will immediately render sensible the electricity which is diffused there, and which must be adhering to the particles of air, and forming minute atmospheres of electricity round them. Thus, when a conductor is electrified, the electricity radiates to the surface, and a small portion of it passing on, attaches itself to the immediately contiguous particles of air, where it is retained by the attraction of the latter, and by its own repulsion prevents the further escape of the fluid. The quantity so accumulated in the adjoining particles of air need be very small to preserve the equilibrium. If we assume the particles of air and of the electric

fluids to be at the same distance under equal pressures, it would be sufficient for one particle of the former to attach itself to one of the latter; hence there is no proof from the experiment of FRANKLIN before mentioned, that such electrified atmospheres do not exist..

The thickness of the shell of fluid which can be retained by a given pressure, affords an excellent means of comparing differ ent electrometers, or rather furnishes a standard by which to compare them. Ift is the thickness of the shell, m a constant co-efficient, and h the height of the barometer, we have

t = m√h or, assuming t = 1 when h 30 inches

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t = 182 √h

An electric machine in good order will readily charge a conductor high enough for the thickness of the shell of fluid on a ball an inch in diameter, and projecting some inches from the conductor, to exceed this limit, and consequently to continue discharging itself into the air.

With the suppositions which I have mentioned, and which there appears every reason to believe correct, the theory of the equilibrium of electricity as improved by Poisson from the prior theories of PINUS and CAVENDISH, leaves little to be wished for on this part of the subject, and may readily be made to embrace both the equilibrium of the magnetic fluids and that of the electricity disturbed by the action of the electromotive apparatus. The first of these applications I made in 1820, in a couple of papers published in the 55th volume of the "Philosophical Magazine;" where, proceeding upon the theory which Poisson had applied with so much success to the distribution of electricity on spheres and ellipsoids, I deduced, by a very easy investigation, the distribution of the magnetic fluids, on similar solids when subject to the action of the earth. The result, which was wholly theoretic, agreed with all the experiments of Mr. BARLOW, and seemed to leave no doubt respecting the truth of the principles. Examining more nearly however, we found an error of one-sixteenth in the constant co-efficient by which the several expressions for the deviation were multiplied. This minute error arises from the resistance which

iron offers to the motion of the magnetic fluids; Mr. BARLOW,

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however, gave it another explanation, and some phenomena; which do not admit of a ready mathematical calculation; seemed to favour the result. I mention this circumstance in consequence of some remarks of an anonymous critic who speaks slightingly of the calculations in Mr. Barlow's work, and gives great praise to a subsequent paper of Poisson.-The results and theory of this paper are precisely those which I had published three years before; the method of investigation is the same as that before employed for determining the equilibrium of electricity; and though not admitting of any definite results, is profound and subtle in the extreme. The question in its greatest generality belongs to a class of problems which have hitherto baffled the powers of the greatest mathematicians of Europe: the condition of equilibrium on bodies of any form. is expressed by a simple equation in definite integrals, or, as such cases might be more properly termed, limited functions, and notwithstanding the talent with which this highly important subject has been treated, it yet remains unexplored, a rich field for some future LAPLACE.

I am not aware whether any one has remarked that the distribution of the electric fluid in Volta's pile, when the ends are isolated, is, with respect to the service, the same as that of the magnetic fluids in a magnet induced by the earth. The constant disturbing force at each surface of copper and zinc must be overcome by the action of the fluids already developed, as the constant action of the earth, minus the constant resistance to motion, has to be overcome in the magnet. Hence a spheroidal pile generated by the revolution of an ellipse will have for the boundary of the shell of fluid, a similar and equal ellipse whose centre is at a very small distance from that of the pile. The quantity of fluid developed will be known immediately we know the motive force of each surface, and if the latter were equal in intensity to the magnetic attraction of the earth, the electrical attraction and repulsion of the extremities of the pile would be equal to the similar magnetic actions of a mass of soft iron of the same shape and size as the pile, and placed in the direction of the dip. It is still a question whether the electric fluid which accumulates in an isolated pile, and that which circulates in one that is closed, are identical;

and one of the difficulties met with in investigating the subject arises from the low intensity of the electricity in the former. This may certainly be greatly increased by making the pile in the form of a double cone, having the ends completely isolated; but such a construction is attended with some practical difficulty. How far it would augment the energy of the current when closed, can scarcely be conjectured in the present state of our knowledge respecting this species of action.

On the French Claim to the Discovery of the Steam-Engine. [In a Letter to the Editor.]

SIR,-I observe in the French Constitutionnel, of last week, that Stuart's work on steam-engines is quoted as an honourable confession, that the steam-engine originated in a Frenchman, and is attributed to Salomon de Caus, of Dauphiny,—and being possessed of that rather scarce work, his Raisons de Forces Mouvantes, it will be not uninteresting to shew the nature of his claims. I therefore subjoin the title and extracts, together with a copy of his figure of the machine he employed; and as it may be amusing to your readers, add the contents of the two other books which accompany it.

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"Les Raisons des Forces Mouvantes, avec diverses Machines, &c. Par Salomon de Caus, Ingénieur de Son Altesse Palatine Electoriale.-A Francfort, 1615.

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"The work is a thin folio with many plates, and is divided into three books-the first of which treats of mechanical powers, with 35 problems and 44 plates, commencing with his definitions of the Elements: in the first theorem he gives a wooden cut of a globe of copper, which has a tube introduced at the side; this descends nearly to the bottom, and has a stop-cock on the end which issues from the side; on the summit is another aperture, closed with a stopper, or valve; when this has been filled with one-third of water, a fire is to be. put under it for three or four minutes, and then the side-cock.. turned, when by the force of the steam, the whole of the water, he adds, will be discharged, and the air remaining

must be discharged by the upper valve or stop-cock, when a portion of the water will be found to have been evaporated.

"Fill the vessel then," says he, "again as before, one-third, closing well both the tube and upper hole, and replacing the fire under for the same space of time-take the vessel off the fire and let it cool, without opening either of the apertures; and on discharging the water it will be found that the quantity is not diminished, since the vapour has been condensed in this experiment.

"We may give a third demonstration of this-which is, that when having replaced the same quantity of water in the vessel, we stop well the stop-cock of the bent tube, which descends to the bottom of the vessel, and open the upper stopper, when placing the vessel again on the fire, and placing a jug below it, the water will boil by the heat, and discharge itself by the stop-cock of the bent tube, but a fifth or sixth part of the water will remain, on account of the violence of the steam which causes the water to rise, and hinders the whole from being discharged -which steam afterwards issues with great violence.--He adds afterwards, that he had tried the same experiment with quicksilver. Here, therefore, we have the steam-boiler in miniature, and its effects-but in his fifth theorem, which he entitles,

"Water raised by fire above its level. "He uses the same boiler in another form, passing the tube from the upper part of the ball of copper, downwards, towards the bottom, instead of passing it by a curve from the side, and thus describes his method.

"The third means of making water ascend is by the aid of fire, in which divers machines may be employed, one of which I here annex.-Make a ball of copper, A, well soldered up, to which there is a cock D to introduce the water by; and a pipe marked B C soldered on to the ball, the end of which at C approaches the bottom without touching it: having injected the

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