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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.

[lo 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 honour: able 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.

Les Raisons des Forces Mouvantes, avec diverses Machines,

&c. Par Salomon de Caus, Ingénieur de Son Altesse

Palatine Electoriale.A Francfort, 1615. “ 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

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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

A marked B C soldered on to the ball, the

Theater:

Librush end of which at C approaches the bottom

C:

an without touching it: having injected the

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B

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water by the stop-cock, close it well, and set the vessel on the fire, when the heat given to the ball will cause the water to ascend by the pipe B C.”

The second part, as well as the first, has handsomely engraved folio frontispieces, ornamented with figures in the style of the times, and contains aviaries, grottoes, and fountains, in twenty plates, containing the machinery used to exhibit them, many of which are very trifling, and are dedicated to Elizabeth, Princess of Great Britain, and Electrice Palatine, &c.

The third book is wholly employed in describing the fabrication of organs, such as casting the pipes, &c., two plates, with several wood-cuts. You see, therefore, that the principle of raising water by steam may, for anything I know, have been first suggested by the French engineer; but that he had any idea of the modern steam-engine must be denied; or that he suggested the machine proposed by the Marquis of Worcester, to raise water for the city of Westminster. I therefore think you will do well to publish it in your next Number, lest, from the manner in which this claim for France is brought forward, some persons might be induced to give credit to the assertion.

I am, Sir, yours, &c.

G. CUMBERLAND.

Bristol, March 2, 1829.

159

ASTRONOMICAL AND NAUTICAL

COLLECTIONS.

i. Elementary View of the UNDULATORY Theory of Light.

By Mr. FRESNEL.
[Concluded from the Number for January.]

Modification of Polarised Light by Reflection. I have found that the total reflection of light in the interior of glass, at an inclination of about 50° from the perpendicular, produced this peculiar modification in the light, when it had first been polarised at an azimuth of 45°, reckoning from the plane of reflection: that is to say, the reflected light was then composed of two equal systems of undulations, polarised at right angles to each other, and differing one-fourth of an undulation in their oscillations. This reflected light, which presents no further trace of polarisation, when it is analysed with a rhomboid of calcarious spar, has the property, like polarised light, of exhibiting very bright colours in crystallised thin plates ; but the colours are of an uncommon nature. It differs also from polarised light in not sensibly developing colours in oil of turpentine, and in plates of rock crystal perpendicular to the axis. When it is subjected again to two total reflections at the same incidence and in the same plane, or in a perpendicular plane, it resumes all the characters and properties of common polarised light: when it is subjected to two more reflections of the same kind in the same directions, it is completely depolarised, and recovers at the same time the other properties which it had derived from the first two reflections, and so forth.

I shall not enter into more particular details of this singular modification of light, which is impressed at once on all the different kinds of rays like polarisation itself; and in this respects exhibits properties equally general. I shall only observe, that it was the nature of the tints developed by light, so modified in crystallised plates, that pointed out to me, that it must be composed of two systems of undulations, polarised at right angles, and differing a quarter of an undulation in their progress; and setting out from this fact, I had no difficulty in explaining and in calculating the various phenomena which it exhibits, with the assist

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