mounted with much brass and steel, were liable to fome unavoidable imperfections. We then hinted that we feared he would find the friction attending them too powerful to be applied to his grand machine; and we here learn that our conjecture was well founded.

Mr. C.'s narrative is followed by fome very ingenious experiments, to illuftrate the manner in which rubbers act; whence it appears that the glafs, while paffing under the cushion, becomes negative on both fides, but moft fo on that fide which is excited; whence we may conclude that plates, which admit of an equal friction on both fides, are more advantageous than cylinders: the furface of the rubber fhould be perfectly level and prefs equally on that of the glafs, and particular care muft be taken that the anterior part does not prefs less than the pofterior.

The author next defcribes the machine, with its rubbers, as he now usually makes it, with the axis turning between mahogany pillars: but he has alfo fitted up fome, in which the axis is infulated, by being fufpended between glafs columns;an account of one of this kind was published, by Meffrs. Deiman and Trooflwyk, about five years ago, and was mentioned in the appendix to the fecond volume of our new feries. To give a particular defcription of the conftruction, without the affiftance of engravings, would be difficult; there are numberlefs little circumftances, which the experienced electrician knows to be of importance, in the form and arrangement of the feveral parts of a machine, but which to others appear trifling; it is fufficient to observe that the machines here defcribed excel all others that we have feen, in the equal and ftrong preffure of the rubbers, in the convenient difpofition of the apparatus, and, as we fhall presently have occafion to thew, in powerful effect,

Mr. CUTHBERTSON proceeds to examine the merits of an electrical machine conftructed under the direction of Dr. Van Marum, which the reader will find mentioned in the appendix to our fifth volume, N. S. In this machine, the plate of which is thirty-two inches in diameter, he obferves that the rubbers, being nine inches long, approach too near to the axis; he then defcribes fome very fimple and ingenious experiments, which he performed in order to afcertain the most advantageous length of the cushions in proportion to the diameter of the plate, and by which he found that, for one of thirty-two inches, the rubbers ought not to be longer than feven inches. Other defects, obferved by our author, are that the baked wood, by which the axis is infulated, is liable to imbibe moisture, notwithstanding all the precautions that can be taken by covering it with electric fubftances, and that, because the axis is fupported only at one end, it is apt to become unsteady, in confequence of fre

003

quent

quent ufe. That the fame conductor may be made to exhibit either pofitive or negative electricity, is not fo advantageous as it may at first appear; for there are many experiments in which feparate conductors are abfolutely neceffary; and these cannot conveniently be applied to this machine, in its prefent ftate; though we conceive that a trifling alteration might remove this defect.

In our account of Dr. Van Marum's rubbers, we described the manner in which Kienmayer's amalgama is made, and in which the Doctor applied it: Mr. CUTHBERTSON finds it more advantageous to mix up the amalgama with lard, to the confiftence of an unguent, which must be fpread on the rubber fo as to form a furface perfectly level. His directions for fixing the rubbers, and for regulating their preffure, are judicious. The preffure recommended for a machine, the plates of which are thirty-two inches in diameter, with four pair of rubbers, is eftimated at nearly eight pounds; or, in other words, the rub bers ought to be fo adjusted, that, when the winch is placed in a horizontal direction, a weight of eight pounds, fufpended to it, fhall draw it down."

The author next defcribes his mode of afcertaining the power of a machine, and of comparing that of different machines: this he does with a jar, containing about 160 fquare inches of coated glafs, mounted with an electrometer, on the fame principle with that invented by Mr. Lane: but, as jars, of equal fize and furface are not always equal with refpect to the charge which they will receive and contain, Mr. CUTHBERTSON always tries his machines by the fame jar, and keeps one, as a ftandard, for this purpose: the brass balls of the electrometer being fet at half an inch diftance from each other, the power of the machine is calculated to be inverfely as the number of revolutions neceffary to make this jar difcharge five times. He has given a table of the powers of his machines thus estimated; which, as it illuftrates the improvements that he has made in them, and the great advantage refulting from Kienmayer's amalgama, we fhall infert:

The other mode of comparing machines is by computing the number of revolutions required to charge a jar, like the former, fo high that its difcharge may melt five inches of the fmalleft iron wire, which is about th of an inch in diameter: for this purpose, Mr. CUTHBERTSON has contrived a very fimple and commodious apparatus. The number of revolutions of his latest machines, requifite for producing this effect, is exactly the fame with what is neceffary to produce the five difcharges of the ftandard jar above mentioned.

In both these ways, Mr. CUTHBERTSON has compared the machine, constructed by Dr. Van Marum, with his own: according to the Doctor's account, four revolutions of his glass plate made the ftandard jar difcharge five times, when the brafs knobs of the electrometer were half an inch distant from each other. Now according to the above table, two plates, of 31 inches each, will do this in a revolution and a half; confequently, a fingle plate will effect it in three revolutions: but, in order to make this comparifon with greater accuracy, our author formed two machines, with each a fingle plate of twentyfour inches diameter, one conftructed in the manner directed by the Doctor,-the other, in his own way. The length of the fpark, drawn from the conductors, appears to have been the fame with both machines; that from the pofitive being 71, and from the negative 5 inches. In order to discharge the ftandard jar five times, when connected with the pofitive conductor, the machine of the Doctor's construction required 4 revolutions, and that of our author's, 4: but, when the jar was connected with the negative conductor, both machines required five revolutions. With the first mentioned machine, feven revolutions charged a jar, placed at the pofitive conductor, fo as to melt five inches of the fmallest iron wire; and, at the negative conductor, twenty-two revolutions were neceffary to melt four inches; with the latter, feven revolutions were fufficient to make the jar, when charged pofitively, melt fix inches; and fifteen, when negatively charged, to melt five inches of the wire. A difadvantage attending the Doctor's conftruction is that it is more expenfive than the other.

One of the most interesting parts of the prefent work is that which relates to the charging of coated glafs, Till lately, it was not known that a fingle jar could be charged fo highly as to melt iron wire; we mentioned this circumftance, together with the neceffity of the uncoated glass being a little damp, in a former article, to which we refer the reader *. Mr. CUTHBERTSON has fince found that, with a machine with two plates of

* Rev. New Series, vol. ii. p. 545.

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eighteen

eighteen inches diameter, he could charge a fingle jar of 169 inches of coated glafs fo highly as to make an inch of the fmallest iron wire vanifh into smoke, and three inches exhibit the phenomenon of filaments floating in the smoke into which the wire was converted; which was mentioned in the defcription of Dr. Van Marum's great machine*: with nine, and, once, with only fix revolutions of the plates, he melted eight inches of wire but this appears to be the greatest effect of fuch a jar, whatever be the fize of the machine with which it is charged. It is fomething remarkable that two, and even three, fuch jars required not more revolutions of the machine to pro duce this effect, than a fingle jar; and that a jar, containing 5t fquare feet of coated glafs, demanded only one revolution more than a common fized one. It is alfo obfervable that jars will bear a much greater charge in fummer than in winter: the fame jar which, in fummer, could be charged fo high as to melt eight inches of wire, would not, in winter, melt more than five, and would break if the charge were increafed beyond this height. The standard jar will, in fummer, discharge itself, when the balls of the electrometer are half, or even three quarters, of an inch diftant from each other, before any flashing is perceived on the uncoated glafs but, in winter, this flashing is feen before it discharges at half an inch. Batteries likewife may be charged much higher in fummer than in winter, and with much less danger of breaking the jars. Farther experiments are neceffary in order to account for this phenomenon in a fatisfactory manner; but, in the mean time, those of Mr. CUTHBERTSON have led to the difcovery that much greater effects may be produced by a fmall battery, if not wiped perfectly dry, than were formerly thought poffible. The fixing of a metallic ftain on paper, by tranfmitting the charge through pieces of wire laid over it, which was effected by Dr. Van Marum's grand machine with a battery of two hundred and twenty-five fquare feet of coated glafs, our author has effected with a furface of fixty square feet, charged by a machine of two plates of thirty-three inches, We have fince feen this experiment performed on all the metals, except Platina, with a battery of fifteen square feet, charged by a machine with a fingle plate, twenty-four inches in diameter, which was made by our author, With the battery, which Mr. CUTHBERTSON here mentions, fix inches of platina wire,

th of an inch in diameter, were melted to globules, On this occafion, he contrived a very ingenious apparatus, in order to confine the wire, through which the explofton was tranftted, in any kind of gas; the diminution of which, by af

Rev. vol. lxxiii. p. 556.

terward

terward opening a communication with a glafs tube filled with water, and furnished with a fcale, might be accurately obferved. The tube, which contained the gas, was 1 inch in diameter, and 15 inches long, and each of the divifions on the scale expreffed the volume of a drachm of water. The tube being filled with oxygene gas, the battery was discharged through two feet of iron wire, sth of an inch in diameter, which was in part melted to globules, and the remainder reduced to a fine duft; the diminution of the gas amounted to a cubic inch: the experiment was then tried with fourteen inches of the platina wire above mentioned, which was reduced to a light-brown powder: but the gas was not at all diminished. Gold and filver wire were also reduced to powder, without the leaft abforption of oxygene. Fourteen inches of leaden wire were, by the explosion, changed into a fine white powder, which was revived by being exposed to the flame of a candle; the gas abforbed was about an ounce measure: the refult was nearly the fame when the experiment was tried in common air, only that the powder was lefs in quantity, inferior in whiteness, and fo damp that it ran into lumps. With tin, the effect was exactly the fame; except that the powder was ftill whiter and in greater quantity. This experiment was repeated in air, in which a red hot coal had been extinguished: but then the wire was melted to globules, and no powder was produced; the diminution of this air was about two drachms.

Mr. CUTHBERTSON's removal to London has prevented him from pursuing thefe interefting experiments any farther:we hope that he will now be able to refume them, and will make the public acquainted with their refult, which may be of great ufe in illuftrating the theory of oxydation.

Sow.

ART. IX. Correspondence, &c. i.e. The Correfpondence between General DU MOURIER and PACHE Minifter of War, during the Campaign in Flanders and Brabant in 1792. 8vo. pp. 165. Paris. 1793.

THE

THESE letters have no other evidence of authenticity than what arifes from their contents, which feem to favour this fuppofition. They are introduced by the editor's de lication to his fellow-citizens, to which no name is fubfcribed: but, from the tenor, it feems to have been written by DUMOURIER; who informs his readers that his motive for publithing this (mall fpecimen of his correfpondence was to convince vem of the many obftacles that he had to encounter, when the command of the French army. This is ico only information that can be collected fro...

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