292] The 2nd experiment shews that this circumstance of the whole redundant fluid being lodged in or near the surface obtains also in other shaped bodies, as well as in the globe, conformably to the supposition made in the remarks at the end of Prop. IX. [Art. 41]. These two experiments, at the same time that they determine the law of electric attraction and repulsion, serve in some measure to confirm the truth of the theory, as it is a circumstance which, if it had not been for the theory, one would by no means have expected.

293] From the 4th experiment it appears, first, that the charge of different bodies of the same shape and size, all ready conductors of electricity, is the same, whatever kind of matter they are composed of; and secondly, that the charge of thin plates is very nearly the same whatever thickness they may be of, provided it is very small in respect of their breadth or smallest diameter; but if their thickness bears any considerable proportion to their breadth, then their charge is considerably greater than if their thickness were very small. These two circumstances are perfectly conformable to the theory, and are a great confirmation of the truth of it.

294] The remaining experiments contain an examination whether the charges of several different sized and different shaped bodies bear the same proportion to each other, which they ought to do according to the attempts made in different parts of these papers to compute their charges by theory, supposing, as we have shewn to be the case, that the electric attraction and repulsion is inversely as the square of the distance: with regard to this it must be observed that, as in computing their charges I was obliged to make use of a supposition, which certainly does not take place in nature, it would be no sign of any error in the theory if their actual charges differed very much from their computed ones; but, on the other hand, if the observed charges agree very nearly with the computed ones, it not only shews that the actual charges of different bodies bear nearly the same proportion to each other that they would do if they were connected by canals of incompressible fluid, but is also a strong confirmation of the truth of the theory. Now this appears to be the case, for, first the charge of a tin plate was found to be nearly, though not quite, the same in whatever part it was touched by the electrifying wire, or in whatever direction it was placed in respect of the jar by which it was electrified.

Secondly, the charge of a single plate or wire was found to bear nearly, though, in the first case, I believe, not quite the same proportion to two similar plates or wires of half the diameter or length which it ought to do according to computation. Thirdly, the proportion which the charges of a thin circular plate and of three cylindrical bodies of different lengths and diameters bear to that of a globe agree with computation; but it must be observed that, as the proportion of the charges of the bodies to that of the globe is determined by the theory within only very wide limits, their agreement cannot be looked upon as so great a confirmation of the theory as it would otherwise be, yet as their shapes are so very different I think that their agreement, even within those limits, may be considered as a considerable confirmation of it.

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[EXPERIMENTS ON COATED PLATES.]

295] This part consists chiefly of experiments made to determine the charges of plates of glass and other electric substances coated in the manner of Leyden vials. The method I used in doing this was nearly of the same nature as that by which I determined the charges of the other sort of bodies in the preceding part, but the apparatus was more compact and portable and is represented in Fig. 20, where Hh is a horizontal board lying on the ground, Ll and Ll are two upright pillars supporting the two horizontal bars Nn and Pp, both at the same height above the ground, and parallel to each other.

To these two bars are fastened four upright sticks of glass covered with sealing wax; they are represented in the figure and shaded black, but are not distinguished by letters to avoid confusion. To these sticks of glass are fastened four horizontal pieces of wire Aa, Bb, Dd, and Ee, and to Bb is fastened another wire mM supported at the further end by a stick of waxed glass.

Rr is a wooden bar reaching from the wire Ee to the pillar Ll, and along the upper edge of this bar runs a wire, one end of which is wound round the wire Ee and the other reaches to the ground and serves to make a communication between Ee and the ground. Cc and Kk are two wires fastened firmly together at k serving to electrify the plate. They are moveable upon K as a center where they communicate with the inside coating of one or more large glass jars, and the same electrometer that was used in the former experiments is fastened to the prime conductor by which the jars

[Not numbered by Cavendish.]

are electrified, in order that they may be charged to the same degree each time.

To the ends C and c of the wire Cc is fastened a silk string, as represented in the figure, passing over the pulley S, with a counterpoise w at the other end which serves to lift Cc from off the wires Aa and Bb, or to let it down upon them at pleasure. Gg is a wire the end G of which is bent into a ring, through which

passes the wire Ee, so that Gg turns upon Ee as a center. Ff is a wire turning in the same manner as Dd. The ends g and ƒ of these wires are fastened by silk strings to C and c as represented in the figure, in such manner that when Cc rests on the wires Aa and Bb, Gg and Ff rest on Dd and Ee, but on lifting up Cc, Gg and Ff are also lifted off from Dd and Ee.

The counterpoise w is so heavy as to overcome the weight of Cc, and to lift it up till the wires Gg and Ff bear against Aa and Bb, which prevents Cc from rising any higher.

[Note. This Figure was found among the MS. It is not numbered, nor does any part of the MS. seem to refer to it, but it is inserted here to show some of the details of a piece of apparatus similar to that described in the text.]

296] In making the experiment one of the plates whose charges we want to compare together, or the plate B as we will call it, is laid on the bars Nn and Pp, between the sticks of glass and end N, the upper coating thereof being made to communicate with Bb and Mm by a wire V resting on Mm, and the lower coating is made to communicate with the ground by a springing wire S fastened to Rr, and by its elasticity bearing against the lower coating of the plate.

Another coated plate is laid on the same bars between the sticks of glass and n by way of trial plate, the upper coating of which communicates with Aa by the wire B, and the lower coating communicates with Dd by the springing wire 8. A pair of pith balls also, such as were used in the former experiments, were suspended from D as represented in the figure.