that of D, each consisting of two plates of glass cemented together and coated on their outside surfaces with circular pieces of tinfoil about 1 inch in diameter*.

I then compared the charge of each of these double plates with that of the globe in the same manner that I compared together the charges of different bodies in the former part, the only difference being that, in trying either of these double plates, I made a communication between the lower coating of the plate and the ground, the wires Mm and Dd (Fig. 14) being contrived so that they were sure to fall on the upper coating †.

By this means the charge of each of these double plates was found to be just equal to that of the globe. The charge of the plate D was then compared with that of the two double plates together, and was found to be less than that in the proportion of 263 to 272, and consequently the charge of the plate D is to that of the globe as 26.3 to 13.6.

334] Before we go further it will be proper to consider what effect the three circumstances taken notice of in Art. 277 will have in altering the proportion of the charge of the double plate to that of the globe. With regard to the two first, it appears that the charge of the globe and double plate will neither of them be sensibly different from what they would be if they were placed at an infinite distance from the jar by which they are electrified, and moreover, in trying the globe, the repulsion of the redundant fluid in the globe increased the deficience of fluid in the trial plate as much as the attraction of the trial plate increased the quantity of redundant fluid in the globe‡, so that it required the same size to be given to the trial plate as it would have done if the globe and trial plate had exerted no attraction or repulsion on each other; and in trying the coated plate, the coated plate could not sensibly increase the deficience in the trial plate, nor could the attraction. of the trial plate sensibly increase the redundance in the coated plate, so that neither of these two causes had any tendency to alter the proportion of the charges of the globe and coated plate to each other.

* If they had been made of a single piece of glass, the coatings must have been so small as would have been inconvenient unless the glass had been of a greater thickness than could have been easily procured. [Arts. 446, 451, 649, 653, 654.] + [Arts. 455, 456, 478.]

+ [Note 17.]

335] But the third cause will have a sensible effect, for in trying the globe the floor and sides of the room near it would be made undercharged, which would increase the charge of the globe, whereas in trying the coated plate the floor would not be made sensibly undercharged, nor, if it was, would it have any sensible effect in increasing the charge of the plate.

So that the charge of the globe bore a sensibly greater proportion to that of the coated plate than it would have done if it had been placed at an infinite distance from any other bodies.

How much the charge of the globe should be increased hereby I can not tell, but I should imagine it should be at least by th part, for if the room had been spherical and 16 feet in diameter (about its real size) and the globe placed in its center, it should have been increased as much as that*, and as the globe was really placed three times as near to the floor as to the ceiling†, I suppose the effect to have been still greater.

* Let the globe Bbß, whose centre is C, be insulated in the hollow globe Ddo concentric with [it]. Let the inner globe be pos. electrified by the canal BE not

communicating with the outer globe, and let the outer globe communicate with the ground. The quant. defic. fluid in the outer globe must be equal to the redundant in the inner globe, and the attraction of the outer globe on the canal BE is to the repulsion of the inner one thereon as

and therefore the quantity of redun. fluid in the inner globe is to that which it would contain if the outer globe were away as

If room was spherical, 16 feet in diameter, globe in middle of it, its charge should be increased in ratio of 16 to 15 by reason of undercharged floor, &c.

[This is the only indication of the height of the room. The circles were suspended by silk strings from a horizontal bar (Art. 466) 87.5 inches from the floor. By Art. 474 the platform 14 inches high diminished the height of the bodies in the

336] In order to find out, if possible, how much the charge of the globe was increased hereby, I made four flat plates of a mixture of rosin and bees wax*, about 4 inches square and 22 thick, and coated each of them with circles of about 18 inches in diameter, and compared the charge of each of them separately with that of a circular plate of tin, 93 inches in diameter. I then compared the charge of two of these plates together with that of a tin circular plate 18 inches in diameter, and lastly I compared the charge of all together with that of a circle of 36 inches diametert.

337] By a mean of the different experiments it appears that the charge of each of the rosin plates was alike, and that the charge of any one of them was to that of the circle of 9.3 inches as 10:34 to 9.3, that the charge of the circle of 18 inches was to that of two of the rosin plates together as 2019 to 21.96, and that the charge of the circle of 36 inches was to that of all four plates as 43.75 to 42.06.

But the charge of the four plates together will not be exactly four times the charge of one plate singly, as some allowance must be made for the charge of the wire connecting their upper surfaces, and, besides that, the charge of the plates when placed close together will not be quite so great as if placed at a distance from each othert.

By trying the charge of all four rosin plates together by the machine, Fig. 20, both when placed close together and at as great a distance from each other as I could, I found their charge when close together to be to their charge when placed at a distance nearly as 41 to 41, and, from some other experiments I made, I am inclined to think that the charge of each of the wires which connected the upper coatings of the plates was to that of one plate alone as 28 to 930§.

ratio of 2 to 3. Hence the height of the center of the bodies from the floor was 42 inches, and the height of the room 4 × 42 inches, or 14 feet. This would agree with the height of the top of the circle of 18 inches being 51 inches from the floor (Art. 472).]

* These plates are non-conductors of electricity, and may be charged as Leyden vials. The manner in which I made them will be described in the following pages [Arts. 373, 514]. My reason for making them of these materials is that the charge of such a plate is much less than that of a plate of glass of the same dimensions.

+ It must be observed that, in the two last mentioned comparisons, the rosin plates were placed close together and their upper surfaces connected by a piece of wire.

[Art. 557.]

§ [Arts. 555, 558, also 443.]

From these circumstances, I am inclined to think that the charge of two plates together is to that of one plate alone as 21.96 to 10:34, and that the charge of the four plates together is to that of one alone as 42:06 to 10:34, and consequently that the charges of the tin circles of 93 inches, 18 inches and 36 inches are to each other as 9-3, 20-19 and 4375*.

338] Though I do not know how to calculate how much the charge of the circles ought to be increased by the attraction of the undercharged ground, yet I think there can be little doubt but that if the charge of the plate of 18 inches is increased in any ratio whatever as that of x to x-18, the charge of the plate of 36 inches will be increased in the ratio of x to x 36, and that of the plate of 9.3 inches in the ratio of x to x-93; therefore if we suppose that the charge of the 18 inch, plate is increased in the ratio of 9 to 8, or of 166 to 166-181, the charges of the three plates should be to each other as

which agrees very nearly with experiment, and nearer so than it would have done if we had supposed the charge of the 18 inch plate to have been increased in any other proportion which can be expressed in small numbers†.

339] I think we may conclude therefore that the charge of the 12.1 inch globe was increased by the attraction of the undercharged ground nearly in the proportion of 9 to 8, for I think there can be little doubt but that the charge of the globe must be increased thereby in nearly the same ratio as that of the 18 inch plate, and therefore we may conclude that the charge of the plate D is to the charge which the 121 inch globe would receive, if it was placed at a great distance from any over or under-charged matter, nearly in the proportion of 26.3 to 121, or, in other words, the charge of the plate D is 26-3, which is rather more than eight times greater than it ought to be if the electric fluid did not penetrate into the glass. I shall speak further as to the cause of this in [Art. 349].

340] In order to try the charge of what Æpinus calls a plate of air, I took two flat circular plates of brass, 8 inches in diameter

#

[Art. 649.] + [Art. 652, and Note 24.]

[Mém. Berl. 1756, p. 119.]

and thick, and placed them on the bars Nn and Pp of the machine (Fig. 20), the two plates being placed one over the other, and kept at a proper distance from each other by three small supports of sealing-wax placed between them, the supports being all of the same height, so that the plates were exactly parallel to each other. Care was also taken to place the plates perpendicularly over each other, or so that the line joining their centers should be perpendicular to their planes.

The lowermost plate communicated with the ground by the wire RS, and the uppermost communicated with Mm by the wire V, just as was done in trying the Leyden vials.

I then found its charge, or the quantity of redundant fluid in the uppermost plate, in the usual manner, by comparing it with the plate D, and found it to be to that of D as

341] As I was desirous of trying larger plates than these, and was unwilling to be at the trouble of getting brass plates made, I took two pieces of plate-glass† 111⁄2 inches square, and coated each of them on one side with a circular plate of tinfoil 11.5 inches in diameter, and placed them on the machine as I did the brass plates in the former experiment, with the tinfoil coatings turned towards each other, and kept at the proper distance by supports of sealing-wax as before, care being taken that the tinfoil coatings should be perpendicularly over each other.

For the more easy making a communication between the circular coating of the lower plate and the ground, and between that of the upper plate and the wire Mm, I stuck a piece of tinfoil on the back of each plate, communicating by a narrow slip of the same metal with the circular coatings on the other side.

I then tried the charge as before, the lower plate communicating with the ground and the upper with the wire Mm.

As glass does not conduct electricity, it is plain that the quantity of electric fluid in the pieces of tinfoil will be just the same that it would be if the glass was taken away, and the pieces of tinfoil kept at the same distance as before.

The memoranda I took of that experiment are lost, but to the best of my remembrance the result agreed very well with the following experiment.

+ [Art. 517.]