from A, or will appear to be repelled by it, just as a body heavier than water tends to descend in it, and, on the contrary, if B is made to contain less than its natural quantity of the fluid, it will have a tendency towards A, or will appear to be attracted by it.

215] Let now the space A be made to contain less than its natural quantity of the fluid (as the fluid in B is now repelled from A with less force than it is in any other direction, id est, apparently attracted towards it), if B also contain less than its natural quantity of the fluid it will tend to recede from A, id est, appear to be repelled by it; but if B contain more than its natural quantity, it will then tend to approach towards A, id est, appear to be attracted by it.

216] If the electric fluid is diffused uniformly through all bodies not appearing electrical and the repulsion of its particles extends to considerable distances, it is plain that the consequences are such as are here described; but how far that supposition will agree with experiment I am in doubt *.

* [Note 18.]

EXPERIMENTS ON ELECTRICITY.

EXPERIMENTAL DETERMINATION OF THE LAW
OF ELECTRIC FORCE.

217] I now proceed to give an account of the experiments, in all of which I shall suppose, according to the received opinion, that the electricity of glass is positive, but it is not at all material to the purpose of this paper whether it is so or not, for if it was negative, all the experiments would agree equally well with the theory.

218] EXPERIMENT I. The intention of the following experiment was to find out whether, when a hollow globe is electrified,

a smaller globe inclosed within it and communicating with the outer one by some conducting substance is rendered at all over or undercharged; and thereby to discover the law of the electric attraction and repulsion.

219] I took a globe 121 inches in diameter, and suspended it by a solid stick of glass run through the middle of it as an axis, and covered with sealing-wax to make it a more perfect nonconductor of electricity. I then inclosed this globe between two hollow pasteboard hemispheres, 13.3 inches in diameter, and about

of an inch thick, in such manner that there could hardly be less than of an inch distance between the globe and the inner surface of the hemispheres in any part, the two hemispheres being applied to each other so as to form a complete sphere, and the edges made to fit as close as possible, notches being cut in each of them so as to form holes for the stick of glass to pass through.

By this means I had an inner globe included within an hollow globe in such manner that there was no communication by which the electricity could pass from one to the other.

I then made a communication between them by a piece of wire run through one of the hemispheres and touching the inner globe, a piece of silk string being fastened to the end of the wire, by which I could draw it out at pleasure.

220] Having done this I electrified the hemispheres by means of a wire communicating with the positive side of a Leyden vial, and then, having withdrawn this wire, immediately drew out the wire which made a communication between the inner globe and the outer one, which, as it was drawn away by a silk string, could not discharge the electricity either of the globe or hemispheres. I then instantly separated the two hemispheres, taking care in doing it that they should not touch the inner globe, and applied a pair of small pith balls, suspended by fine linen threads, to the inner globe, to see whether it was at all over or undercharged.

221] For the more convenient performing this operation, I made use of the following apparatus. It is more complicated, indeed, than was necessary, but as the experiment was of great importance to my purpose, I was willing to try it in the most

accurate manner.

ABCDEF and AbcDef (Fig. 12) are two frames of wood of the same size and shape, supported by hinges at A and D in such manner that each frame is moveable on the horizontal line AD as an axis. H is one of the hemispheres, fastened to the frame ABCD by the four sticks of glass Mm, Nn, Pp, and Rr, covered with sealing-wax. h is the other hemisphere fastened in the same manner to the frame AbcD. G is the inner globe, suspended by the horizontal stick of glass Ss, the frame of wood by which Ss and the hinges at A and D are supported being not represented in the figure to avoid confusion.

Tt is a stick of glass with a slip of tinfoil bound round it at x, the place where it is intended to touch the globe, and the pith balls are suspended from the tinfoil.

The hemispheres were fixed within their frames in such manner that when the frames were brought near together the edges of the hemispheres touched each other all round as near as might be, so as to form a complete sphere, and so that the inner globe was inclosed within them without anywhere touching them, but on the contrary being at nearly the same distance from them in all parts.

222] It was also so contrived, by means of different strings, that the same motion of the hand which drew away the wire by which the hemispheres were electrified, immediately after that was done, drew out the wire which made the communication between the hemispheres and the inner globe, and immediately after that was drawn out, separated the hemispheres from each other and approached the stick of glass Tt to the inner globe. It was also contrived so that the electricity of the hemispheres and of the wire by which they were electrified was discharged as soon as they were separated from each other, as otherwise their repulsion might have made the pith balls to separate, though the inner globe was not at all overcharged.

The inner globe and hemispheres were also both coated with tinfoil to make them the more perfect conductors of electricity.

223] In trying the experiments a coated glass jar was connected to the wire by which the hemispheres were electrified, and this wire was withdrawn so as not to touch the hemispheres till the jar was sufficiently charged. It was then suffered to rest on them for two or three seconds and then withdrawn, and the hemispheres separated as above described.

224] An electrometer also was fastened to the prime conductor by which the coated jar was electrified, by which means the jar and consequently the hemispheres were always electrified in the same degree. This electrometer as well as the pith balls will be described in [Arts. 244 and 248]; the strength of the electricity was the same as was commonly used in the following experiments, and is described in [Arts. 263, 329, 359, 520].

225] My reason for using the glass jar was that without it it would have been difficult either to have known to what degree the hemispheres were electrified or to have kept the electricity of the same strength for a second or two together, and if the wire had been suffered to have rested on the hemispheres while the jar was charging, I was afraid that the electricity might have spread itself gradually on the sticks of glass which supported the globe and hemispheres, which might have made some error in the experiment.

226] From this manner of trying the experiment it appears: First, that at the time the hemispheres are electrified, there is