Th. 55°. Smeaton's Hygrometer 181. Common Hygrometer + 5.

469] Monday, Dec. 16 [1771]. Trials of time in which electricity of stone square, &c. was destroyed.

The squares were supported on glass, and a piece of tin foil about 1 inch square fastened on each corner. On one of these pieces was fastened a wire from which the pith balls were suspended. The square was then electrified by applying a charged vial, and then a wire communicating with the wall was applied to the other piece of tin foil.

With slate the corks closed in 10"

The stones had been kept in fore room for several days. The gummed glass had been kept in fore room till the gum began to crack. It was then kept in back room for about 5 hours, and then kept in fore room about 1 hour.

Hygrometer corks closed in 4. The glass being then wiped they closed in 7. Being then suffered to stand uncovered for 2 or 3 hours, the corks closed in 5'.

470] Tuesday, Dec. 17 [1771]. Th. 53. S. H. 20. C. H. +9.

Experiments of [Art. 468] continued.

471] Wednesday, Dec. 18 [1771]. Th. 50°. S. H. 17. C. H. +3.

Trials of flat plates of different substances about 1 foot square*.

472] Comparison of two tin circles 9.3 inches in diameter with one of 18.5; the two circles being placed in vertical planes parallel to each other and perpendicular to the vertical plane joining their centers and the trial plate, their centers being both in the above-mentioned plane*.

There was a distinct touching wire to each plate meeting each other at R, the two wires were kept asunder by a slender glass tube, and about 1 inch of the end of the wires bent at right angles horizontally in order to touch the plates by being let fall on their edges. When the large circle was tried, this double touching wire was removed and a single one used in its room, which was sometimes fastened to the middle of the glass tube, and sometimes used without it, as will be expressed.

The height of the top of the circles above floor = 4'. 3".

The center of the large circle when that was used, or the middle point between the centers of the two small circles when they were used, (8'. 10")

was

from 17. 3′′]

(vial

middle of trial plate)'

The circles were suspended by silk strings. The length of the touching wires for the circles was 36 inches.

[Exp. v., Art. 273.]

473] Monday Dec. 30 [1771]. Th. 50°. S. H. 18.

Two small circles at 18 inches from each other.

Large circle, touching wire being fastened to glass tube.

The Proportion by theory, vide P. 14 of calculations*, are as fol'lowst.

474] The same experiments repeated in the same manner except that the distance of the center of the large circle, or of the middle point between the centers of the small ones was 5'. 3" from the vial, and the middle point of the trial plate 8'. 2" from vial, and that some boards forming a floor about 4 or 5 feet square was placed under the circles 14 inches from the ground, and that a perpendicular bar of the same breadth as those of the frame was placed 5 inches nearer to the circles than the other, so that the distance of the center of the large circle from the vial and the ground, and also the distance of the nearest small

* ["P. 14 of Calculations" refers to a rough calculation in parcel No. 6, which is an early form of Props. XXIX, and XXX. See Arts. 140-143. "P. 14" contains the following remark, which fixes its date after Art. 456, "By exp. P. 15 [Art. 456] quant. el. in circle is to that in globe of same diam. as 1: 1·56 :: :·78, 2+ n therefore 2+2n

=78." Here n is the reciprocal of p in Art. 140.]

+ [See Art. 681 and Notes 11 and 21.]

477]

TWO WIRES.

235

circle from the perpendicular bar when they were placed at 36 inches. distance, were diminished in about the ratio of 2 to 3*.

475] Tu. Dec. 31 [1771]. Th. 51o. S. H. 18.

Small circles at 18 inches distance.

476] Comparison of 2 wires 3 feet long and inch in diameter with 1 of 6 feet long and 185 in diametert.

The wires were placed parallel to each other, horizontal and perpendicular to the horizontal bar. They were touched almost close to one extremity by the same wires and in the same manner as the circles in the former experiment.

That end of the wires near the part which was touched was suspended by silk, the other end was supported on waxed glass. The distances were the same as in [Art. 472].

477] Fr. Jan. 3 (1772). Th. 50°. S. H. 191. C. H. +2.