142 
DE. EVERETT ON THE RIGIDITY OE GLASS. 
ratio of torsion to flexure, we must divide the numbers' in the first column by those in 
the second, and diminish the quotients by yg-g of their amounts. The quotients thus 
corrected are 
1 0) (b), 1-222 ; 2 (a) (b), 1-2,30 ; 3 (a)(b), 1-221, 
whence we obtain at once for Poisson’s ratio (or) the values -222, -230, -221. Some small 
corrections will be applied to these values hereafter, only affecting the third decimal 
place ; but we deem it important thus early to direct attention to the strength of evidence 
showing that Poisson’s ratio for the substance in hand is less than y. 
An earlier set of observations, in only four positions of the rod, were taken July 13th, 
14th, and 16th, the apparatus being at this time less favourably arranged, inasmuch as 
the rod was more distant from a vertical through the centre of the scale than in the later 
set. The following were the results obtained : — 
1(a). 
Pointer at 90° 
Torsion 555y 
Flexure 452 
1(b). 
0° 
„ 550 
„ 430 
11(a). 
„ 45° 
„ 550 
„ 4591 
np}. 
„ 135° 
„ 5441 
* 437| 
Giving the following means, 
I (a) (b). Torsion 552-f Flexure 441 
11(a)(1). „ 547-g- „ 448|, 
whence we obtain, after correcting for inequality of divisions, the values of Poisson’s 
ratio -246, -220, the largeness of the former number being due to the large angle made 
by the rays of light with the vertical plane containing the rod. A correction for this 
defect will be applied in the sequel. 
After the observations of July 17th and 18th, the rod was removed from its place, and 
cut at the places where the mirrors had been attached. The length of the central 
portion was found to be 235-6 millims., and its weights in air and water respectively 
32-002 and 21-112 grms., the temperature of the water being 13-3 Reaum. 
The distances S S', T T' were 558-2 and 557-2 millims., so that the mean arm of couple 
was 557‘7 millims., the force being the weight of 100 grms. 
The height of the scale above the mirrors was 4327 millims. ; but since the deviation 
of a reflected ray is double of the angle turned by mirror, it will be necessary to divide 
the arcs traversed on the scale by twice this distance, or 8654 millims., in order to find 
the angles turned. 
The scale-divisions for torsion were - 7 - 5 - millim., but as they were subdivided by esti- 
mation to tenths, and it is in these tenths that the above torsion-numbers are expressed, 
the unit is to be regarded as the yff of a millimetre. In like manner the unit for the 
flexure-numbers is the yf§ of a millimetre. We shall denote the torsion-numbers and 
flexure-numbers, expressed in these units, by the letters T and F. 
From the observations of July 17th and 18th we have the mean values T=546, 
