DR. EVERETT ON THE RIGIDITY OE GLASS. 
149 
Hence we find for brass, 
M=^=l,094,800,000, 
71 T 4 5 7 5 7 
n~= 372,890,000, 
7rr 
*=3lCT)= 5 ’ 700 ’ 700 ’ 000 ’ 
<r~-l = *469. 
2 n 
From comparing the above value of Jc with its values for the two glass rods experi- 
mented on, it would appear that brass is from 13^ to 16 times more incompressible 
than glass ; but this result is to be received with caution, for reasons which will be stated 
further on. 
A rod of cast steel was next operated on, with the following results, the weights used 
being the same as for the brass rod. 
i(4 
Pointer at 310° 
Torsion 2041 
Flexure 155|, 
313 
n(4 
o 
O 
GO 
CM 
„ 205 
„ 155 , 
307 
III (a). 
„ 250° 
„ 207 
„ 154 , 
316 
1(4 
„ 220° 
„ 206 
„ 157 , 
313 
II (b). 
„ 190° 
„ 206f 
„ 154 , 
313i 
HI (4 
„ 160° 
„ 206f 
„ 156 , 
3131 
Hence we have the following means : — 
1 4) (4 Torsion 205*1 Flexure 156*2, 313*0 
11(a)(5). „ 205*9 „ 154*5, 310*1 
111(a)(5). ■ „ 206*9 „ 155*0, 314*7 
Correcting for difference of scale-divisions, we obtain the following determinations of 
Poisson’s ratio. 
From torsion at 100 grms. compared with flexure at 100 grms., 
*305, *325, *327. 
From torsion at 100 grms. compared with flexure at 200 grms., 
*304, *321, *308. . 
As-the apparatus was disturbed in my absence, and the mirrors were moved from their 
places before any measurements had been made of their positions, it is impossible to 
determine with accuracy from the foregoing observations the torsional and flexural 
rigidities of the rod. In order to determine Poisson’s ratio as accurately as the data 
permit, we shall assume (what is known to be near the truth) that the ratio of the whole 
length operated on to the length between mirrors was the same as for the brass rod, and 
that the optical corrections are the same. From these data, the mechanical corrections 
are found to be 
+ *0084 T, + *0064 F w + *0078 F 2 , 
MDCCCLXVII. X 
