146 
DE. EVEEETT OH THE KIGTDITY OE GLASS. 
The differences in these determinations of M and n, being only about five per cent, in 
the former case and two in the latter, are probably real, the denser specimen being also 
the more rigid. The values of 1c and <r are liable to a larger percentage of error ; but 
this remark is more especially applicable to last year’s results, as our present apparatus 
affords greatly increased facilities for determining the ratio of flexural to torsional 
rigidity. 
With respect to the composition of the two specimens, I am unable to give precise 
information, as the ingredients are mixed according to no definite rule. 
The glass rod having been taken down, a rod of drawn brass was mounted in its place, 
the apparatus remaining in precisely the same position as in the experiments of July 
17th and 18th. The following results were furnished by the first set of observations, 
July 27th and 28 th : — 
i(4 
Pointer at 
0° 
Torsion 408 
Flexure 276 
ii(4 
5? 
30° 
„ 406 
„ 274- 
in (4 
99 
60° 
„ 404 
„ 275 
1(b) ■ 
„ 
90° 
„ 404 
to 
CO 
o 
11(b). 
99 
120° 
„ 404 
„ 276- 
111(b). 
99 
150°, 
„ 407 
„ 275; 
From these we obtain the following means 
1(a) (b). Torsion 406 Flexure 278, 
II (a)(b). „ 405 „ 275-5, 
111(a) (b). „ 405-5 „ 275-1, 
whence, after correcting as before for difference of scale-divisions, we obtain for Poisson’s 
ratio the values -451, -461, -465. 
The weights used in these observations w r ere the same as for the glass rod. 
A second set of observations were made July 31st, August 1st and 2nd, in which, 
besides the old weights, which were each 100 grms., weights of 200 grins, were also 
employed. These latter, however, could only be used for flexure, as when the attempt 
was made to employ them for torsion, it was found impossible to prevent the rod from 
turning in its socket. In consequence of turning which took place from this cause at 
the commencement of this set of observations, the following pointer-readings are not 
precisely comparable with the foregoing, that is to say, the zero-point may be regarded 
as having shifted between the two sets of observations. A slight change was also made 
in the position of one of the telescopes, between observations 3 (a) and 1 (b), for the 
purpose of obtaining better light, and at the same time a string was attached to the 
“ damper” in such a manner that the observer could pull the damper away from the rod 
without removing his. eye from the telescope. 
The following were the results, the two sets of flexure-numbers being obtained with 
weights of 100 and 200 grms. respectively. 
