152 
DR. EVERETT ON THE RIGIDITY OE GLASS. 
and as the experiments in the Lecture-room gave *315 as the value of <7, I adopt the 
value -310. Kiechhoff’s value for steel is *294, and Cleek Maxwell’s for iron -267. 
The following are the collected results of the experiments described in both this and 
the former paper, the values of M, n, and k being reduced to kilogrammes’ weight per 
square millimetre. 
M . . . . . 
Flint Glass, 
1865. 
. 6143 
Flint Glass, 
1866. 
5851 
Drawn Brass, 
1866. 
10948 
Cast Steel, 
1866. 
21793 
n 
2442 
2390 
3729 
8341 
k 
. 4230 
3533 
57007 
18756 
a 
•258 
•229 
•469 
•310 
Specific gravity 
. 2-942 
2-935 
8-471 
7-849 
Strictly speaking, the above values of M are the measures of resistance to longitudinal 
extension parallel to the length of the rods, and the above values of n are the measures 
of resistance to shearing in planes parallel or perpendicular to the length. The values of 
k and a have been deduced on the hypothesis that the materials of the rods are isotropic. 
If, however, as is probably the case, this hypothesis is not fulfilled, and if the deviation 
from isotropy be such that the resistance to shearing in planes parallel or perpendicular to 
the length is less than for intermediate planes, then the values of k and a above calculated 
are too large ; for longitudinal extension (especially if accompanied by lateral contraction) 
involves a certain amount of shearing in planes oblique to the length, and the resistance 
to this shearing is one of the constituents of M, whereas the shearing which takes place 
in torsion is perpendicular to the length. Such a deviation from isotropy as we are now 
considering will therefore increase the ratio of M to n, and will therefore increase <r, 
M 
which is equal to — — 1. It will also increase k, since the value of k may be written 
— This caution is specially important in the case of the brass rod, both because 
3(1 — 2 tr) 
the operation of “ drawing ” appears likely to produce such a deviation from isotropy 
as we have been describing, and also because the value of a for this rod comes out so 
nearly equal to ^ that the factor 1— 2<r in the denominator of k will be greatly affected 
by small errors in the value of a. For these reasons we are not disposed to attach 
much weight to the very large value of k which we have found for brass. 
We append for comparison some of the principal results obtained by previous expe- 
rimenters. 
The values obtained by Weetheim for different specimens of glass (crystal) were, — 
M 3481 to 4429, mean 4039, 
n 1288 to 1687, mean 1518, 
k 3569 to 4476, mean 3968; 
and for different specimens of brass, 
M 9665 to 10645, mean 10054, 
n 3600 to 3973, mean 3745, 
k 10216 to 11058, mean 10631. 
