428 Proceedings of the Royal Society of Edinburgh. [Sess. 
a number of stresses lying between p, 0,—p, and p, O, p. Internal pressure 
causes the intermediate principal stress to have a small negative value 
(compression), but this is of no importance. The criterion of strength for 
the steel tubes was taken as the yield-point, and the results of the experi- 
ments are given in a series of tables attached to the paper. The conclusion 
was drawn that the strength of a material is determined by the maximum 
shearing stress, and not by the maximum principal stress, or by the maxi- 
mum principal strain. 
The writer, by taking Guest’s data for the principal stresses, and 
assuming /a = 0T4 for the steel tubes, found that the numerical values of 
the minimum resistance to sliding were sometimes less uniform, and some- 
times more uniform, than the values of the maximum shearing stress. The 
figures are not sufficiently interesting to reproduce, but the experiments 
could not be said to give results more in favour of the minimum resistance 
theory than of the maximum shear theory. 
Another point of interest was the determination of the values of jjl 
which would accord with the experimental results. The values of this 
coefficient, obtained from the simple tension and simple torsion results only, 
are given in the table below, from which the amount of variation can be 
judged. The variations found for each separate bar, under the different 
conditions of loading, were quite as great. It should be pointed out that 
small errors in measuring stresses lead to relatively large errors in the 
determination of /x. No measurements of the direction of sliding at the 
yield-point seem to have been made by Guest. These experiments, there- 
fore, do not permit one to assign any definite value to /u. 
Table. 
Form of Specimen. 
Diameter. 
Thickness. 
t u. 
Remarks. 
| Guest — 
I. Round tube . 
D316 
0-029 
- 0-242 
II. „ . . 
1-250 
0-028 
. « • 
Several different results. 
III. 
0-499 
0-025 
-0-071 
IV. „ . . 
1-250 
0-025 
+ 0-093 
V. „ . . 
1-250 
0*025 
... 
No torsion test. 
VI. 
1-250 
0-025 
+ 0-015 
VII. „ . . 
1 *250 
0-025 
+ 0-160 
VIII. 
1-250 
0-025 
+ 0-090 
IX. 
1-250 
0-025 
+ 0-144 
Hancock — 
Round tube 
rooo 
0-050 
+ 0-146 
rooo 
0-075 
+ 0-380 
« 
* 
1-000 
0-250 
+ 0-204 
Round bar 
0-500 
... 
+ 1-189 
