ELASTIC RADIAL DEFORMATIONS OF FLYWHEELS. 265 
the experiments, only a few trials were made with it as 
it was considered advisable to complete them with the 
original rod (fig. 3). Its advantages are absence from bend- 
ing, and extreme lightness; moreover, when different 
wheels are tested it is at once adaptable, whereas in the 
case of the pointed rod, it is necessary to trim the end of 
the spindle in order that the rod may rest on the rim and 
on the prism at the same time. All the tests described 
in the following were carried out with the distance rod 
shewn in fig. 3, as it was the most satisfactory of the steel 
rods, owing to its rigidity and to its having the smallest 
extension, as seen later. 
To find the lineal deflection equivalent to one scale 
division: The correct expression for the extension as 
measured by the Marten’s mirror apparatus is given by 
Mr. G. H. Knibbs, F.R.A.s.,’ and it is directly applicable to 
the method in question. It is 
ayecl cS 2° +57 sino t 2 
R 2L cos» 
Where e = extension. 
R= depth of prism. 
L = distance between scale and mirror. 
E= half length of distance rod. 
1 = distance between knife-edges. 
» = angle of rotation of line joining them. 
In the present case, R=0°300 inch, L=1010mm. Thus, 
taking the approximate value for the deflection equivalent 
to one scale division, we have 
ve 0°3 
2x2x1010 
since each division = 0°5 mm., i.e., e = 0°000075 inch. 
* Journ. Royal Society, N.S.W., Vol. xxx1., 1897 p. 98. 
R—Aug. 5, 1903, 
