354 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The Rigidity Modulus . — Although the rigidity modulus does not enter 
directly into the calculations for deflection, yet it was necessary to 
determine it for the purpose of finding Poisson’s ratio. The multiplying 
factor in the deflection equation is 
m 2 — 1 
m 2 
; if m is 4, this amounts to fg, 
and if m is 3, it amounts to §, a ratio of 135 to 128, or 1*06 to 1. Although 
this is not a very serious matter, as S almost certainly would lie between 
i and yet it was adjudged best to find m experimentally. Owing, 
however, to the greatest thickness of plate obtainable being ^ in., this 
became a difficult matter. 
A length of about 10 in., of square section T b in. side, was first prepared. 
A heavy disc whose moment of inertia was known was attached to one 
end, and the other end was gripped in a clamp. The method of oscillation 
