XII . — Experiments on the modulus of Torsion. By Benjamin Bevan, Esq. 
Communicated by the President. 
Read December 18, 1828. 
]\UMEROUS experiments have already been published on the strength of 
wood and other substances, as far as regards their cohesion and elasticity ; but 
I am not aware of any extensive table of the modulus of torsion of different 
species of wood, deduced from experiments conducted upon a proper scale, 
and with the necessary care. 
To supply this defect, and to furnish the practical engineer and mechanic 
with useful data, and with rules for their application, is the object of the pre- 
sent communication, consisting of a copious table of the results of my experi- 
ments, made at various times, and upon substances of considerable variety of 
dimensions within the ordinary limits of practice. 
It is proper to observe, that the various specimens of wood upon which my 
experiments were made, were sound and dry, except it is otherwise expressed 
or described, and were in general free from all large knots. 
Considerable care was used to obtain correct dimensions of the specimens 
under experiment, by means of a simple instrument, which answers the pur- 
pose of improved callipers, by which the dimensions of the specimens were 
measured, and read off by a magnifying-glass to the 400 dth part of an inch. 
Previous to trial, each specimen was brought to a prismatic form, as near as 
could be wrought by the ordinary means, and the dimensions afterwards taken 
by means of the improved callipers above mentioned, at equal distances ; and 
the mean breadth and thickness thus obtained, were used in the calculations 
for obtaining the modulus. My experiments were often repeated on the same 
species of wood, under considerable variations of length, breadth, and thick- 
ness ; and always with the most satisfactory results ; viz. from nine to ninety 
