420 Mr. Bevan's Expeiiments on the Modulus of Torsion. 



To supply this defect, and to furnish the practical engineer 

 and mechanic with useful data, and with rules for their appli- 

 cation, is the object of the present communication, consisting 

 of a copious table of the results of my experiments, 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 ge- 

 neral free from all large knots. 



Considerable care was used to obtain correct dimensions of 

 the specimens under experiment, by means of a simple instru- 

 ment, which answers the purpose of improved callipers, by 

 which the dimensions of the specimens were measured, and 

 read off by a magnifying-glass to the 400dth 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 calcula- 

 tions for obtaining the modulus. My experiments were often 

 repeated on the same species of wood, under considerable va- 

 riations of length, breadth, and thickness; and always with the 

 most satisfactory results ; viz. from nine to ninety inches in 

 length, and from three inches to three-tenths of an inch in 

 thickness. Due care was observed to prevent any error in the 

 apparent torsion or twist arising from compression at the ends 

 of the prisms, both at the clamp by which they were fixed, 

 and at the radial lever at which the successive weights were 

 applied; two sources of error which have materially affected 

 former experiments on this subject, in other respects care- 

 fully made. 



To every specimen under experiment I attached two in- 

 dexes ; one a few inches from the end fixed in the clamp or 

 vice, and the other at a small distance from the attachment of 

 the lever or wheel, where the weight or straining power was 

 applied ; and the distance between the two indexes was used 

 as the length for calculatinc'. Another error of less majrni- 

 tude I have been able to avoid by fixing a pivot or small 

 gudgeon at the supported end, in the line of the axis of the 

 prism, instead of making the lower side or angle of the prism 

 at the supported end the revolving point. 



My experiments were made upon pi'isms of very different 

 proportions as to breadth and depth, viz. from J^^th to equality. 



In general practice, the square or cylindrical shaft is usually 



adopted, 



