23 



centre of suspension. The pendulum was pulled just far enough 

 to overcome friction alone. After several trials, it was found to be 

 equal to 0-25 pounds. Thus, the friction of the machine is 12-5 

 inch-pounds, which friction is added, in every case, to the moments 

 recorded by the pencil. 



The absolute errors in the size of the test pieces vary by excess 

 from above one-thousandth (yoinr) ^ an mc h U P to a little above 

 one-hundredth (ooi) of an inch. 



We have put down the moments at the elastic limit and at the 

 maximum before rupture ; also the angles of torsion at the elastic 

 limit, maximum before rupture, and final rupture with the cor- 

 responding elongations of the outermost fibres for the first three 

 angles. As a matter of detail, we have observed those which 

 reached the final rupture with a higher moment after the first 

 rupture, and those that did not offer a higher resistance. The 

 names used express the character of the four critical points to 

 which a material is subjected under an increasing stress. 



Not being able to put in the strain diagram, we have endeav- 

 ored to write out for illustration the behavior of several pieces 

 under stress, and their mode of rupture as concisely as possible in 

 the details of torsion tests. These may be more valuable than the 

 numerical results. The torsion tests give a better and more 

 general idea of all the properties of strength and elasticity of the 

 woods than any other test, as, in this case, they are compelled to 

 write for themselves their " own story." 



DETAILS OF TORSION TESTS. 



(i.) Carbon, d = 0-6271. Test piece turned from a large 

 sample near the heart. Very sound in the turned part. Elastic 

 limit, maximum and rupture occurred under the same moment. 

 From this and by breaking in two splinters it shows it is a brittle 

 wood. It reached a moment at the latter points of 200-87 inch- 

 pounds, at an angle of 7-5 and gave away completely at !3-2 

 with no higher moment. Its curve was at 45 from the axis of 

 angles and was short-wavy all along. 



(2.) . With a short wavy and parabolic curve 



reached its elastic limit at 7-ii under a moment of 186 inch- 

 pounds, and reached a maximum of 190 inch -pounds moment, 

 which kept constant for about 8, when it came down and ruptured. 

 Fairly regular stiff rupture. 



