68 



THE HUMAN MOTOR 



2. Work of flexure : 



= ~ & (forTa cylindrical rod). 



O 7C/ -tl* 



3. Work of torsion : 



T =; I- M X = 



M 2 T, 



a cylindrical rod) . 



Thus elastic energy is a form of energy ( 37) equivalent to the 

 others \ shock imparts to an elastic body a certain quantity 

 of energy, the body taking a deformation determined by its 

 section.^h's length and its " Young's modulus." That energy 

 is transmitted, more or less completely, by the .elastic body, and 

 specially, more or less slowly, so that if several shocks succeed 



each other rapidly, they will be 

 f~. transformed into a continuous 

 action. Thus Marey showed 

 that the application of rubber 

 ropes to the drawing of hand 

 carriages effected an economy 

 of 26 per cent, of the work done, 

 the shocks not being transmitted 

 to the shoulders of the man, 

 excessive effort and discomfort 

 were avoided. The motive 

 power of the heart is, in the 

 same way, rendered regular and 



FIO. 82. 



10 economical. Such is also the law 



of the respiration, etc. An in- 

 teresting experiment of Marey 's 

 is worthy of mention ; AB is the 

 beam of a balance (fig. 82). A 

 ball G weighing 100 grammes is 



attached to A, and to B a ball p weighing 10 grammes, by a string 

 whose length is h. The beam is held horizontal by resting on a 

 special stop S, so that it can only move in the direction of the 

 arrow. The raising and letting fall from a height h' of the little 

 ball p will not cause any movement of the beam. 



Replace the string of the ball G by an elastic thread and the 

 beam will be displaced if the little ball is repeatedly dropped. 

 This is a clear example of the transmission and accumulation of 

 the work of shocks, so rapid that in some way they produce a 

 tensile stress, capable of overcoming the weight of the larger 

 ball, and causes a certain displacement of it. 



The tension thus produced is the result of the elastic work of 

 the string. The work done in falling by the ball p is p 

 (h' + h + /), / being the elongation of the string L ; and the 



