62 THE HUMAN MOTOR 



IT LXF. 

 Thus E = -^ 



Letting L = / and S = 1 square millimetre, E = F, that is to 

 say that Young's Modulus expresses an effort capable of doubling 

 the length of a rod of 1 square millimetre. In practice, this hypo- 

 thesis is never true, because the matter cannot be deformed to 

 that point, but breaks when its tenacity, which is the cohesion 

 between its molecules is overcome. The formula (1) is only true 

 for efforts not overstepping the limit of elasticity. It is only 

 within the limit that we can apply Robert Hook's ( l ) law : ut 

 tensio, sic vis (similar deformation for similar effort.) 



When the stress has reached an amount sufficient to overcome 

 the cohesion of the material, the limit of elasticity is exceeded 

 and the body breaks. This point, the breaking stress, whether 

 it be due to break by tension or compression (crushing) is 

 found by experiment, and depends in an irregular way on the 

 dimensions of the body. Thus Rondelet and Hodgkinson found, 

 for different woods, that it increases as the square of the section, 

 and inversely, that it diminishes as the square of the length. 

 Then 



the section being squared, and the length equal at the most to 

 15 times the side of the section, the co-efficient K = 2'565 is 

 expressed in kilogrammes, S in square centimetres and h in 

 decimetres. 



It has been found that circular sections resist the most, 

 hence the advantages of round bodies. Cubic bodies aje also 

 very resistant. Finally, the modulus E and the resistance R 

 diminish as the matter becomes fatigued, but increase, in woods, 

 according to density, age, and dryness. 



The following figures illustrate this point : 



(*) Robert Hooke, an English natural philosopher (1635-1703), 



