130 THE HUMAN MOTOR 



Ox 70 contractions per minute 



Horse 75 



Man 90 to 100 



Hunting dog 102 



Cat 162 



Rabbit 240 



Guinea pig 300 



White mouse 350 



The same law is exemplified in the rate of the wing beats of 

 various birds and insects which vary from the eagle 50 to 60 

 per minute to the fly : nearly 20,000 (Marey). The speed is, 

 as a rule, less in the female. 



The causes which determine the rhythm are twofold : firstly, 

 the nervous excitation accelerates or decelerates according to 

 the resistance to be overcome ; secondly, the muscular fibre 

 repairs itself in the intervals of repose, and the conditions of that 

 reparation are not the same for all the muscles. According to 

 M. Chauveau, the reparative work consists purely of re-establish- 

 ing the elastic properties of the matter, of creating elastic force, 

 or the force of contraction. The internal " physiological work," 

 in the living cells, which never stops, achieves this definite result. 



92. Similitude of Machines One often uses, in the calculation 

 of machines the idea of " similitude " (similis in Latin : similar) 

 introduced into science by Newton ( l ), and of which Galileo had 

 dreamed. It permits the study of a machine by a " reduced 

 model." This is what is done, for instance, for the hulls of ships, 

 for the plane of aeroplanes, etc. The reduced models of ships 

 are generally to the scale of -^. Suppose L to be the length of 

 the arm of a statue, and / that of the same arm in the reduced 



model. The quotient y = r is called the " relation of similitude." 



Between the " surfaces " it is obvious the relation will be r 2 , 



and between the volumes it will be r 3 . Therefore : 

 s , v 



_ __ y& yd 



S V 



For example, the surface of the human body varies as the 

 square of its height, but the volume varies as the " cube " ; so 

 that the volume decreases less rapidly than the surface, when 

 considering decreasing statures. ( 91). 



Considering two machines made of identical materials, there 

 is the same relation for the " masses " or the " weights " as for 



(*) Newton, Philosophia Naturalis, ii., Section 1, propos. 32. 



