FOOT AND FOREARM AS LEVERS 61 



power which is reached when the piston is in the position 

 shown in fig. 15, B. Then the leverage decreases until 

 the second dead centre is reached (fig. 15, C) ; from that 

 point the leverage is increased until the second maximum 

 is reached (fig. 15, D), whereafter it decreases until the 

 arrival at the first position completes the cycle. Thus 

 in each revolution there are two points where all leverage 

 or power is lost, points which are surmounted because 



% 



^ 



A. 



B. C. 



Fig. 15. — Showing the crank-pin of an engine at : 

 A, First dead centre. C, Second dead-centre. 



D. 



B, First maximum leverage. 



D, Second maximum leverage. 



of the momentum given by the fly-wheel. Clearly we 

 should get most out of an engine if it could be kept 

 working near the points of maximum leverage — with 

 the lever as nearly as possible at right angles to the 

 crank-pin. 



Now, we have seen that the tendon of Achilles is the 

 piston cord, and the heel the crank-pin, of the muscular 

 engine represented by the gastrocnemius and soleus. In 

 the standing posture the heel slopes downwards and 

 backwards, and is thus in a position, as regards its piston 



