150 MOVEMENT 



amount of vertical foot-pressure on the platform is 

 altered in amount, and is recorded on the tracing by 

 the lever. The following is the law which governs 

 the variations in pressure : — 



All muscular actions ivhicli alter the centre of gravity 

 of the body in such a manner as to raise it augment the 

 foot-pressure on the ground. 



All actions tending to loiver the centre of gravity 

 diminish the foot -pressure* 



The dynamograph which measures the force can be 

 employed conjointly with an apparatus for recording 

 the actual movement. It can be seen, then, that 

 mechanical actions accomplished by living creatures 

 obey general laws — amongst others those of ballistics. 



Now, in order to determine the phases of a move- 

 ment, two methods may be employed, either that of 

 mechanical registration or that of geometrical chrono- 

 photography. 



Mechanical Record of a Movement. — Movements, such 

 as we shall have to deal with, are generally too 

 extensive to be directly recorded by means of a 

 tracing needle. In jumping, for instance, the head 

 may be elevated any distance between 30 and 50 

 centimetres in a vertical direction. This movement 

 must be reduced to such proportions as can be regis- 

 tered on the surface of a revolving cylinder. This 

 reduction can be effected by means of the elastic- 

 thread method previously described (Fig. 29). 



If a man stands on the platform of a dynamograph, 

 and wears a very tight-fitting cap, the elastic thread 

 may be fastened at one end to the cap, and at the 

 other to a solid support by means of a clip (Fig. 101). 

 This thread may be fixed near its upper end to the lever 



* All effects which are produced during the performance of a 

 movement are followed, when the movement is concluded, by counter 

 effects in the opposite direction. 



