MOVEMENTS OF THE WINGS OF BIRDS. 245 



Now let us raise the ball to the position B ; the elasticity 

 of the india-rubber thread will cause the lever to rise. Thus 

 it is acted upon alternately by two forces, sometimes by the 

 traction exerted by the silk thread, which lowers it ; at others, 

 by the retraction of the india-rubber, which re-acts as soon as 

 the tractile force ceases. Thus the lever will follow faithfully 

 all the movements which are given to the extremity of the 

 thread which draws it down. 



FIG. 10'2 Transmission of a to anrl-fro movement by means of a simple 



traction-cord. 



The lever I/, which is to trace on the cylinder the 

 ments transmitted to it, moves in an opposite direction to the 

 course of the cord C C. The tracing will thus be reversed, 

 and if it were important to obtain it in the same direction, it 

 would be necessary to turn the registering 1 drum, so as to place 

 the membrane downwards.* 



With two instruments of this kind, one acted upon by th'e 



* As many instruments of this kind are required as there are move- 

 ments to be studied. But three connected levers will always be sufficient 

 to ascertain the movements of a point in space, since each of the posi- 

 tions of this point is defined when it has been determined with reference 

 to three axes at right angles to each other. 



