THROWING THE FLY. 95 



.arc B i H to H. The fly is thus converted into 

 a simple projectile, and, if it were not for gravi- 

 tation, would traverse in a straight line from c 

 to D ; because, the moment the spring is given 

 at B, the rod is, or rather ought to be, at right 

 angles with the lines c B D. But, in obedience 

 with the laws of every projectile, being unre- 

 strained by the rod or line, it describes the para- 

 bola c G F E (E H being the extended line). 

 The force of the backward throw being expended, 

 it falls below E of its own gravity : the motion of 

 the rod being renewed, forwards at H, and, since 

 the rod is raised in passing towards i, the fly 

 also must again rise in following its course, and, 

 when the spring forward is communicated at i, 

 should be found at j, in the parallel E i H, and 

 the rod, i A, at right angles with j i N (i j being 

 the length of the line). Here again, but for 

 gravitation, the fly would proceed towards N ; 

 instead of which, it describes the parabola j K L 

 M, falling on the water at M. 



Secondly, after the spring at B is given, the 

 force being great enough to send the fly to E, 

 suppose, instead of moving the rod backwards 

 to H, it were retained at B ; as the line, from its 

 insufficient length, would stop it at F, while 

 much force is still operating in the fly, it would 

 rebound, and so slacken the line and prevent it 

 falling fully extended. Then, in attempting to 



