o-C FORCE AND MOTION. 



dience to the muscular impulse only, its motion would be 

 in a straight line ; but the force of gravity is ever active, 

 and constantly turns it from that line, and forces it to 

 move in a graceful curve instead. 



74. Centrifugal Force. Although it is obviously 

 impossible to give any direct experimental proof of the first 



FIG. 7. 



law of motion, we see many illustrations of the tendency 

 of moving bodies to move in straight lines even when 

 forced to move in curved lines. A curved line may be 

 considered a series of infinitely small straight lines. A 

 body moving in a curve has, by virtue of its inertia, a 

 tendency to follow the prolongation of the small straight 

 line in which it chances to be moving. Such a prolonga- 

 tion becomes a tangent to the curve, to move in which a 

 body must fly further from the centre. This tendency 



