MECHANICS 59 



reflexion equals the angle of incidence " a truth of the 

 greatest value to billiard players though they have to 

 allow for the friction and other conditions which prevent 

 this equality being attained on any billiard table with 

 absolute exactness. 



When a body is once in motion, force is not needed 

 to maintain the motion. When, however, there is any 

 change in the direction or speed of a moving body, then 

 we have evidence of the existence of force. This is only 

 another way of stating the first law of motion. 



Therefore any continued force must produce a con- 

 tinuous change, either in direction or velocity. A sudden 

 change of either kind is produced by impact, i.e., by an 

 instantaneous exertion of force. 



From what we saw with respect to quantity* of 

 motion, it may be approximately deduced that a charge of 

 gunpowder which would impart to a bullet of a certain 

 size a velocity which we may express by 100, would 

 impart to a bullet ten times that size only a velocity 

 of 10. 



Various interesting apparatuses have been invented 

 to illustrate these laws of motion, but for their 

 description and a vast mass of further information, the 

 reader must have recourse to distinct treatises on 

 dynamics. 



When a force acts continuously upon a body, the 

 effect is necessarily cumulative, and in that case its 

 velocity will be constantly quickened and accelerated. 

 If we let fall from our hands at the same time a 

 feather and a marble, the latter falls at once very 

 quickly to the ground, while the other falls very slowly 

 and with many oscillations. 



* See ante, p. 54. 



