The Laws of Motion. 225 



be so much longer than the other. For if one 

 force singly would carry the body through the 

 space A E, in the same time that the other would 

 have carried it through the space A D, the joint 

 action of both will carry it in the same time 

 through the space A H F, which is the diagonal 

 of the oblique parallelogram A D E F. 



If both forces act upon the body in such a 

 manner, as to move it uniformly, the diagonal 

 described will be a straight line ; but if one of 

 the forces acts in such a manner as to make the 

 body move faster and faster, then the line de- 

 scribed will be a curve. And this is the case of 

 all bodies which are projected in rectilinear direc- 

 ^tions, and at the same time acted upon by the 

 power of gravity, which has a constant tendency 

 to accelerate their motions in the direction wherein 

 it acts. 



This last is an observation of great importance, 

 as it is the foundation of the beautiful system of 

 Newton concerning the planetary motions. The 

 force which impels these bodies forward in a rec- 

 tilinear direction, is called the projectile or the 

 centrifugal force, as driving them from the centre ; 

 and the force which draws it towards the centre, 

 or the power of gravity, is called the centripetal 

 force. Thus, if the body A (fig. 90) is projected 

 along the straight line A F H in open space, where 

 it meets with no resistance, and is not drawn aside 

 by any power, it will go on for ever with the 

 same velocity, and in the same direction. But 



L5 



