OF CENTRAL FORCES. 25 



Therefore, when we see a body move in a curve of any LECT. 

 kind whatever, we conclude it must be acted upon by ^^-^^ 

 two powers at least ; one putting it in motion, and ano- 

 ther drawing it off from the rectilineal course it would 

 otherwise have continued to move in : and whenever 

 that power which bent the motion of the body from a 

 straight line into a curve, ceases to act, the body will 

 again move on in a straight line touching that point of 

 the curve in which it was when the action of that power 

 ceased. For example, a pebble moved round in a sling 

 ever so long a time, will fly off the moment it is set at The effect* 

 liberty, by slipping one end of the sling cord : and will ^ force* 11 

 go on in a line touching the circle it described before : 

 which line would actually be a straight one, if the earth's 

 attraction did not affect the pebble, and bring it down 

 to the ground. This shews that the natural tendency 

 of the pebble, when put into motion, is to continue mov- 

 ing in a straight line, although by the force that moves 

 the sling it be made to revolve in a circle. 



The change of motion produced is in proportion to 

 the force impressed : for the effects of natural causes 

 are always proportionate to the force or power of those 

 causes. 



By these laws it is easy to prove that a body will de- 

 scribe the diagonal of a square or parallelogram, by two 

 forces conjoined, in the same time that it would describe 

 either of the sides, by one force singly. 



Thus, suppose the body A to represent D| 

 a ship at sea ; and that it is driven by 

 the wind, in the right line A B, with such 

 a force as would carry it uniformly from A 

 to B in a minute : then, suppose a stream ^^A. B 



or currrent of water running in the direction A D, with 

 be impelled. If a bullet be fired from the mouth of a cannon, its patk 

 will be a parabola, as the effect produced by the powder will contin- 

 ually diminish, while the tendency to descend, must of necessity rn- 

 crease with the diminished projectile force. 



