The Laws dfMotfai. 



diverted its course. When therefore we see a 

 body move in a curve of any kind whatever, we 

 conclude it must be acted upon by two powers 

 at least ; one putting it in motion, and another 

 drawing it away from the rectilinear course in 

 which it would otherwise have continued to move : 

 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 liberty, by slipping one end of the 

 sling cord : and will 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 shows that the natural tendency of 

 the pebble, when put into motion, is to continue 

 moving in a straight line, although by the force 

 that moves the sling it is made to revolve in a 

 circle. 



From this maxim it will evidently appear, that 

 when two forces act at once upon the same body, 

 in different directions, it will go in neither, but 

 in a course between both. If the billiard ball a 

 (fig. 86) is struck at once by the two cues b and 

 f, it will be impelled forward in the diagonal or 

 middle line, whereas b would have impelled it in 

 the line e, and c in the line d. 



