226 Experimental Philosophy. [Lecture 1 5. 



if, at the same moment the projectile force is 

 given it at A, the body S begins to attract it 

 with u force duly adjusted*, and perpendicular 

 to its motion at A, it will then be drawn from 

 the straight line AFH, and forced to revolve 

 about S in the circle ATW; in the same manner, 

 and by the same law, that a pebble is moved 

 round in a sling. And if, when the body is in 

 any part of its orbit (as suppose at K), a smaller 

 body, as L, within the sphere of attraction of 

 the body K, is projected in the right line LM, 

 with a force duly adjusted, and perpendicular 

 to the line of attraction LK; then the small 

 body L will revolve about the large body K in 

 the orbit NO, and accompany it in its whole 

 course round the yet larger body S. Here S 

 may represent the sun, K the earth, and L the 

 moon. But of this we shall treat more at large 

 in the lectures on astronomy. 



These principles will serve to explain many 

 facts which will come from time to time under your 

 observation. Thus if a leaden ball is dropt from 

 the mast-head of a ship, under swift sail, you 

 would suppose, before the ball would reach the 

 deck, the ship would be slid from under it, and 

 that it would fall behind the ship into the sea, 



* To make the projectile force a just balance to the 

 gravitating power, so as to keep the planet moving in a 

 circle, it must give such a velocity as the pl.met would 

 acquire by gravity, when it had fallen through half the 

 femidiameter of that circle. 



