28 



OF CENTRAL FORCES. 



LECT. .From the uniform projectile motion of bodies in 

 v^v^, straight lines, and the universal power of gravity or at- 

 The laws of traction, arises the curvilineal motion of all the heaven- 

 tary ^mo- 6 ty bodies. If the body A be projected along the straight 

 turns.' line A FH in open space, where it meets with no resist- 



ance, and is not drawn aside by any power, it will go 

 on for ever with the same velocity, and in the same di- 

 rection. But if, at the same moment, the projectile force 

 is given it at A, the body S begins to attract it with a 

 force duly adjusted, 19 and perpendicular to its motion at 

 A, it will then be drawn from the straight line AF H, 

 and forced to revolve about /S in the circle A T W ; 

 in the same manner, and by the same law, that a peb- 

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



Note 19. 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 llie planet would acquire by gravity when it had fallen through 

 half the eemidiameter of that circle. 



