Chap. XLII.] FALLING BODIES. 503 



through the point of support. But there are two 

 positions in which this may occur : one in which the 

 centre of gravity is at its highest point, as, for instance, 

 when an egg is balanced on its long axis ; and the 

 other when the centre of gravity is at its lowest point, 

 as when the egg is lying on its side. In the former 

 case, the egg is in the position of unstable, and, in the 

 latter case, of stable, equilibrium. 



If a body be supported on a base, the vertical 

 from the centre of gravity must fall within the base 

 if the body is to be in equilibrium. 



The laws of falling bodies. It is owing to 

 the action of gravity that bodies fall to the earth. 

 We have already seen (page 488) that a force can be 

 estimated by the velocity it confers on a body of unit 

 mass in a unit of time. The intensity of the action of 

 gravity may then be calculated by the velocity which 

 a body, falling freely, will acquire at the end of one 

 second. The laws that prevail in falling bodies were 

 first investigated by Galileo, who experimented by 

 letting bodies fall from the leaning tower of Pisa. 

 He found that the action of gravity was independent 

 of the nature of the body. Balls of different sub- 

 stances fell with equal rapidity. This has been 

 proved since Galileo's time by observing that in a 

 vacuum a piece of clown will fall as fast as a piece of 

 metal. Bodies fall with different rapidities in air, 

 because of the resistance which air offers, and which, 

 of course, affects a large surface more than a small one. 

 The laws of falling bodies have been very accurately 

 determined by the well-known Attwood's machine, 

 and other instruments. When a body falls freely 

 in air its motion is not uniform, that is, it does not 

 pass through equal spaces in equal times. If a force 

 acted on a body for a certain time, and then suddenly 

 ceased acting, it would confer a certain velocity on 

 the body proportional to the time during which it 



