68 WELLS'S NATURAL PHILOSOrHY. 



or metal, against which the blows of the hammer will be directed- liy 

 adopting this plan, however, no increased resistance is opposed to the blows 

 of the hammer, the momentum, or moving force of which is equally imparted 

 iu both cases ; but in the first case, the momentum is received bj^ the board 

 alone, which, having little weight, is driven by it through so great a space 

 as to produce considerable flexure, or even fracture ; but iu the second case, 

 the same momentum being shared between the board and the block behind it, 

 will produce a flexure of the board as much less as the weight of the board 

 and block applied to it together, is greater than the weight of the board alone. 

 The same principle serves to explain a trick sometimes exhibited in 

 feats of strength, where a man in a horizontal position, his legs and 

 shoulders being supported, sustains a heavy anvil upon his chest, which 

 is then struck by sledge-hammers. The reason the exhibitor sustains no 

 injury from the blows, is that the momentum of the sledge is distributed 

 equally through the great mass of the anvil, and gives to the anvil a down- 

 ward motion, just as much less than the motion of the sledge, as the mass of 

 the sledge is less than the mass of the anvil. Thus, if the weight of the an- 

 vil be 100 times greater than the weight of the sledge, its downward motion 

 upon the body of the exhibitor will be 100 times less than the motion with 

 which the sledge strikes it, and the body of the exhibitor easily yielding to 

 BO slight a movement, and also resisting it by means of the elasticity of the 

 body, derived from its peculiar position, escapes without injury. 



When is the ^^l. When two bodics come in contact, the 

 bodic8°"a[d'to collision is said to be direct, when a right line 

 be direct? passing tlirough their centers of gravity jDasses 



also through the point of contact. 



The center of gravity in such cases corresponds with the center of col- 

 lision ; and if such a center come against an obstaclCj the whole momentum 

 of the body acts there, and is destroyed ; but if any other part hit, the body 

 only loses a portion of its momentum, and revolves round the obstacle as a 

 pivot, or center of motion. 



When two in- 142. When two non-elastic bodics, moving 

 come'into''ciri! in opposltc dircctions, come into direct collision, 

 fcion,whatoc- ^^qj ^.jjj ^q^^]^ jQgg ^n cqual amount of mo- 



( mentum. 



Hence, the momentum of both after contact, will be equal to the difference 

 of the momenta of the two before contact, and the velocity after contact will 

 be equal to the diflerence of the momenta divided by the whole quantity of 

 matter. Let the quantity of matter in A be 2, and its velocity 12 ; its mo- 

 mentum is, therefore, 24. Let the quantity of matter in B be 4, its velocity 

 3; its momentum will be 12. The momentum of the mass after contact, on 

 the supposition they move in opposite directions and come in direct col- 

 lision, will be the difl"erence of the two momenta, or 12 ; and the velocity of 



