30 MECHANICS, 



the machinery of a locomotive was placed upon springs, 

 it would endure the wear and tear of use four times as 

 long as without them. 



For this reason, a ton of stone, brick, or of sand is 

 more severe for a team than a ton of wool or hay, wliich 

 possesses considerable elasticity. 



ESTIMATING THE QUANTITY OF MOMENTUM. 



The quantity of momentum is estimated by the ve- 

 locity and weight of the body taken together. Thus 

 a ball of two pounds' weight moves with twice the force 

 of a one-pound ball, the speed being equal ; a ten-pound 

 ball with ten times the force, and so on. A body mov- 

 ing at the rate of two feet per second possesses twice 

 the momentum of another of equal size with a velocity 

 of only one foot per second. A musket ball, weighing 

 one ounce, flying with fifty times the speed of a cannon 

 ball, weighing fifty ounces, would strike any object 

 with equal force ; or, if they should meet each other 

 from opposite directions, the momentum of both would 

 be mutually destroyed, and they would drop to the 

 earth. 



Where the mass is very great, even if the motion is 

 slow, the momentum is enormous, A large ship float- 

 ing near a pier wall may approach it with so small a 

 velocity as to be scarcely perceptible, and yet the force 

 would be enough to crush a small boat. When great 

 weight and speed are combined, as in a rail-way loco- 

 motive, the force is almost irresistible. This circum- 

 stance often insures the safety of the passengers ; for 

 as nothing is capable of instantly overcoming so pow- 

 erful a momentum, when accidents occur the speed is 



