18 MECHANICS. 



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 harder 

 for a team tlian a ton of wool or hay, which possesses con- 

 siderable elasticity. 



ESTIMATING THE QUANTITY OP MOMENTUM. 



The quantity of momentum is estimated by the Telocity 

 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 moving 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 floating 

 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 locomotive, the force 

 is almost irresistible. This circumstance often insures the 

 safety of the passengers ; for as nothing is capable of 

 instantly overcoming so powerful a momentum, when 

 accidents occur the speed is more gradually slackened, 

 and the passengers are not pitched suddenly forward. A 

 light wagon, rapidly driven, possessing but little compara- 

 tive force, is more suddenly arrested, and the danger is 

 greater. 



W^hen two bodies meet from opposite directions, each 



