64 WELLS'S NATUr.AL PHILOSOPHY. 



Thus, if a body moves 10 miles in 2 hours, its velocity is found by di- 

 viding the space, 10, by the time, 2 ; the ansvs'er, 5, gives the velocity per 

 hour. 



How can we l^l. To RscGrtain the Space passed over by 

 space''"paFsed ^ moving bocly, multiply the velocity by the 



over by a body f i tyi p 

 in motion? time. 



Thus, if the velocity be 10 miles per hour, and the time 15 



hours, the space will be 10 multiplied by 15, or 150 miles. 



How is the 13^- To ascertain the Time employed by a 



by'l bmly 'la l>ody in motion, divide the space passed over 

 "ainlT? ""'"' ^y the velocity. 



Thus, if the space passed over be 150 miles, and the ve- 

 locity 10 miles per hour, the whole time empjloyed will be 150 divided by 

 10 — 15 hours. 



■What is Mo- 133. The Momentum of a body is its quan- 

 mentumf ^j^y ^f motioH. Momcntum expresses the 

 force with, which one body in motion would strike against 

 another. 



That a mass of matter moving in any manner exerts a cer- 

 IlluBtrations of ^^j^j f^j.^^ against any object with which it may come in eon- 

 tact, is a principle of Natural Philosophy which experience 

 teaches us most frequently and most readily. The child has hardly emerged 

 from the nurse's arms, before it becomes conscious of the force with which 

 it would strike the ground if it fell. We take advantage of momentum, or 

 the force of a moving body, in almost all mechanical operations. The mov- 

 ing mass of a hammer-head drives or forces in the nail, shapes the iron, breaks 

 the stone ; the force of a moving mass of water gives strength to a torrent, 

 and turns the wheel ; the force of a moving mass of air gives strength to the 

 wind, carries the ship over the ocean, forces round the arms of a wind-mill. 



Is motion im- 134. Whcu a body is caused to move, the 

 t'hr'V'tici''cs motion is not imparted simultaneously to 

 of a body at gyerv particlc of the body, but at first only to 



the same in- J l -J ' J 



Btant? the particles which are directly exposed to the 



influence of the force — for instance, of a blow. From 

 these particles, it spreads to the rest. 



A slight blow is sufficient to smash a whole pane of glass, 

 How can you x a 7 



illustrate this while a bullet from a gun will only make a small round hole 

 ''"^'^ in it, because, in the latter case, the particles of glass that re- 



ceive the blow are torn away from the remainder with such rapidity, that the 

 motion imparted to them has no time to spread further. A door standing open, 

 which would readily yield on its hinges to a gentle push, is not moved by a 

 caauon-ball passing through it. The ball, in passing through, overcomes the 



