Fig. 2. 



suddenly as to give it sudden motion. An unusual 

 exertion of the team is required in starting a loaded 

 wagon ; but when once on its way, it would require 

 the same effort of the horses to stop it as to back it 

 when at rest. 



The force of inertia is finely exhibited by means of a 

 little instrument called the iner- 

 tia apparatus {Fig. 2). A mar- 

 ble or small ball is placed on a 

 card (c) resting on a concave 

 stand. A spring snap is then 

 made to strike the card, tlirowing 

 it to a distance, but leaving the 

 ball upon the hollow end of the stand. The same ex- 

 periment may be easily performed by placing a very 



Inertia Apparatu 



on a common sand-box, or even the hollow of the hand. 

 A sudden snap with the finger will throw the card 

 away, while the apple will drop into the cavity. The 

 following experiment is still more striking: Procure 

 a thread just strong enough to bear three 

 pounds, and hang upon it a weight of 

 two pounds and a half Another half 

 pomid would break it. Now tie an- 

 other thread, strong enough to bear one 

 pound, to the lower hook of the weight. 

 If the lower thread bo pulled grad- 

 ually^ the upper thread will of course 

 break ; but if it be pulled with a jerk., 

 the loiver thread will break. If the 

 jerk be very sudden, the lower string 

 will break, even if it be considerably 



