GRAVITATION. 37 



its velocity will be doubled ; nine times as far in three 

 seconds, while its velocity will he tripled, &o. 



If a stone is thrown upward, its motion continues 

 gradually to decrease, at the same rate as it increases 

 in falling; hence the same time is required to reach 

 its highest point, as to fall from that point back to the 

 earth. Therefore the velocity with which it is first 

 projected upward is equal to the velocity wliich it at- 

 tains at the moment of strildng the ground. There is 

 an exception, however, to this general rule. In a vac- 

 uum it would be perfectly correct, but in ordinary prac- 

 tice the resistance of the air tends to diminish the ve- 

 locity while as cending, and still further to retard it while 

 descending. For this reason, it wiU fall with less speed 

 than it first arose. For heavy bodies and small dis- 

 tances, this exception would be imperceptible ; but 

 with small bodies falling from great heights, the difier- 

 ence will be considerable. 



The velocity of a stone after falling one second, or 

 sixteen feet, is at the rate of thirty -two feet per second ; 

 hence, if thrown upward at that rate, it will rise just 

 sixteen feet high. After falling tlnree seconds, the rate 

 is ninety-six feet; and hence, if projected upward at 

 ninety-six feet per second, it will rise nine times sixteen 

 feet, or one hundred and forty-four feet high. And so 

 of other heights. 



Were it not for the resistance of the air, a feather 

 would fall as swiftly as a leaden baU. Tliis is conclu- 

 sively shown by an interesting experiment. A tall glass 

 jar [Fig-. 11), open at the bottom, is covered with a 

 brass cap, fitting it air-tight. Through this cap passes 

 an air-tight wire, which, by turning, opens a small pair 



