38 



MECHANICS. 



Fig. 11. 



I 



of pincers. Within these are placed 

 a feather and a half dollar, and the 

 air is then thoroughly drawn from the 

 receiver by means of an air-pump. 

 The wire is turned, and the feather 

 and coin both drop at once, and strike 

 the bottom at the same moment. 



measuring the velocity. ^at- 



wood's machine. 



In consequence of the swiftness of 

 falling bodies, it is not easy to meas- 

 ure the exact distance through which 

 they fall in a given time. There is an 

 instrument, however, known as At- 

 woocVs, Machine^ which renders their 

 motion much slower, at the same time 

 Feather andcomfaiiing^^^'^ the rate of iucreasc iu velocity is 



ame in a vacuum, precisely the samc, and it 

 therefore admits of an exact measurement of 

 the descent. The principle of this machine 

 may be easily understood by Fig. 12, where 

 two weights, hung on a fine silk cord run- 

 ning over a wheel, exactly balance each oth- 

 er. If now a small additional weight be 

 placed on one of these, it will destroy the bal- 

 ance, and the weight will begin to move 

 downward. As this little weight has to impart mo- 

 mentum to both the other larger weights, they will 

 move as much slower than ordinary falling as the 

 smaller weight is less than they. On this principle 

 Atwood's Machine, represented by Fig. 13, is made. 



