MECHANICS 



FIG. 13. 



n 

 S 



tude is measured by degrees, minutes, and seconds of 

 an arc which may be placed so as to measure it. 360 

 .degrees have been adopted as subdivisions of a circle, 

 each such "degree" being subdivided into 60 minutes 

 and each such minute into 60 seconds. 



The time occupied by a pendulum in one oscillation 

 constitutes the duration of a vibration. Were it not 

 for friction and the re- 

 sistance of the air, the 

 weight would ascend al- 

 ways to the same height 

 on either side as that 

 whence it first started, 

 and so would constitute 

 an instrument with per- 

 petual motion, since the 

 action of the force of 

 gravity is incessant. 

 Within certain limits, 

 the time occupied by a 

 vibration is not altered 

 by increasing its ampli- 

 tude, because the more 

 the weight be elevated, 

 the more the speed of 

 its descent will be increased, and in exact proportion to 

 the degree of elevation. 



The vibration of pendulums being thus a simple and 

 direct effect of the force of gravity, they have been made 

 use of to measure variations in that force at different 

 places, to estimate the density of matter beneath the 

 surface of the ground, and even to determine the shape 

 of the earth. 



It has been ascertained that the time occupied by a 



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