THE PENDULUM. 9 



the time of each swing must be shorter or longer, 

 as the power of gravity increases or decreases. 

 The beats of a given pendulum, whose length 

 remains unchanged, mark, as you know, equal 

 lengths of time, which may be measured with sur- 

 prising accuracy. The slightest changes, there- 

 fore, of the power which causes them, may be 

 discovered by the alteration of the time of the swing. 



Let then such an instrument — for instance, 

 one that beats seconds, and which is so made as to 

 set right the changes wrought in it by the rise and 

 fall of temperature (called a compensating pendulum) 

 — be put up at any place. It will, whenever and 

 however often the time of its swing may be ascer- 

 tained by a chronometer, always beat seconds. 

 From this constancy of the swing-time at any 

 given place, we must conclude that the force which 

 causes these motions is equally unchangeable. 



However, every change of place alters the swing- 

 time of a given pendulum,, Placed on a high 

 mountain its beats are perceptibly slower, and they 

 are quicker the nearer it is taken to the poles. The 

 increase, therefore, of the swing-time on rising to a 

 height, and its decrease by removal, on the level, 

 from the equator to the poles, give us a sure proof 

 of the decrease of gravitation as the distance from 

 the centre of the earth increases, as well as of the 

 flattening of the earth at the poles. 



By means of the swing-time of the pendulum, 



