468 



THE EARTH. 



But this is not the only test by which the figure of the earth has been ascer- 

 tained. If the earth were a true globe revolving on its axis in twenty-four hours, 

 the effect of its revolution would cause gravity to diminish on approaching the 

 equator, and increase on approaching the poles ; for the centrifugal force due 

 to the rotation increasing toward the equator would cause a greater diminution 

 of gravity there than toward the poles, where it lessens. Now, it is possible 

 to calculate the effect of such centrifugal force upon the earth if it had the 

 figure of a true globe. The effect of this diminution of gravity will be ascer- 

 tained with great exactness by observing the vibration of a pendulum in differ- 

 ent parts of the earth. It has been already explained that the motion of a 

 pendulum is produced by the gravity of the earth acting upon the ball of the 

 pendulous body, and that the greater the attraction of gravity, the more rapid 

 will b@ the vibration ; and vice versa. We carry, then, a pendulum alternately 

 toward the equator and toward the poles, and find invariably that its vibration 

 is slower when taken toward the equator, and more rapid when taken toward 

 the poles. But we find that this variation in its vibration does not correspond 

 to that which it ought to have if the earth were an exact globe. It is just the 

 variation which ought to take place if the earth were an oblate spheroid, of the 

 form already described. 



Thus we have two independent tests of the figure of the earth, which give 

 accordant results. 



