iv The Earth a Spinning Ball 53 



least two different kinds of motion. The first convincing 

 proof of the Earth's motion was the discovery that a weight 

 dropped from the top of a high tower did not reach the 

 Earth's surface perpendicularly under the point from which 

 it was let go, but always a little to the east ( 93). 



87. Rotation of the Earth. The old difficulty that the 

 Earth could not be moving because we do not feel it, and 

 that the star-dome could not be fixed because we see it 

 move, no longer troubles people who are familiar with the 

 imperceptible motion of a well-started train, and the apparent 

 gliding away of the platform in the opposite direction. The 

 Earth spins uniformly and regularly from west to east, as 

 may be inferred from the uniform and regular apparent 

 rotation of the starry sky at night. The first Law of Motion 

 ( 50) enables us to understand how the rotation of the 

 Earth has been actually proved, and what the immediate 

 consequences of rotation are. The French physicist 

 Foucault showed how a large pendulum once set swinging 

 changed the plane of its swing slowly and regularly. If 

 started, for instance, swinging above a table from north to 

 south, at the end of twelve hours it would be found swinging 

 from east to west, and in twenty-four hours it would have 

 changed its plane still farther and be 



swinging from south to north again. 



Since the only force which could act on 



a moving pendulum hung from the solid 



roof of a building is the rotation of the 



Earth, this change in the direction of 



the pendulum proves it. The pendulum 



does not really change its direction of 



swinging in space ; it remains in a state 



of uniform motion, and the apparent FIG. 10. Direction of 



twisting is produced by the house and Kfe'and^^ctS'nS- 



the whole Earth turning while the pen- deviation of moving 



dulum marks out its invariable line. Sphere. S Uthem 



88. Polarity. A ball at rest has no 



ends or natural points from which to reckon position, but as 

 soon as the ball is made to spin two opposite points on its 

 surface become different from all others, although there may 



