GALILEO AND HIS JUDGES. 147 



were shaped like a cylinder no change would be 

 detected ; but the spherical form of the Earth, as 

 it rotates on its axis, here makes the whole difference ; 

 the floor of the room where the pendulum vibrates is 

 carried round the axis of rotation, as everything else 

 is, but the plane of oscillation remaining the same 

 or parallel to the original one it no longer points 

 north and south. At the equator this phenomenon 

 would disappear, and in the southern hemisphere it 

 would be the other way : that is, the pendulum would 

 vibrate north-west and south-east. 



The same thing is exemplified by the small 

 machine called the gyroscope, where a heavy disc, 

 so adjusted as to revolve freely in any given 

 direction, independently of the frame in which it 

 is placed, will continue, when once set in rapid 

 motion, to spin in the same plane, directed, for 

 instance, to any one star that happens at the time 

 to be due north or due south of us, while the frame 

 moves round it with the rotation of the Earth. 



I think, then, on the whole, we may say that those 

 persons who, in the present state of our knowledge 

 on the subject, are not convinced that the Earth 

 revolves on its own axis, would not be satisfied by 

 any evidence whatever. 



Returning now to the general question of Coperni- 

 canism, we find that for some time after the trial of 

 Galileo, things remained much in statu quo; unless 

 we except the observation of the transit of Venus, in 

 1639 ; but, as that eventful seventeenth century 



