FORMAL ASTRONOMY. 525 



of the water, if it point to the centre of the circle at first, does not con- 

 tinue to do so, but remains parallel to itself during the whole revo- 

 lution. Now there is no cause to make the water (and therefore the 

 straw) rotate on its axis ; and therefore it is not a clear or convenient 

 way of speaking, to say that the water in this case does revolve on its 

 axis. But if the water in this case do not revolve on its axis, a body 

 in the case of the Moon does revolve on its axis. 



The difficulty, as I have said in the text, is of the same nature as 

 that which the Copernicans at first found in the parallel motion of the 

 Earth's axis. In order to make the axis of the Earth's rotation remain 

 parallel to itself while the Earth revolves about the Sun, in a mechan- 

 ical representation, some machinery is needed in addition to the ma- 

 chinery which produces the revolution round the centre (the Sun) : 

 but the simplest way of regarding the parallel motion is, to conceive 

 that the axis has no motion except that which carries it round the 

 central Sun. And it was seen, when the science of Mechanics was 

 established, that no force was needed in nature to produce this paral- 

 lelism of the Earth's axis. It was therefore the only scientific course, 

 to conceive this parallelism as not being a rotation : and in like manner 

 we are to conceive the parallelism of a revolving body as not being a 

 rotation. 



M. FoucauUs Proofs of the EartKs Motion. 



It was hardly to be expected that we should discover, in our own 

 day, a new physical proof of the earth's motion, yet so it has been. 

 The experiments of M. Foucault have enabled us to see the Rotation 

 of the Earth on its axis, as taking place, we may say, before our eyes. 

 These experiments are, in fact, a result of what has been said in speak- 

 ing of the Moon's rotation : namely, That the mechanical causes of 

 motion operate with reference to absolute, not relative, space ; so that 

 where there is no cause operating to change a motion, it will retain its 

 direction in absolute space ; and may on that account seem to change, 

 if regarded relatively in a limited space. 



In M. Foucault's first experiment, the motion employed was that of 

 a pendulum. If a pendulum oscillate quite freely, there is no cause 

 acting to change the vertical plane of oscillation absolutely ; for the 

 forces which produce the oscillation are in the vertical plane. But if 

 the vertical plane remain the same absolutely, at a spot on the surface 

 of the revolving Earth, it will change relatively to the spectator. He 

 will see the pendulum oscillate in a vertical plane which gradually 



