THE EARTH. 453 



such a motion, being placed in the centre and at rest, we should see the sun 

 ( progressively moving round us ; we should project his disk among the stars, 

 5 and the apparent motion would be to us what it is. But it is most necessary 

 to reflect that the very same effect would be produced without a single change 

 of circumstance, if, instead of the earth being at rest and the sun moving round 

 it, the sun were at rest, and the earth were carried annually round it. Such a 

 motion of the earth would cause the sun to be successively seen at all points 

 of the ecliptic at which it is seen throughout the year ; and, in short, would 

 give to the sun exactly the same apparent motion which it appears to us to 

 have. It is therefore evident that the annual motion of the sun will be ex r 

 plained with equal clearness, and would be equally produced or caused, either 

 by its own motion round the earth, or by the annual motion of the earth round 

 it, There is nothing in the appearance of the sun itself which would give a 

 preference or confer a greater probability on either of these suppositions rather 

 than the other. If we are to choose between them, we must therefore seek 

 the grounds of choice in some other circumstances. 



But the long-continued and deeply-rooted opinion that the sun and not the 

 earth moves, must have had some natural and intelligible grounds. These 

 grounds undoubtedly arose from impressions that if the earth moved, we should 

 in some way or other be sensible of its motion ; more especially if that motion 

 had the enormous velocity which must be imputed to the earth if it be granted 

 that it moves round the sun at all. 



But, on the other hand, it must be considered that we are conscious of mo- 

 tion through the senses only by observing the relative change of position of some 

 external sensible objects. We see the mutual distance and relative position 

 of two or more visible objects change, and we infer immediately that some one 

 or other of them must have moved. We can be rendered sensible of the mo- 

 tion of the room we occupy, or of the ground upon which we stand, only by 

 some derangement of the position of these relative to our own body. But 

 li we could conceive all the objects that surround us moving with perfect uni- 

 formity in a fixed direction, and that our own bodies should participate in the 

 motion, we should thea have no evidence by which we could ascertain the ex- 

 istence of that motion at all. This will be clear to every one by considering 

 the effect produced when we are in the cabin of a boat which is drawn uni- 

 formly on smooth water. If we cannot look at the banks of the river or canal, 

 we then shall be entirely unconscious that the boat is moving; but if we 

 are enabled to look out from a window from which we can see the banks, the 

 first impression will be that the banks are moving in the contrary direction to 

 the boat, and it is only by reason and reflection that this impression will be 

 corrected. If we are in the cabin of a steamboat from which we cannot look 

 abroad, the only motion of which we are conscious is the tremulous motion 

 produced by the working of the machinery, and we are only conscious of this 

 because it changes in a slight degree, and momentarily, the relative position 

 of the frame of the boat and our own bodies. But we are even then uncon- / 

 scious of the progressive motion of the boat. : * i 



It will, then, be easily conceived that the motion of the globe of the earth 

 through space being perfectly smooth and uniform, we can have no sensible 

 means of knowing it, except the same which -we possess in the case of a boat 

 moving smoothly along a river : that is, by looking abroad at some external 

 objects which do not participate in the motion imputed to the earth. Now, 

 when we do look abroad at such objects, we find that they appear to more 

 exactly as stationary objects would appear to move, seen from a moveable sta- 

 tion like the earth. It is plain, then, even if it be true that, the earth really 

 has the annual motion round the sun which is contended for, that we cannot 



