ox MOTION. \Q 



cessary to constitute motion; that in all cases when we are inquiring whether 

 or no any body or point is in motion, we must recur to some other point 

 which we can compare with it, and that if a single atom existed alone in the 

 universe, it could neither be said to be in motion nor at rest. This may seem 

 in some measure paradoxical, but it is the necessary consequence of our de- 

 finition, and the paradox is only owing to the difficulty of imagining the ex- 

 istence of a single atom, unsurrounded by innumerable points of a space which 

 we represent to ourselves as immoveable. 



It has been for want of a precise definition of the term motion, that many 

 authors have fallen into confusion with respect to absolute and relative mo- 

 tion. For the definition of motion, as the change of rectilinear distance be- 

 tween two points, appears to be the definition of what is commonly called re- 

 lative motion ; but, on a strict examination, we shall find, that what we usu- 

 ally call absolute motion is merely relative to some space, which we imagine 

 to be without motion, but which is so in imagination only. The space which 

 we call quiescent, is in general the earth's surface; yet we well know, from 

 astronomical considerations, that every point of the earth's surface is perpe- 

 tually in motion, and that in very various directions : nor are any material 

 objects accessible to our senses, which we can consider as absolutely motion- 

 less, or even as motionless with regard to each other; since the continual va- 

 riation of temperature to which all bodies are liable, and the minute agitations 

 arising from the motions of other bodies with which they are connected, will 

 always tend to produce some imperceptible change of their distances. 



When therefore we assert, that a body is absolutely at rest, we only mean to 

 compare it with some large space in which it is contained : for that there ex- 

 ists a body absolutely at rest, in as strict a sense as an absolutely straight line 

 may be conceived to exist, we cannot positively affinn; and if such a quies- 

 cent body did exist, we have no criterion by which it coidd be distinguished. 

 Supposing a ship to move at the rate of three miles in an hour, and a person 

 on board to walk or to be drawn towards the stern at the same rate, he would 

 be relatively in motion, with respect to the ship, yet we might very properly 

 consider him as absolutely at rest: but he would, on a more extended view, 

 be at rest only in relation to the earth's surface ; for he would still be revolv- 

 ing round the axis of the earth, and with the earth round the sun; and with 



