OF UNDISTURBED MOTION. 6\ 



mathematical demonstration, "that the length of a straight 

 line be capable of being identified, whether by the effect 

 of any object on the senses, or merely in imagination, so 

 that it may remain invariable" (76) : although this postulate 

 has more generally been tacitly understood than expressed. 

 Scholium 4. When, therefore, we assert that a body 

 is absolutely at rest, we only mean to express its relation to 

 some comparatively large space in which it is contained : for 

 that there exists a body, or even a point, absolutely at rest, 

 in as strict a sense as an absolutely straight line may be 

 conceived to exist, we cannot positively affirm; and if such 

 a quiescent body or point did exist, we have no criterion 

 by which it could 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 abso- 

 lutely 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 revolving round the axis of the earth with that sur- 

 face, and with the whole earth round the sun: and with the 

 sun and the whole solar system he would perhaps be slowly 

 moving among the starry worlds which surround us. Now 

 with respect to any effects within the ship, all the subse- 

 quent relations to exterior objects are of no consequence 

 whatever, and the change of his rectihnear distance, from 

 the various parts of the ship, is all that needs to be consi- 

 dered in determining those effects. In the same manner, 

 if the ship appear, by comparison with the water only, to 

 be moving through it with the velocity of three miles an 

 hour, and the water be moving at the same time in a con- 

 trary direction at the same rate, in consequence of a tide 

 or current, the ship will be at rest with respect to the shore, 



