THE LAWS OF MOTION. 235 



continue the same from one instant to another, 

 for there is nothing to change it. This appears 

 to be taking refuge in words. We may call the 

 velocity, that is the speed of a body, its motion ; 

 but we cannot, by giving it this name, make it a 

 tiling which has any a priori claim to perma- 

 nence, much less any self-evident constancy. 

 Why must the speed of a body, left to itself, con- 

 tinue the same, any more than its temperature. 

 Hot bodies grow cooler when left to themselves, 

 why should not quick bodies go slower when left 

 to themselves? Why must a body describe 

 1000 feet in the next second because it has de- 

 scribed 1000 feet in the last? Nothing but ex- 

 perience, under proper circumstances, can inform 

 us whether bodies, abstracting from external 

 agency, do move according to such a rule. We 

 find that they do so : we learn that all diminution 

 of their speed which ever takes place, can be 

 traced to external causes. Contrary to all that 

 men had guessed, motion appears to be of itself 

 endless and unwearied. In order to account for 

 the unalterable permanence of the length of our 

 day, all that is requisite is to show that there is 

 no let or hindrance in the way of the earth's 

 rotation ; no resisting medium or alteration of 

 size she " spinning sleeps" on her axle, as the 

 poet expresses it, and may go on sleeping with 

 the same regularity for ever, so far as the ex- 

 perimental properties of motion are concerned. 

 Such ib the necessary consequence of the first 



