THE LAWS OF MOTION. 233 



true. It might be difficult to discuss this point 

 in general terms with any clearness ; but let us 

 take the only example which we know of a 

 motion absolutely uniform, in consequence of the 

 absence of any force to accelerate or retard it ; 

 this motion is the rotation of the earth on its 

 axis. 



1. It is scarcely possible that discussions on 

 such subjects should not have a repulsive and 

 scholastic aspect, and appear like disputes about 

 words rather than things. For mechanical writers 

 have exercised all their ingenuity so to circum- 

 scribe their notions and so to define their terms, 

 that these fundamental truths should be expressed 

 in the simplest manner : the consequence of 

 which has been, that they have been made to 

 assume the appearance rather of identical as- 

 sertions than of general facts of experience. But 

 in order to avoid this inconvenience, as far as 

 may be, we take the first law of motion as 

 exemplified in a particular case, the rotation 

 of the earth. Of all the motions with which we 

 are acquainted, this alone is invariable. Each 

 day, measured by the passages of the stars, is so 

 precisely of the same length, that, according to 

 Laplace's calculations, it is impossible that a 

 difference of one hundredth of a second of time 

 should have obtained between the length of the 

 day in the earliest ages and at the present time. 

 Now why is this? How is this very remarkable 

 uniformity preserved in this particular pheno- 



