3G2 THE PRINCIPLES OF SCIENCE. 



body which seems to us to move uniformly is not doing 

 so, but is subject to fits and starts unknown to us, because 

 we have no absolute standard of time, then all other 

 bodies must be subject to exactly the same arbitrary fits 

 and starts, otherwise there would be a discrepancy be 

 tween them disclosing the irregularities. Just as in com 

 paring together a number of chronometers, we should 

 soon detect bad ones by their irregular going, as measured 

 by the others, so in nature we detect disturbed movement 

 by its discrepancy from that of other bodies, which we 

 believe to be undisturbed, and which agree very nearly 

 among themselves. But inasmuch as the measure of motion 



O 



involves time, and the measure of time involves motion, 

 there must be ultimately an assumption. We may define 

 equal times, as times during which a moving body under 

 the influence of no force describes equal spaces e, but all 

 we can say in its support is, that it leads us into no 

 known difficulties, and that to the best of our experience, 

 one freely moving body gives exactly the same results as 

 any other. 



When we inquire where the freely moving body is, no 

 satisfactory answer can be given. Practically the rotating 

 globe is sufficiently accurate, and Thomson and Tait say : 

 Equal times are times during which the earth turns 

 through equal angles 11 . No long time has passed since 

 astronomers thought it impossible to detect any inequality 

 in its movement. Poisson was supposed to have proved 

 that a change in the length of the sidereal day, amounting 

 to one ten-millionth part in 2500 years, was incompatible 

 with an ancient eclipse recorded by the Chaldeans, and 

 similar calculations were made by Laplace. But it is now 

 known that these calculations were somewhat in error, 



8 Rankine, Philosophical Magazine/ Feb. 1867, vol. xxxiii. p. 91. 

 b Treatise on Natural Philosophy, vol. i. p. 179. 



