310 THE PRINCIPLES OF SCIENCE. [CHAP. 



and Tait say : " Equal times are times during which the 

 earth turns through equal angles." 1 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 2,500 years was 

 incompatible with an ancient eclipse recorded by the 

 Chaldteans, and similar calculations were made by Laplace. 

 But it is now known that these calculations were some- 

 what in error, and that the dissipation of energy arising 

 out of the friction of tidal waves, and the radiation of the 

 heat into space, has slightly decreased the rapidity of the 

 earth's rotatory motion. The sidereal day is now longer by 

 one part in 2,700,000, than it was in 720 B.C. Even before 

 this discovery, it was known that invariability of rotation 

 depended upon the perfect maintenance of the earth's 

 internal heat, which is requisite in order that the earth's 

 dimensions shall be unaltered. Now the earth being 

 superior in temperature to empty space, must cool more or 

 less rapidly, so that it cannot furnish an absolute measure 

 of time. Similar objections could be raised to all other 

 rotating bodies within our cognisance. 



The moon's motion round the earth, and the earth's 

 motion round the sun, form the next best measure of 

 time. They are subject, indeed, to disturbance from other 

 planets, but it is believed that these perturbations must 

 in the course of time run through their rhythmical courses, 

 leaving the mean distances unaffected, and consequently, 

 by the third Law of Kepler, the periodic times unchanged. 

 But there is more reason than not to believe that the earth 

 encounters a slight resistance in passing through space, 

 like that which is so apparent in Encke's comet. There 

 may also be dissipation of energy in the electrical relations 

 of the earth to the sun, possibly identical with that which 

 is manifested in the retardation of comets. 2 It is probably 

 an untrue assumption then, that the earth's orbit remains 

 quite invariable. It is just possible that some other body 

 may be found in the course of time to furnish a better 



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

 8 Proceedings of the Manchester Philosophical Society, 28th Nov. 

 1871, voL xi. p. 33. 



