UNITS AND STANDARDS OF MEASUREMENT. 363 



and that the dissipation of energy arising out of the fric- 

 tion 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 certain that the invariable 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 far 

 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 cognizance. 



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 all kinds of disturb- 

 ance from other planets, but it is believed that these 

 must in the course of time run through their rhythmical 

 courses, and leave the mean distances unaffected, and con- 

 sequently, by the third Law of Kepler, the periodic times 

 unchanged. But there is more reason than not to believe 

 that the earth encounters a certain slight resistance in 

 passing through space, like that which is so apparent in 

 Encke's comet. There may also be a certain 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 *. It is probably an untrue 

 assumption then, that the earth's orbit remains quite 

 invariable, and if so our last hope of getting a really 

 uniform measure of time disappears, and we are reduced 

 to accepting such as are sufficient for all practical pur- 

 poses. 



1 ' Proceedings of the Manchester Philosophical Society,' 28th Nov. 

 1871, vol. xi. p. 33. 



