so THE POPULAR SCIENCE MONTHLY. 



fact, necessarily putting a limit to the accuracy of all prediction, until 

 some other unchanging and convenient measure of time shall be found 

 to replace the " day " and " second." 



The question at once presents itself. How can the constancy of the 

 day be tested ? The lunar motions furnish grounds of suspicion, but 

 nothing more ; since it is at least as likely that the mathematical 

 theory is minutely incorrect or incomplete as that the day is sensibly 

 variable. Up to the present time the most effective tests suggested 

 are fi'om the transits of Mercury and from the eclipses of Jupiter's 

 satellites. On the whole, the result of Professor Newcomb's elaborate 

 and exhaustive investigation of all the observed transits, together with 

 all the available eclipses and occupations of stars, tends rather to 

 establish the sensible constancy of the day, and to make it pretty cer- 

 tain (to use his own language) that "inequalities in the lunar motions, 

 " not accounted for by the theory of gravitation, really exist, and in 

 "such a way that the mean motion of the moon between 1800 and 1875 

 "was really less (i. e., slower) than between 1720 and 1800." Until 

 lately, the observations of Jupiter's satellites have not been made with 

 sufficient accuracy to be of any use in settling so delicate a question ; but 

 at present the observation of their eclipses is being carried on at Cam- 

 bridge, Massachusetts, and elsewhere, by methods that promise a great 

 increase of accuracy over anything preceding. Of course, no speedy 

 solution of the problem is possible through such observations, and 

 their result will not be so free from mathematical complications as 

 desirable — complications arising from the mutual action of the satel- 

 lites and the ellipsoidal form of the planet. On account of its free- 

 dom from all sensible disturbances, the remote and lonely satellite 

 of Neptune may possibly some time contribute useful data to the 

 problem. 



We have not time, and it lies outside my present scope, to discuss 

 whether, and, if so, how, it may be possible to find units of time 

 and length which shall be independent of the earth's conditions and 

 dimensions, free from all local considerations, cosmical, and as appli- 

 cable in the planetary system of the remotest star as in our own. 

 At present we can postpone its consideration ; but the time must un- 

 questionably come when the accuracy of scientific observation will be 

 so far increased that the irregularities of the earth's rotation, pro- 

 duced by the causes alluded to a few minutes ago, will protrude and 

 become intolerable. Then a new unit of time will have to be found 

 for scientific purposes, founded, perhaps, as has been already suggested 

 by many physicists, upon the vibrations or motion of light, or upon 

 some other physical action which pervades the universe. 



Another problem of terrestrial astronomy relates to the constancy 

 of the position of the earth's axis in the globe. Just as displacements 

 of matter upon the siirface or in the interior of the earth would pro- 

 duce changes in the time of rotation, so also would they cause corre- 



