194 



5 fENCE, 



[Vol. IV., No. 8& 



triangulation meets the American at Bering Strait, it 

 is not unlikely that the accuracy of lunar observa- 

 tions will be greatly increased. 



The present uncertainty as to the earth's dimen- 

 sions is not, however, a sensible embarrassment to 

 astronomers, except in dealing with the moon, espe- 

 cially in attempting to employ observations made at 

 remote and ocean-separated stations for the deter- 

 mination of her parallax. 



As to the form of the earth, it seems pretty evident 

 that before long it will be wise to give up further at- 

 tempts to determine exactly what spheroid or ellip- 

 soid most nearly corresponds to the actual figure of 

 the earth; since every new continental survey will 

 require a modification of the elements of this sphe- 

 roid in order to take account of the new data. It will 

 be better to assume some closely approximate sphe- 

 roid as a finality ; its elements to be forever retained 

 unchanged, while the deviations of the actual surface 

 from this ideal standard will be the subject of con- 

 tinued investigation and measurement. 



A more important and anxious question of the 

 modern astronomer is, Is the earth's rotation uni- 

 form, and, if not, in what way and to what extent 

 does it vary ? The importance, of course, lies in the 

 fact that this rotation furnishes our fundamental 

 measure and unit of time. 



Up to a comparatively recent date, there has not 

 been reason to suspect this unit of any variation suf- 

 ficient to be detected by human observation. It has 

 long been perceived, of course, that any changes in 

 the earth's form or dimensions must alter the length 

 of the day. The displacement of the surface or 

 strata by earthquakes or by more gradual elevation 

 and subsidence, the transportation of matter towards 

 or from the equator by rivers or ocean currents, the 

 accumulation or removal of ice in the polar regions 

 or on mountain-tops, — any such causes must neces- 

 sarily produce a real effect. So, also, must the fric- 

 tion of tides and trade-winds. But it has been 

 supposed that these effects were so minute, and to 

 such an extent mutually compensatory, as to be quite 

 beyond the reach of observation; nor is it yet certain 

 that they are not. All that can be said is, that it is 

 now beginning to be questionable whether they are, 

 or are not. 



The reason for suspecting perceptible variation in 

 the earth's revolution, lies mainly in certain unex- 

 plained irregularities in the apparent motions of the 

 moon. She alone, of all the heavenly bodies, changes 

 her place in the sky so rapidly, that minute inaccura- 

 cies of a second or two in the time of observation 

 would lead to sensible discrepancies in the observed 

 position ; an error of one second, in the time, corre- 

 sponding to about half a second in her place, — a 

 quantity minute, certainly, but perfectly observable. 

 No other heavenly body has an apparent movement 

 anywhere nearly as rapid, excepting only the inner 

 satellite of Mars ; and this body is so minute that its 

 accurate observation is impracticable, except with the 

 largest telescopes, and at the times when Mars is 

 unusually near the earth. 



Now, of late, the motions of the moon have been 



very carefully investigated, both theoretically and 

 observationally; and, in spite of every thing, there 

 remain discrepancies which defy explanation. We 

 are compelled to admit one of three things, — either 

 the lunar theory is in some degree mathematically 

 incomplete, and fails to represent accurately the 

 gravitational action of the earth and sun, and other 

 known heavenly bodies, upon her movements ; or some 

 unknown force other than the gravitational attrac- 

 tions of these bodies is operating in the case; or else,, 

 finally, the earth's rotational motion is more or less, 

 irregular, and so affects the time-reckoning, and con- 

 founds prediction. 



If the last is really the case, it is in some sense a 

 most discouraging fact, necessarily putting a limit to 

 the accuracy of all prediction, unless some other un- 

 changing 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 from the transits of Mercury and from 

 the eclipses of Jupiter's satellites. On the whole, the 

 result of Professor Newcomb's elaborate and exhaus- 

 tive investigation of all the observed transits, together 

 with all the available eclipses and occultations of 

 stars, tends rather to establish the sensible constancy 

 of the day, and to make it pretty certain (to use hi& 

 own language) that "inequalities in the lunar mo- 

 tions, 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 Cambridge, Mass., and elsewhere, by methods that 

 promise a great increase of accuracy over any thing 

 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 satellites, and the ellip- 

 soidal 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 a unit of time (and length) which 

 shall be independent of the earth's conditions and 

 dimensions (free from all local considerations), cos- 

 mical, and as applicable 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 irregu- 

 larities of the earth's rotation, produced by the causes- 



