5°2 



NA TURE 



{Sept. 1 8, 1884 



easily evaded ; but. as the matter stands, it looks as if nothing 

 short of a complete geodetic triangulation of the whole ear'h 

 would ever answer the purpose, — a triangulation covering Asia 

 1, as well as Europe, and brought into America by 

 way of Siberia and Behring's Straits. 



It is indeed theoretically possible, and just conceivable, that 

 the problem may some day be reversed, and that the geodesist 

 may come to owe some of his most important data to the 

 observers of the lunar motions. When the relative position of 

 two or more remote observatories shall have been precisely 

 determined by triangulation (for instance, Greenwich, Madias, 

 and the Cape of Good Hope), and when, by improved methods, 

 and observations made at these fundamental stations, the moon's 

 position and motion relative to them shall have been determined 

 with an accuracy mich exceeding anything now attainable, then 

 by similar observations, made simultaneously at any station in 

 this hemisphere, it will be theoretically possible to determine 

 in oflhis station, and so, by wayof the moon, to bridge 

 the ocean, and ascertain how other stations are related to those 

 which were taken as primary. I do not, of course, mean to 

 imply that, in the p< esent slate of observational astronomy, any 

 such p ocedure would lead to results of much value ; but, before 

 the Asiatic triangulation meets the American at Behring's 

 Straits, it is not unlikely that the accuracy of lunar observations 

 will be greatly increased. 



The present uncertainty as to the earth's dimensions is not, 

 however, a sensible embarrassment to astronomers, except in 

 dealing with the moon, especially in attempting to employ 

 observations made at remote ami ocean-separated stations for 

 the determination of her parallax. 



As to the form of die earth, it seems pretty evident that before 

 long it will be wise to give up further attempts to determine 

 exactly what spheroid or ellipsoid most nearly correspond* to the 

 actual figure of the earth, since every new continental survey 

 will require a modification of the elements of this spheroid in 

 order to take account of the new data. It will be better to 

 assume some closely approximate spheroid as a finality ; its 

 elements to be for ever retained unchanged, while the deviations 

 of the actual surface from this ideal standard will be the subject 

 of continued investigation and measurement. 



A more important and anxious question of the modern 

 astronomer is, whether is the earth's rotation uniform, 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 sufficient to be delected by 

 human observation. It has long been perceived, of course, that 

 any changes in the earth's form or dimensi ins n list 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 mi mountain-tops, — any such causes must 

 necessarily produce a real effect. So, also, must the friction of 

 tides and trade-winds. But it has been supposed that these 

 effects were so minute, and to such an extent mutually compen- 

 satory, 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. 



Hie reason for suspecting perceptible variation in the earths 

 revolution, lies mainly in certain unexplained 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 

 inaccuracies ..fa sei id or two in the time of observation would 

 lead to sensible discrepancies in the observed position ; an error of 

 one second, in the time, corresponding to about half a second in 

 her place, — a quantity minute, certainly, but perfectly observa I :. 

 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 imprac- 

 ticable, except with the largest telescopes, and at the times when 

 Mars 1, 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 everything, there remain discrepancies which 

 defy explanation. We are compelled (.. admit one of three 

 things, — either the lunar theory is in some degree mathematically 

 incomplete, and fads 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 attractions 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 confounds 

 predict! m. 



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 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 

 from the transits of Mercury and from the eclipses of Jupiter's 

 satellites. On the whole, the result of Prof. 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 certain (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 1S00 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 ; bat at present the observation of their 

 ecli :s 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 sp.edy solution of the problem 

 is possible through such observations, and their result will not 

 be so free from mathematical complications as desirable, — com- 

 plications arising from the mutual action of the satellites, and 

 the ellipsoidal form of the planet. On account of its freedom 

 from ill 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 cope, 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 condii ions and dimensions (free from all local considera- 

 tions), cosmical, 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 unquestionably come when 

 the accuracy of scientific observation will be so far increased 

 that the irregularities of the earth's rotation, produced by the 

 cause-, 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 

 . ted i.y 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 con- 

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

 displacements of matter upon the surface or in the interior of the 

 earth would produce changes in the time of rotation, so also 

 would they cause corresponding alterations in the position of the 

 axis and in the places of the poles, — changes certainly very 

 minute. The only question is, whether they are so minute as to 

 defy detection. It is easy to see that any such displacements of 

 the earth's axis will be indicated by changes in the latitudes of 

 our observatories. If, for instance, the Pole were moved a 

 hundred feet from its present position, towards the continent of 

 Europe, the latitudes of European observatories would be in- 

 creased about one second, while in Asia and America the effects 

 would be trifling. 



The only observational evidence of such movements of the 

 Pole, which thus far amounts to anything, is found in the results 

 obtained by Nyren in reducing the determinations of the latitude 

 of Pulkowa, made with the great vertical circle, during the last 

 twenty-five years. They seem to show a slow, steady diminution 

 of the latitude of this Observatory, amounting to about a second 

 in a century 1 as if the North Pole were drifting away, and increas- 

 ing its distance from Pulkowa at the rate of about one foot a 

 year. 



The Greenwich and Paris observations do not show any such 

 result ; but they are not conclusive, on account of the difference 

 of longitude, to say nothing of their inferior precision. The 

 [uestion is certainly a doubtful one; but it is considered of so 



