♦ KNOWLEDGE ♦ 



[Se. 



Hogarth pilloried liim in the cdobrited madhouse scene 

 of the"Rake'« Progress." Of course, Wliistonhad perceived 

 the essential feature of all methods intended for deter- 

 mining the lonp-itnde. Any si2:n;i.l which is recognisalle, 



no mutter l^y ( y,- ' r ,n-. (r in wIi;itsocver way, at both 

 statimiN, tlir iil'i i-eiiri .si;[ii..ii mill the station whose 

 iiitisi iirirsMuiiy suffice to convey 



the 1 



of o 



Til 



.rdif 



Whiston's scheme L.y in tlic impliLil 



any form of ordnance raiM I'lnjii nckct signals },a- 



enough to be scm er I,, aid in niid-.u-raii. .\l;alil\■^tly 



tion is imiHi--; . , ■ - i j n ,1. in tlic'celrNtial S[':u'us ; fur 

 these alone en: I • , ^e, m- <1 Mn.ultaneeuNly fnan widely 

 distant jiartM'fila eirih. It has been to .such signals, 

 then, that men uf science have turned for the required 

 means of determining longitude. 



Galileo was the first to point out that the satellites of 

 Jiqnter supply a series of signals which might serve to 

 determine the longitude. AVhen ene fif these bodies is 

 eclipsed in Jupiter's sliadnw, oi' passes oiit of sight be- 

 hind Jupitei-'s disc, or reappears fn.m eclipse or occulta- 

 tion, the phenomenon is one which can be seen from a 

 whole hemisphere of the earth's surface. It is as truly 

 a signal as the appearance or disappearance of a light in 

 ordinary night-signalling. If it can be calculated before- 

 hand that one of these events will take jilace at any 

 given hour of Greenwich time, then, freni whatever spot 

 the phenomenon is observed, it is Icnown there that the 

 Greenwich hour is that indicated. Theoretically this is 

 a solution of the fnneus jm-blem ; and Galileo, the dis- 

 coverer of Jnpit.rs feiir satellites, thought he had found 

 the means uf determining the longitude with great 

 accuracy. Unfortunately these hopes have not been 

 realised. At sea, indeed, except in the calmest weather, 

 it is impossible to observe the phenomenon of Jupiter's 

 satellites, simply becaiisethe telescope cannot be directed 

 .steadily upon the planet. But even on land Jupiter's 

 satellites afEord but imperfect means of guessing at the 

 longitude. For, at present, their motions have not been 

 thoroughly mastered by astronomers, and though the 

 " Nautical Almanac " gives the estimated epochs for the 

 various phenomena of the four satellites, yet, owing to 

 the imperfection of these tables, these epochs are often 

 found to be appreciably in error. There is yet another 

 difficulty. The satellites are not mere points, but being 

 in reality as large as or larger than oui' moon, they have 

 discs of appreciable though small dimensions. Accordingly 

 they do not vanish or reappear instantaneously, but 

 gradually, the process lasting in reality several seconds 

 (a longer or shorter time according to the particular 

 satellite considered), and the estimated moment of the 

 phenomenon thus comes to depend on the power of the 

 telescope employed, on the skill or the visual powers of 

 the observer, on the condition of the atmosphere, and 

 so on. Accordingly, very little reliance could be placed 

 on such observations as a means for determining the 

 longitude with any considerable degree of exactness. 



No other celestial phenomena present themselves 

 except thtS3 depending on the moon's motions.* All 



learn of him witliout contraclictiDg him when I differed in opinion 

 from him : he could not in his old age bear such contradiction, and 

 so he was afraid of me tlie last thirteen years of his life." 



* If but one star or a few would periodically (and quite regularly) 

 " go out " for a few moments, the intervals between such vanisbings 

 l>eing long enough to ensure that one would not be mistaken in 

 point of time for the nest or following one, then it wou i be pos- 

 sible to determine Greenwich or other reference time with great 

 i-^gg ^jj^ 2jgj.g ^yg cannot but recognise an argument against 



the planets, as well as the stm and moon, traverse at 

 various rates and in different paths the sphere of the 

 fixed stars. But the moon alone moves with sufficient 

 rapidity to act as a time-indicator for terrestrial voya- 

 gers. It is hardly necessary to explain why rapidity of 

 motion is important ; but the following illustration may 

 be given for the jmrpose. The hour-hand of a clock 

 does in reality indicate the minnie as uell as the Imur; 



-et . 



e.f i 







So with the more slowly - moving celestial bodies. 

 They would serve well enoiigh, at least some among 

 them would, to show the day of the year, if we 

 could only imagine that such information were eycr 

 required from celestial bodies. But it would be hope- 

 less to attempt to ascertain the true time with any 

 degree of accuracy from their motions. Now the 

 moon really moves with considerable rapidity among 

 the stars.* She completes the circuit of the celestial 

 sphere in 27^ days (a period less than the common luna- 

 tion), so that in one day she traverses about thirteen 

 degrees, — or her own diameter (which is rather more 

 than half a degree) in about an hour. This, astronomi- 



the singular tlieory that the stars were intended simply as lights 

 to adorn our heavens and to be of use to mankind. The teUologists 

 who have adopted this strange view can hardly show how the theory 

 is consistent with the fact that quite readily the stars (or a few 

 of them) might have been so contrived as to give man the means 

 of travelling with much more security over the length and breadth 

 of his domain than is at present possible. In this connection I 

 venture to quote a passage in which Sir John Herschel has touched 

 on the vse/uliicss of tlie" stars, in terms which were they not cor- 

 rected by other and better known passages in his writings, might 

 suggest that he had adopted the theory I have just mentioned: — 

 " The stars," he said, in an address to the Astronomical Society, 

 in 1827, "are landmarks of the universe; and amidst the endless 

 and complicated fluctuations of our sj-stem, seem placed by its 

 Creator as guides and records, not merely to elevate our minds 

 by the contemplation of what is vast, but to teach us to direct 

 our actions by reference to what is immutable in His works. It 

 is indeed hardly possible to over-appreciate their value in this 

 point of view. Every well-determined star, from the moment its 

 place is registered, becomes to the astronomer, the geographer, 

 the navigator, the surveyor, a point of departure which can never 

 deceive or fail him,— the same for ever and in .ill places, of a 

 delicacy so extreme as to be a test for every instrument yet in- 

 vented by man, yet equally adapted for the most ordinary purposes ; 

 as available for regulating a town clock as for conducting a navy 

 to the Indies ; as eftective for mapping down the intricacies of a 

 petty barony as for adjusting the boundaries of transatlantic 

 empires. When once its place has been thoroughly ascertained- 

 and carefully recorded, the brazen circle with which the useful 

 work was done may moulder, the marble pillar may totter OB its 

 base, and the astronomer himself sui-vive only in the gratitude of 

 posterity ; but the record remains, and transfuses all its own 

 exactness into every determination which takes it for a ground- 

 work, giving to inferior instruments, nay, even to temporary con- 

 trivances, and to the observations of a few weeks or days, all the 

 precision attained originally at the cost of so much time, labour, 

 and expense." It is only necessary as a corrective to the erroneous 

 ideas whieli u.i^lii etlienvise be suggested by this somewhat high- 

 tiowii 1 ,1 I !,!■ following remarks from the work which 



repre i : I rschel's more matured views, his well- 



kno\M iiomy." " For what purpose arc we to 



supije-' II bodies scattered through the abyss ot 



space. ....... .... . ..iuminate our nights, which an additional 



moon of llie tl'ieii-undih part of the size of our own world would 

 do much better : nor to sparkle as a pageant void of meaning and 

 reality, and bewilder us among vain conjectures. Useful, it is true, 

 they are to man as points of exact and permanent reference, but he 

 must have studied astronomy to Uttle purpose, who can suppose 

 man to be the only object of his Creator's care ; or who does not 

 see in the vast and wonderful apparatus around us, provision for 

 other races of animated beings." 



* It was this, doubtless, which led to the distinction recognisecJ 

 in the book of Job, where the moon is described as " nalhng in 



