March, 1903.] 



KNOWLEDGE. 



59 



into a formal rationale of celestial appearauces, but iu a 

 large and general way. He made no attempt to realise 

 the particularities of a process vaguely conceived of as one 

 of growth by absorption or assimilation. He aud Laplace 

 thought out their separate schemes quite irrespectively 

 one of the other. There is no evidence of their having met 

 or corresponded, nor does their mutvial influence appear to 

 have been appreciable. Yet Laplace needed, as the raw 

 material for his solar system, precisely the " shining fluid " 

 elaborated, one might say, by Herschel, partly through 

 the revelations of his telescopes, jmrtly as the outcome of 

 his reasonings concerning the chevelure of the star in 

 Taurus. Halley, it is true, had by a sagacious intuition 

 surmised the composition of nebul® out of a " lucid 

 medium." But the ineffectual phrase remained stranded 

 in the pages of the Philosophical Transactions, and has only 

 of late been set floating on the streatti of scientific litera- 

 ture. Down to the end of the eighteenth century, world- 

 building had been a purely speculative undertaking. It 

 lacked actuality ; it was concerned with operations thought 

 of as belonging exclusively to a past order of things, now 

 over and done with, and lying wholly outside the range of 

 experience. Through Herschel's synthesis, however, those 

 dimly apprehended operations were brought into view as 

 variously progressing even now in different parts of the 

 Cosmos, as incipient in some regions, advanced in others, 

 the rubbish of the workshop here half masking the rising 

 edifice, while elsewhere signs of decay and exhaustion 

 give legible presage of an appointed end. And this 

 stupendous vision of a forming universe has not vanished 

 on critical scrutiny. It is no dream-tissue ; it cannot 

 dissolve into airy nothingness ; it is based upon a firm 

 substratum of reality. The immeasurable purposes of 

 Creative Wisdom are still only in part fulfilled. It has 

 become the strange privilege of humanity to contemplate, 

 from its little shoal of time, the oceanic flow of their 

 development. Thus, in the swing of the ages, Laplace's 

 thought was caught up and vitalised. He himself was 

 scarcely sensible of their movement. He recognised very 

 imperfectly, if at all, his obligations to Herschel's nebulous 

 star. His means were inadequate ; his field of view 

 narrow ; his knowledge, though co-extensive with that of 

 his time, fell short of what his boundless task demanded. 

 In some respects his mode of procedure was faulty ; his 

 forecasts have been belied ; the behaviour imputed by him 

 to a nebula such as he devised is questionable, if not 

 impossible. But, with the instinct of genius, he hit off' the 

 "psychological moment" ; and, divining the genetic import 

 of harmonies of construction obvious to perception, but 

 arduous of interpretation, he laid down with masterly 

 simj)licity the ground-plan of a structure likely to main- 

 tain its substantial integrity despite innumerable additions 

 and modifications. 



THE PATH OF THE MOON.-II. 



]iy A. C. D. CUOM.MELIN. 



Wii now pass on to the second part of the subject— viz., 

 the more accurate study of the shape of the Moon's path 

 round the Earth. If we suppose her to have been so pro- 

 jected as to describe a circle round the Earth in the plane 

 iif the ecliptic, then the Sun's disturbing action would 

 distort this circle into an oval with its shorter a.\is in the 

 direction of the iSuu and its longer one at right angles to 

 this direction. The diameters of the oval are as (39 to 70, 

 which implies that the Moon is ou the average 3400 miles 

 nearer at syzygies (New or Full) than at quadrature. We 



can verify this by taking the Moon's Parallax at the 4 

 quarters for the " Nautical Almanac " for several years and 

 taking the average value of each. Thus for the nine years 

 1883 — 1891 the average parallaxes are as follow — 

 New Moon 57'-62, Full Moon 57'-65, First and 

 Last Quarter 56''81. 



This oval is known as the Variation curve, and in the 

 researches on the Moon's motion commenced by Prof. Hill, 

 and now being continued by Prof. B. W. Brown, it is 

 taken as the starting point on which all the other distur- 

 bances are su]>erposed. This method promises to give at 

 once greater facility and greater accuracy in the study of 

 the Moon's motion. 



It will be noticed that the New Moon is slightly further 

 away (about 120 miles) than the Full Moon. It is also 

 found that the time from Last Quarter to First Quarter 

 is (on the average of a large number of lunations) a 

 quarter of an hour longer than that from First Quarter to 

 Last Quarter. These are known as the Parallactic Inequali- 

 ties in the Moon's distance and longitude, since they 

 depend on the Sun's distance, and would disappear if this 

 were sensibly infinite compared with the Moon's. The 

 inequality in longitude can be determined observationally 

 by observations near the First and Last Quarters, extend- 

 ing over many years, and the Sun's distance can be 

 deduced from the result. Prof. Hansen, in this way, found 

 that Encke's value of the Sun's distance (95.^ millions of 



Fio 3.— The Variation Oval. 

 O = llouii Moon. • = Moon affected b_v variation. 



miles) was too great. Hansen, however, reduced it too 

 much, his value being about 91 ^ millions, instead of the 

 now accepted value of 9'S millions. The diagram (Fig. 3) 

 gives an idea of the shape of the Variation curve ; the 

 departure from circularity, as also the slight displacement 

 of the Earth from the centre of the oval, are very greatly 

 exaggerated. The white circles indicate the position of 

 the Mean Moon, while the black ones indicate that of the 

 Moon aftecti'd l)y the Variation. If we leave the Paral- 

 lactic Inequality "out of consideration we see that the effect 

 of the Variation on her direction vanishes at all 4 quarters, 

 aud roaches a maximum at the intermediate points or 

 octants, where it amounts to 39V. At the first aud fifth 

 octant the Moou is iu advance of the Mean Moon by this 

 amotmt, while at the third and seventh it is the same 

 amount behind it. 



The ancient astronomers trusted chiefly to eclipses for 



