154 



KNOWLEDGE 



[July, 1901. 



as that which hems in this cuiious island in the great 

 celestial river of light. Lastly the southern heavens 

 generally seem much i-icher in stars just within the 

 ordinary range of vision than the northern, the barren 

 circumpolar region being almost surrounded by a broad 

 belt, especially rich in the regions of Eridanus and 

 Argo, in which sixth magnitude stars are to be found 

 in unexampled abundance. 



For Cape Town and the colonies of Victoria and 

 New Zealand, the Southern Cross is a circumpolar con- 

 stellation, always above the horizon, and at midnight 

 on March 28th it stands vertically above the south 

 pole, Canopus being then on the same level as the pole 

 and west of it, whilst Alpha Endani, Achernar, " the 

 end of the river," almost touches the horizon in the 

 south. At the present time of the year, the Cross at 

 midnight is at the same elevation as the south pole but 

 west of it, balancing Acheniar at the same distance 

 on the east; whilst Canopus is below the south horizon 

 for South Africa, but just visible for Tasmania and 

 South New Zealand. 



These great leader stars enable the general trend 

 of the southern constellations to be easily made out. 

 When the Cross is seen erect, the foot of the Cross is 

 marked by Alpha, the head by Gamma, and the Cross 



LYNX 



' • URSA *, 



'CAHELOPARDUS . . „^„^ 



URSA 

 • MINOR 



CANES 



^, .'i^gf DRACO , 



• .^^ ••^' • HERCULES • 

 • PEGASUS %^US •• : . • • 



-••-• ^%* LYRA •CORONA** 



•■ ■•..." ■ fevife .:• ,• •••serpens - 



• • • • -V •., •.* 



• • v,ywpfcuw5 ! 



\ajuarius 



mu'eus y^quiuAv.,* , 





the Southern Cross, we have Chameleon and Musca; 

 close to Beta Argus is Volans, the Flying Fish ; Pictor 

 lies by the side of Canopus; the Greater Magellanic 

 Cloud' borders on four constellations, Mensa for "Table 

 Mountain," Dorado, Reticulum and Hydrus; the 

 Lesser Cloud lies between Hydrus and Toucan. Five 

 constellations meet round Achernar; Eridanus a primi- 

 tive constellation ; Horologium and Phoenix ; and the 

 two just mentioned, Hydivis and Toucan. Passing over 

 to the Southern Cross, the Milky Way between that con- 

 stellation and the Scoi-pion sweeps over the feet of the 

 Centaur and six other constellations beside, — two of 

 them primitive. The three constellations immediately 

 below the Scorpion ai-e Lupus, Noiina and Ara, of which 

 only Norma, placed in the middle of the Milky Way, 

 is new. Circinus below Norma, and the Southern 

 Triangle below Ara are both new, as also is Apns, which 

 lies between them and Octaus. The Southern Triangle 

 is a neat little figure on the edge of the Milky Way, 

 its three principal stars making a diamond with Alpha 

 Centaui-i. Alpha, the brightest star, and the one furthest 

 from the Centaur, lies midway between Alpha Crucis 

 and Alpha Pavonis, a second magnitude stai-, the 

 brightest of Pavo, a constellation mvich brighter than 

 any of the new constellations we have yet noted, and 

 one covering a considerable portion of the zone of small 

 stars to which allusion has ah'eady been made. Indus, 

 a fainter constellation, lies between Pavo and Toucan, 

 and completes the band of small asterisms surrounding 

 Octans. 



The Midnight Sky for London, 1901. .hdy 1. 



beam by Delta and Beta, Beta being mucli the brighter 

 of the two. Moving from Delta through Beta, we come 

 at no gi-eat distance to the first magnitude star, Beta 

 Centauri, and Alpha Centaviri lies but a little further 

 on, almost on the same straight line. Nearly midway 

 between Canopus and Alpha Cnicis lies Beta Argus, 

 a star but little below the first magnitude. Not far 

 from Beta is the False Cross, four stai-s. in a trapezium, 

 not quite so bright and a little more widely spread 

 th-an those of the true Cross. Epsilon Argus forms 

 the foot, Kappa the head, and Delta and Iota the cross- 

 beam. 



The many obscure constellations which have been 

 formed in this region may now be pretty easily picked 

 up. 



Between Octans, which surrounds the south pole, and 



PROF. ADAMS' LECTURES ON THE LUNAR 

 THEORY.* 



By P. H. CowELL. 



Probably few courses of lectures have enjoyed as great 

 a reputation as those which Professor Sampson, after 

 including them among the collected papers of Professor 

 Adams, has now published in book form. The lectures 

 deal with the most interesting of all problems of api^lied 

 mathematics, and at the time of their delivery they 

 formed the only adequate attempt to present the subject 

 to a student in a form so that, while a comprehensive 

 view of the whole subject is achieved, numerical labour 

 is as far as possible avoided. As Professor Sampson 

 says, several treatises exist that are intended to form 

 the basis of tables in which completeness is the fii-st 

 object and manner of presentation the secondary. 

 During the period 1860-1889, when Professor Adams was 

 lecturing on the subject, there were in existence elemen- 

 tary theories, of which Professor Sampson truly says 

 that they leave off when the diflicidties of the subject 

 begin, that is to say, the various cases of slow conver- 

 gence have been exposed and not dealt with. To 

 present the same idea in slightly different language : 

 It is conceivable that a computer might repeat the 

 whole of the calculations of a lunar theorist, and verify 

 his numerical accuracy (or detect his errors, as the case 

 may be), and at the end of his work, which would 

 probably take quite ten years, he might not have a 

 clear geometrical conception of what he had been doing. 

 Again, one of the elementary treatises referred to might 

 awaken in a reader some faint glimmering of the 



♦Lectures on the Lunar Theory, by John Couch Adams, m. A., 

 F.K.S., late Lowndean Professor of Astronomy and Geometry in the 

 University of Cambridge. Edited by R. A. Sampson, m.a.. Professor 

 of Mathematics in the University of Durham. (Cambridge: .^.t the 

 University Press. 1900. London : C. J. Clay and Sons.) 



