124 



KNOWLEDGE 



[July 1, 1898. 



extinct, while their less specialized allies continue to 

 flourish. 



Finally, as regards tusks in general, it appears that while 

 in the land carnivores these are always canines, are present 

 in both jaws, and have closed roots, in the other orders of 

 mammals their development is apparently somewhat capri- 

 cious, while they are very frequently present in only one 

 jaw — almost invariably the upper — and continue to grow 

 permanently. Moreover, they are almost as frequently 

 incisors as they are canines ; so that apparently similar 

 tusks may be in nowise homologous with one another. 

 Never developed to any size in animals with large cranial 

 appendages in the form of antlers or horns, tusks are 

 frequently wanting in those lacking the latter. Primitively 

 their use was undoubtedly as weapons of attack and 

 defence, or to aid in procuring vegetable food ; but in 

 many cases they have subsequently undergone a frequently 

 sexual development beyond the needs of such purposes, 

 and are thus in this respect analogous to the antlers of 

 many stags. In other instances, however — and this in all 

 the groups in which they occur — they have undergone a 

 still further semi-monstrous development, rendering them if 

 not actually harmful, probably in some cases inconvenient 

 to their owners. Finally, the independent acquisition in 

 closely allied or widely separated groups of mammals of 

 tusks of very similar structure and appearance shows 

 how little reliance is to be placed on external characters as 

 indicative of relationship. 



THE STORY OF fi ERIDANI. 



By the Rev. T. D. Anderson,- D.Sc. 



AT a distance of 32° from the South Pole, and with 

 a right ascension of Ih. 34m., shines a Eridani, 

 the eighth in brightness, photometry assures us, 

 of all the stars on the sphere. Those who have 

 occasion to consult star maps and catalogues will 

 have noticed that to this star is given, besides its letter a, 

 the formidable - looking name of Achernar. Achernar 

 means in Arabic " the last in the river," and is a transla- 

 tion of the eschatos tou potumou of Ptolemy, such being 

 the descriptive appellation given by that astronomer to 

 a star catalogued by him as the 34th in Eridanus, and 

 estimated by him as of the first magnitude — one of fifteen 

 in all which he places in that rank. 



That a, Eridani is the fschatns tou potamnu of Ptolemy 

 has been almost invariably accepted by astronomers, for 

 except it, there is no first magnitude star anywhere near 

 the southern end of the great celestial river. And yet the 

 difficulties attending this identification are insuperable. 

 Ptolemy made his observations at Alexandria (lat. 31° N.) 

 in the first half of the second century of our era, at which 

 time a. Eridani, which has, be it remarked, almost no 

 proper motion, was only 22° distant from the South Pole, 

 so that Ptolemy, to see it even on the horizon, would have 

 had to ascend the Nile to the Second Cataract. A usual 

 explanation has been that given by F>ode in hi= edition of 

 Ptolemy's Catalogue (page 84) : " It may be asked how 

 Ptolemy knew anything of a Eridani, since this star in his 

 time never rose above the horizon at Alexandria, but even 

 when on the meridian remained nearly 10^ below the 

 horizon. He must have learned about it only from the 

 uncertain reports of travellers who had visited the southern 

 regions of the then known world, and have inserted it in 



• It may interest readers of KifOWi.EnGE to he reminded that \)r. 

 Anderson is the keen obserrer -n-ho first deteeted tlie Xova .^urigie as 

 a stranger in the heavens ou Fcbruarv 1st, 1H92, when it was only a 

 little above the fifth magnitude. He modestly announeed the ajjpear- 

 anee of the new star to Dr. Copeland on an anonymous postcard, 

 and the discovery was onh' debited ti> Dr. Anderson after subsequent 

 inquiries. — A. C. R.vxvard. 



his catalogue accordingly." He consequently rejects as 

 entirely erroneous the longitude and latitude of the 

 eschatos tou potd moil as given by Ptolemy. 



But unfortunately for this view, we have received from 

 antiquity a good deal more information about the Last in 

 the River than the bare mention of its longitude and 

 latitude in Ptolemy's Catalogue. It is one of the fifteen 

 first magnitude stars whose risings and settings are given 

 by Ptolemy in his Calendar, and in that work it is expressly 

 stated (98ii) that although it did not rise above the horizon 

 where the sun's diurnal arc at midsummer is 15} hours 

 (i.e., lat. 4.5° N.), it did rise where the sun's diurnal arc at 

 midsummer is only 15 hours (i.e., lat. 41° N.) Nay more, 

 Hipparchus, in his Commentary on Aratus, states that at 

 lat. 37° N. the Last in the River, or as he calls it — his 

 star nomenclature is much fuller and more sonorous than 

 that of Ptolemy — the brightest and preceding and southern- 

 most of all in the River, rose when 351° of the ecliptic 

 was culminating, and set when 03° of the ecliptic was cul- 

 minating (ed. Petavius, pages 241a and 244b) ; or in other 

 words, it was in his time 42^° distant from the South Pole. 



When we bear in mind these statements of Hipparchus 

 and Ptolemy, and remember also that a Eridani — the 

 so-called Achernar — was in the time of Hipparchus only 

 21° from the South Pole, the belief becomes irresistible 

 that we have here the case of a first magnitude star either 

 ceasing to shine altogether or becoming reduced to a much 

 lower magnitude. As for the latter alternative of a serious 

 reduction in splendour during the course of 2000 years, it 

 is certainly not unknown in the annals of astronomy. 



Thus ^ Leonis is given by Ptolemy, both in the Almagest 

 and in the Calendar, as of the first magnitude ; at the end 

 of last century it was still bright enough for Bode to 

 classify it as of the 1 — 2 magnitude ; but now it is only of 

 the second. Thus also a Sagittarii, given by Ptolemy in 

 his Calendar as of the second magnitude, and in the 

 Almagest as of the 2 — 3 magnitude, was in Sufi's time 

 (the tenth century) only of the 4 — 5 magnitude, and that 

 astronomer was so much astonished at the discrepancy 

 between his own estimate and that of Ptolemy that he 

 thought some error must have crept into the current 

 copies of the Almagest. For this opinion, however, although 

 it is adopted by Schjellerup in his notes on Sufi, there 

 appears to be absolutely no foundation. By the time of 

 Lacaille a Sagittarii had recovered to the 3 — 4 magnitude. 

 It is now usually estimated at 4-0. 



The fate of Ptolemy's Last in the River has been much 

 the same as that of f3 Leonis and a Sagittarii. The Last 

 in the River— the real Achernar — is undoubtedly Eridani, 

 a star estimated by Halley and Lacaille as of the third 

 magnitude, and still remaining of that magnitude. 



The great difficulty in the way of identifying 6 Eridani 

 with the cscluifos tou potumou is that the position which we 

 find in Ptolemy — and the same remark applies to the 

 position deducible from the remarks of Hipparchus — does 

 not answer to that of 0. The latitude which Ptolemy 

 gives is almost exactly that of 6, but the longitude is 

 nearly four degrees too great. Instead of 356^-°, Ptolemy 

 gives 0° 10'. How the longitude should be so much 

 in error would be very difficult to ascertain. The fact 

 may be noted, however, that Ptolemy is more than 

 once similarly wrong as regards stars that lie far south. 

 His longitude of /3 Centauri is wrong to the extent of 

 2.^°, while the error in the case of Fomalhaut is about the 

 same as in the case of the Last in the River, its latitude 

 being given 2 " too great — 23° S. instead of 21° S. 



But in the catalogue of Ulugh Beg (1437 a.d.) we find 

 the Last in the River laid down as a first magnitude 

 star, in a position answering to that of with almost 



