512 



NATURE 



\_Sept. 25, 1879 



ever, of very different kinds are provided with prehensile 

 tails. 



Amongst others may be mentioned tree-porcupines, 

 certain opossums, and a small ant-eater. All these 

 animals live on trees. But the mylodon and megatherium 

 — though forest animals living in all probability exclu- 

 sively on the foliage of trees, were far too bulky to climb 

 them, or to be supported by their branches. They appear 

 to have fed thus : raising themselves on their hind legs 

 and tail (as on a tripod — like the kangaroos) they 

 embraced trees with their powerful arms, and swaying 

 them to and fro, gradually prostrated them in order to 

 feed upon their leaves. It has been objected to this view 

 of their probable habits, that if they acted in this way 

 they must often get their heads broken. Well, strange to 

 say, the heads of some fossils have had their heads broken 

 and healed again, and their skull was specially constructed 

 so as to obviate to a considerable extent the danger of 

 fatal consequences ensuing from accidents of that kind. 



The tails of some beasts are, as I have said, exception- 

 ally naked. The tails of others, however, are exception- 

 ally hairy. Such is the case with the horse, which is 

 called "long-tailed" when the tail is adorned with a 

 clothing of very long hairs. 



{To be continued.) 



OUR ASTRONOMICAL COLUMN 

 Biela'S Comet in 1852.— In view of the probable 

 approach to the earth's orbit of the two heads of Biela's 

 comet in the present year, it is not without interest to 

 recall the circumstances under which these bodies were 

 last observed in the autumn of 1852. As soon as the 

 calculated place of what was assumed to be the principal 

 comet of 1846, according to Santini, was sufficiently 

 removed from the sun's place to afford a chance of dis- 

 covery, a search was commenced at several European 

 observatories, notably by Secchi, at the Observatory of 

 the Collegio Romano at Rome. The comet was not 

 found in its computed position, and the cause of this is 

 now known to have been the abandonment by Santini of his 

 old semi-axis major, founded originally upon Damoiseau's 

 calculation of the perturbations of mean motion between 

 the appearances in 1805-6 and 1826, and the observations 

 of those years and the substitution of a value deduced by 

 Plantamour from the observations in 1845-46; had the 

 original semi-axis been retained the comet would have 

 been readily found by means of Santini's computations. 

 Extending the limits of the search, therefore, Secchi 

 detected a faint comet on the morning of August 26, 

 1 85 2, some 6° from the calculated place, which Prof. Peters 

 of Altona immediately pointed out as probably one portion 

 of Biela's comet, from the rate and direction of its 

 motion, as, indeed, it proved to be. (In Metnorie delP 

 Osservaiorio del Collegio Romano, anni 1852-55, the dis- 

 covery is dated, by a misprint, August 16, civil reckoning, 

 the first observation was made on August 25, at i6h. 14m. 

 M.T.) This object was observed on several subsequent 

 mornings, and on September 16 Secchi found the other 

 head of the comet, following that previously observed 

 about two minutes of time, and about half a degree to 

 the south. With the great refractor at Pulkowa, M. Otto 

 Struve found Secchi's comet of August 26, on September 

 18 (astronomical), or immediately after the notice reached 

 him, and two mornings later, he observed both heads. 

 Mr. James Breen, to whom Prof. Challis had intrusted 

 the Northumberland equatorial at Cambridge for a search 

 for the comet, found one portion of it on September 8, 

 and observed it further on September 16 and 21. At 

 Berlin one head was detected on September 17, and 

 reobserved on September 22. M. Otto Struve, in his 

 account of the Pulkowa observations, calls that head of 

 the comet which was first observed by Secchi on August 

 25, A, and that found on September 15, using now 

 astronomical dates, he calls B ; the latter was the north- 



preceding comet, the former the south-following one. A 

 discussion of the observations of both heads, twenty-two 

 in number, showed that those at Cambridge referred to A 

 on all three mornings, and those at Berlin to B ; both 

 nuclei were observed at Rome on September 19 and 20, 

 and at Pulkowa on September 20, 23, and 25. The 

 appearance of the two portions of the comet is best 

 described in M. Otto Struve's memoir, which is also 

 accompanied by two admirably executed drawings, 

 depicting their relative aspect on September 20 and 25, 

 B on September 18 was at least 30" in diameter, with 

 sensible brightening in the centre, but no decided nucleus, 

 and the light of the comet was about equal to that of a 

 star of Argelander's ninth magnitude. On September 20 

 A was easily seen with the finder of the large refractor, 

 both heads were of about equal brightness, B might be a 

 little the brighter, and exhibited a distinct nucleus ; the 

 nucleus of A was not so distinct as that of B, and there 

 was a greater brightness of the nebulosity, as well as an ex- 

 tension of it in the direction of B ; the apparent diameters 

 about i' and 40"; the diameter of B, which was circular, 

 was estimated 40". On September 23 A was notably 

 fainter than B, and without nucleus ; the lengthened 

 form of A was only seen with difficulty, but the sky was 

 not quite transparent. On September 25 there was a 

 remarkable change as compared with the relative appear- 

 ance of the two heads five days before ; A was materially 

 fainter than B ; the latter was very distinct in the finder, 

 while the place of the former was hardly suspected ; 

 diameter of A about 30", that of B from 50" to 60". A 

 was round, B slightly oblong ; the brightest part of A was 

 not in the centre of the nebulosity, but in the direction of 

 B, and the nucleus of B was in the opposite direction to 

 A, the brightest part of the nebulosity unequally distri- 

 buted about the nucleus of B being turned away from A ; 

 the position-angle of this direction was 286°. On Sep- 

 tember 28, the last day of observation, the moonlight was 

 strong, and B only was seen with difficulty. We give 

 these details, not remembering to have seen them repro- 

 duced in this country ; but the description fails to convey 

 the impression made by comparing M. Otto Struve's 

 drawings of September 20 and 25 ; were it not that we 

 know to the contrary, it might almost be inferred there- 

 from that one portion of the comet had revolved round 

 the other to the extent of 180°; their relative appearance 

 had been wholly interchanged, and it will be remembered 

 that about February 12, 1846, the secondary comet much 

 exceeded in brightness the primary one, though this con- 

 tinued only three or four days, when the latter resumed its 

 previous decided superiority. There was thus, as M. 

 Struve remarks, the same interchange of brightness 

 between the two nuclei at both appearances, and this he 

 is inclined to attribute to a mutual action. It may, how- 

 ever, be remarked that the distance between them in 1852 

 was, according to Hubbard, 00193, or about 1,750,000 

 miles, which seems to militate against such an explana- 

 tion, and rather to induce an idea of action inherent in 

 the separate comets, or of influence exercised upon them 

 through their approach to the sun. At M. Struve's ob- 

 servations of September 20, using Hubb rd's elements, 

 we find the distance of A from the earth was i "492, and 

 that of B, I "483 ; while on September 25, the distance of A 

 was I '525, and of B, l"5ii ; so that there was no marked 

 change of distance between the dates of his drawings. 



Variable Stars. — The foUowingare Greenwich mean 

 times of geocentric minima of Algol observable in this 

 country during the last three months of the present 



