February 1, 1894.] 



KNOWLEDGE 



37 



that the velocity of comets near to their periheHon dis- 

 tance is frequently large compared with the velocity of 

 the earth in its orbit, and that the head or nucleus of 

 comparatively small comets frequently greatly exceeds the 

 earth in size, and would consequently sweep through a 

 larger area. 



But it seems more probable that the evolution of gas 

 from a mass of- stones, held together by the feeble action 

 of their mutual gravity, may, as the mass is slowly heated, 

 be sufficiently energetic to displace the stones, and thus 

 bring fresh material to the surface to be acted on by the 

 direct radiation of the sun. 



If this is the case it would account for the irregularities 

 in the structure of small comets being more marked than 

 in large comets. For the larger the group of stones the 

 greater would be the mutual attraction of its various parts, 

 and the less would be the liability to its disturbance by 

 the evolution of gas at a pressure corresponding to a given 

 temperature ; and the larger the group of stones the 

 greater would be the density of the gaseous envelope sur- 

 rounding it, and the greater would be the resistance to 

 outrushes of gaseous matter from the nucleus. 



We have at present diligently to collect facts. The 

 branching structures shown in the photographs of Swift's 

 comet, as well as the more striking branching structures 

 shown in the photograph of Brooks' comet taken on the 

 21st of October, and the flattened heads of the small tree- 

 like structures which project and form the notches at the 

 eastern edge of the tail, present forms that remind one of 

 the forms of solar prominences — that is, forms which are 

 assumed by uprushing masses of gas passing through a 

 resisting medium — and the conclusion pointed at, is that 

 these similar forms have a similar origin, though spectro- 

 scopic and polariscopic observation of the light derived 

 from comets' tails leads to the conclusion that the matter 

 of which they are composed is not in the gaseous state. 



The bright line spectrum seen in comets is generally 

 only recognizable in the immediate neighbourhood of the 

 nucleus, and the light from the tail is fouud to be polarized 

 in a manner which indicates that its brightness is due to 

 the light of the sun dispersed by a cloud of fine dust, the 

 particles of which are on the average small compared with 

 the wave-length of light. It would seem, therefore, that 

 the gaseous matter evaporated from the nucleus is precipi- 

 tated and then driven away from the sun in a cloud-like 

 mass that retains the general outline or form which the 

 gaseous matter had acquired during its outrush through 

 the loosely-packed atmosphere surrounding the cometary 

 nucleus. 



Even large comets have exhibited irregularities in their 

 tails, notably the great comet of 18H2 (//), in which the 

 want of uniformity in the density of the cloudy matter of 

 the tail was noted by several observers. I append a 

 picture of this comet from a sketch by Mr. .lohn Brett, 

 which seems to lie supported by the photographs obtained 

 by Dr. Gill at the Cape of Good Hope with a portrait 

 lens of two and a half inches aperture and eleven inches 

 focal length. The mass projecting nearly at right angles 

 from the end of the tail was noticed by several observers, 

 while only a few noticed the faint veil of light shown 

 in the photograph extending in a direction presumably 

 towards the sun from the brighter parts of the comet near 

 the nucleus. 



Xottcr of Boofe. 



Horns and llonfs : or Chapters on Hoofed Animals. By 

 R. Lydekker. London: 1893; 8vo,pp. 411. Illustrated, 



Horace Cox. — Mr. Lydekker's articles on big game, which 

 have appeared during the last few years in the columns of 

 the Field and Lawl and ]Vater, have been very generally 

 appreciated and we are glad to welcome their re-issue in 

 volume form. In an ever -advancing science like zoology, 

 progress is continually being made in our knowledge of 

 animals, and we are glad to see that Mr. Lydekker has done 

 his best to bring his chapters well abreast of the present 

 time in regard to the numbers and characters of the species 

 in the various groups treated of, though there are many 

 groups left unnoticed. Thus the wapiti, and other horned 

 animals of North America, are left without notice, and we 

 look in vain for any description of such well-known 

 animals as the European chamois, the American prong- 

 horned antelope, the Rocky Mountain white "goat," and 

 the extinct Irish elk. 



It is obviously unreasonable to expect to find a complete 

 treatise on horned and hoofed animals published in a 

 series of articles intended for popular reading, but one 

 may safely assert that no book which has hitherto 

 appeared has contained such a complete list of horned 

 and hoofed animals in the groups and regions treated of. 

 The value of the work to naturalists would have been 

 still further increased if Mr. Lydekker had given the 

 various names or synonyms used by different authors in 

 describing the same species. In a second edition he 

 might, without detracting from the popular character of 

 the book, give such synonyms in the index, with the 

 authors who have used them, in chronological order, 

 after the plan adopted by Sir Victor Brooke in his 

 various papers on gazelles, sheep, and deer in the 

 Proceedini/s of the Zooloijical Soeiety. 



In their original form, some of the chapters were not 

 illustrated, but this deficiency has now been amended ; 

 while some of the figures which appeared in the columns 



^s;„,rsi^m 



The Musk Ox {Ovibof: moschaln.i). 



of the Field and Land and Water have been replaced 

 liy new ones, in every way worthy of Mr. Lydekker and 

 of the artists by whom they are executed. We notice, 

 however, that there are still some outline figures which 

 compare but poorly with the finished wood engravings ; 

 and if, as we hope it may, the book soon reaches a second 

 edition, we would recommend the publisher to supply 

 fresh ones. 



Treating of the musk ox, which, as ^Iv. Lydekker points 

 out, cannot be rightly classed as an ox, though its horns 

 are at first sight so bufl'alo-like that the animal is almost 

 always popularly regarded as a member of the bovine 

 group, Mr. Lydekker remarks that, though in size and 

 general appearance the musk ox recalls our smaller High- 

 land cattle, its short tail, which is completely concealed 

 among the long hair, and hairy muzzle, as well as the 



