270 



NA TURE 



\Aug. 6, 1874 



I must call your earnest attention, for it is verified in 

 nature in the most striking way and upon the largest 

 scale. All molecules of the same density must naturally 

 group themselves together in the vicinity of the curvi- 

 linear axis of the tail iifi w" vfi. . . . and thus form the 

 open plume to which we have referred ; but if the comet 

 emit molecules of very unequal densities, on which the 

 repulsive force acts with different energies, there ought to 

 be several distinct tails, more or less curved, all situated 

 behind the radius vector. This is precisely the case with 

 Donati's comet. Fig. 1 1 proves the truth of this ; it 

 shows the comet with three distinct tails. The two 

 smaller tails were almost straight, but ahvays in rear 

 of the radius vector ; they presented their less marked 

 conve.xity in the same direction as the bright tail. 



The great comet of 1861 had also two tails. When we 

 saw it lor the first time, on June 30, it appeared to have 

 only one, 11 8° long and perfectly straight, except a singu- 

 lar irregularity for which we could not at first account 

 (see Fig. 12). But soon the two tails separated, and it be- 

 came evident that we had been deceived by a simple play 

 of perspective. The earth, in fact, on June 30 was in the 

 plane of the orbit of this comet, and as the curvilinear 

 axes of the tails are always situated in this plane, they 

 were united, from our point of view, into one and the 

 same straight line, or at least into one and the same arc of 

 the great circle of the celestial vault. The sketch of the 

 same comet (Fig. 13) seen a fortnight previously by ob- 

 servers in the southern hemisphere, shows clearly the dis- 

 position of this double tail, the most curved half of which 

 almost touched the eanh with its extremity. 



iH^ 



These singular elTects of the repulsive force are easily 

 explained by a comparison which will appear at first to 

 be far removed from our subject, but the fundamental 

 analogy of which is palpable : 1 refer to the winnowing 

 of corn. In fact, wc cannot better compare the entire 

 surface-action of the repulsive force than to that of a puff 

 of air which repels light bodies and has no sensible action 

 upon denser bodies. When we wish to separate the grain 

 from the chaff by means of the winnowing fan, we allow 

 both to fall gradually into a current of air ; the grain 

 escapes from its action and falls at the feet of the win- 

 nower, while the chaff, much lighter, is carried to a dis- 

 tance, and forms upon the ground a separate heap (see 

 Fig. 14). If a third material, still lighter than the chaff, 

 is found mixed with the grains placed upon the fan, 

 it will be drawn away still farther, and will form 

 a third heap beyond the second. Evidently the fall 

 into space, under the sole inlluence of terrestrial attrac- 

 tion, would not operate with such discrimination, for all 

 matters placed upon the fan would fall at the same rate 

 and along the same curve, whatever might be their 

 density. 



Well, the repulsive force of the sun — a surface- action, 

 and not one of mass, like attraction — winnows, so to 

 speak, the materials which are separated from the come- 

 tary nucleus by being rarefied ; it picks them out and 

 distributes them, according to their density, into tails of 

 different curvatures. The lightest form the straightest tails, 

 and those nearest to the prolonged radius vector, while 

 the nucleus, escaping the repulsive action on account of 



its relatively enormous density, continues to obey, almost 

 rigorously, the Keplerian laws of attraction. 



We need not believe that the phenomenon of muliple 

 tails is rare ; without speaking of the horrible dragon de- 

 picted in Fig. 2, many comets have had several tails. 

 The facility with which the almost straight but very feebly 

 luminous tails of Donati's comet escaped observers in 

 France, leads us to believe that the phenomenon may be 

 general, and that by careful inspection several tails may 

 almost always be found to each comet. And according 

 to theory, a perfect homogeneity of materials, the neces- 

 sary condition for a single tail, must be, for any celestial 

 body, rather the exception than the rule. 



But then, it may be said, if very dense matters are 

 drawn away by nucleal emission on the side next the sun, 

 ought these materials escaping the repulsive action not to 

 take the lead of the nucleus and form a sort of tail on the 

 side next to the sun ? Yes, without doubt ; and this case 

 is effectually fulfilled, for some rare comets have presented 

 it, such as those of 1S23, 1845, and 1851. I would not 

 insist upon these exceptional but not abnormal tails, situ- 

 ated on the side nearest to the sun, almost lying upon the 

 orbit, or at least forming an obtuse angle with the initial 

 direction of the ordinary tails. 



( To be continued.) 



DR. BHAU DAJEE 

 'T^HIS very remarkable native of India, the true friend 

 -'- of his fellow-countrymen as well as of science and 

 learning, died on May 31 at the comparatively early age 

 of 51 years. As many of our readers may be ignorant of 

 the claims of Dr. Bhau Dajee to notice, we give a brief 

 sketch of his career, for which we are indebted to the 

 Times of India. 



He was born in 1823 in the village of Manjeren, near 

 Sawunt Warree. His parents were in poor circumstances, 

 and when he was about seven years of age they came to 

 Bombay, bringing him with them. He was first placed 

 in the native Education Society's Schools in Bombay, and 

 afterwards went to the Elphinstone College. There he 

 took a foremost place amongst the scholars, and was 

 noted for his ability and unremitting application to 

 his studies. The highest scholarships were taken by him, 

 and he was specially rewarded with a gold medal. When 

 his studies were concluded he was appointed assistant 

 professor of chemistry and natural philosophy at the col- 

 lege. About this time (1S42) a prize of 600 rupees was 

 offered by Government for the best essay in English and 

 Guzerathi on Female Infanticide. This prize Bhau 

 Dajee gained, and the essay, which has since been 

 published, has always been looked upon as one of the 

 best contributions on that subject. He commenced his 

 studies at the Grant Medical College, under Dr. More- 

 head, in 1845. The college had only then been esta- 

 blished for a short time. His success here was again 

 most marked, and gained for him the lasting friendship 

 of many distinguished members of the medical profession. 

 He received his diploma in 1S51. He soon created a 

 name for himself as a clever and rising medical prac- 

 titioner, and quickly found himself in possession of an 

 extensive practice amongst all classes. His time was 

 divided between his medical duties and his historical and 

 philological researches. From the first he took a great 

 interest in all public questions, especially those, which 

 affected the interests of his fellow-countrymen. He, with 

 Dr. Birdwood, was instrumental in the establishment of the 

 Gardens and Victoria and Albert Museum, Bombay. The 

 Bombay Association too may be said to owe its existence 

 to his energy ; he was the first secretary, and always took 

 a deep interest in the discussions of the society on Indian 

 affairs and measures. A considerable portion of his in- 

 come was expended in procuring rare and valuable MSS. 

 from Cashmere, Orissa, Benares, and Southern India 



