ASTRONOMICAL PHENOMENA AND PROGRESS. 



47 



planet. Before entering the solar domain they 

 were doubtless members of a cometary system ; 

 passing near Neptune at the same time, and at 

 some distance from each other, their different 

 relative positions, with regard to the disturb- 

 ing body, may account for the slight differ- 

 ences in the elements of their orbits. The 

 question "at what epoch did they enter the 

 solar system," Professor Kirkwood answers 

 thus : 



The mean between the longitudes of the aphelia 

 of the two comets is 271 41'. Neptune had this lon- 

 gitude in 1775; the comet of 1812, in 1777 ; and that 

 of 1846, in 1809. Now, with the known period of 

 Neptune and the periods of the comets as determined 

 byEnckeand Pierce, we find (neglecting perturba- 

 tions) that 



Neptune was in longitude 271 41' in the year 694 

 B. o. ; the comet of 1812, longitude 271 41' in the year 

 696 B. c. ; the comet of 1846, IV., longitude 271 41' 

 in the year 696 B. c. 



It seems, therefore, that the three bodies were very 

 nearly together about 695 years before the Christian 

 era. It is consequently not improbable that the ellip- 

 tical form of the two cometary orbits dates from this 

 epoch. 



The Constitution of Comets. Professor 

 Tyndall advances a new theory of comets, 

 after a careful investigation of the phenomena 

 of those bodies. He regards the cometic tail 

 as not matter projected from the head, but 

 matter precipitated on the solar beams which 

 have traversed the head. He shows that such 

 precipitation may occur either with compara- 

 tive slowness along the beam, or with the ve- 

 locity with which the beam actually traverses 

 space. Thus the amazing rapidity noticed in 

 the development of the tail is accounted for. 

 As the comet sweeps round the perihelion, the 

 tail is not composed of the same matter, but 

 new matter is precipitated on the solar beams, 

 the part of the old tail which is not protected 

 by the head of the comet being dissipated by 

 the sun's calorific rays, and, tfye dissipation not 

 being necessarily instantaneous, the tail leans 

 toward that portion of space last quitted by 

 the comet. Occasional lateral streamers are 

 explained as possibly due to the temporary 

 mastery of the actinic rays in parts of the-coni- 

 etary atmosphere not screened by the nucleus. 

 The shrinking of the comet's head as it ap- 

 proaches the sun is due to the beating of the 

 heat-rays against the attenuated fringe of the 

 head which is thus dissipated. 



The Quarterly Journal of Science for Octo- 

 ber, alluding to Professor Tyndall's views, says 

 that it cannot at present admit his explanation 

 of lateral streamers, because it leaves us in as 

 much perplexity as we have ever been with 

 respect to that strange phenomenon. When a 

 tail is seen extending in a right line from the 

 head, but at an angle of 60 or so to the radial 

 line from the sun, more is required to account 

 for the peculiarity than the bare possibility 

 that along that line the actinic rays may 

 temporarily have obtained a mastery; and 

 the appearance of six distinct tails spreading 

 from the head in the shape of a fan is still 



more difficult to explain on the Tyndall theory. 

 The writer in the Quarterly Journal concedes, 

 however, that the apparent swinging round of 

 the comet's tail is undoubtedly explained by 

 the new theory. 



Professor Tyndall's theory is the subject of 

 two interesting communications to the Philo- 

 sophical Magazine from Mr. Ernest Carpmert 

 and Mr. "W. B. Gibbs, F. R. A. S. The former 

 remarks that the theory turns on an assump- 

 tion hardly compatible with the laws of mo- 

 tion, viz., that the tail of a comet is "matter 

 precipitated on the solar beams traversing the 

 cometary atmosphere." If so, the lineal dimen- 

 sions of a comet must exceed the length of its 

 tail that is, in some cases, sixty millions of 

 miles or more. As it seems incredible that 

 such a bulk of vapor can whirl round the sun 

 at the perihelion passage unbroken, the author 

 suggests a slight modification of the Tyndall 

 theory, to avoid this difficulty, and explain 

 equally well all the observed appearances. If 

 there be an extremely thin solar atmosphere, 

 extending considerably farther than the- earth's 

 orbit and if, when a comet approaches the 

 sun, and is therefore exposed to intense heat, 

 its volume becomes comparable with the vol- 

 ume of the sun the heat of the sun will be 

 shut off from that portion of the solar atmos- 

 phere in the shade of the comet (which, though 

 transparent to light, is opaque to heat), and 

 actinic clouds will be formed in the solar at- 

 mosphere, thus giving the appearance of a tail 

 to the comet ; and, if the shape of the comet 

 be irregular, there may be more than one tail. 

 After the comet makes its perihelion passage, it 

 recedes from the sun, and, at the same time, 

 gradually cools and contracts, and the tail 

 would therefore slowly diminish and fade away. 

 Mr. Gibbs does not think that the theory ex- 

 plains some of the cometary phenomena ob- 

 served by the telescope. Immediately behind 

 the nucleus, where, according to the theory, we 

 should expect a very luminous region, we com- 

 monly have a dark space. Also, the matter 

 which forms the tail frequently streams out from 

 the head toward the sun, as was the case with 

 Halley's comet in 1836. The author cites Pro- 

 fessor Bond's description of Donati's comet, 

 where he says that "the material, after being 

 thrown off from the nucleus, instead of being 

 at once driven into the tail, formed a dense 

 cloud of nebulosity, into which the luminous 

 matter continued for some time to stream. 

 This cloud extended itself on the sunward side, 

 remaining in its vicinity for several days. When 

 it had acquired a certain stage, the discharge 

 took place mainly from the corners or cusps on 

 either side in two streams, which, coalescing 

 with those issuing from other envelopes, formed 

 the two branches of the tail." Then, too, some 

 comets have several series of envelopes, which 

 rise up toward the sun. In Donati's comet 

 seven were detected, and in the great comet of 

 1861 no less than eleven. The force which 

 causes the ascent of these envelopes is intermit- 



