1818.] Scientific Intelligence* 227 



Wednesdays, and Fridays. — The diseases of the eye, although 

 forming an integral part of the lectures on surgery, will, for the 

 convenience of illustration, be delivered every Thursday evening- 

 until completed. 



II. Remarks on the Tails of Comets. By H. Flaugergues.* 



A series of papers by M. Flaugergues, has been lately pub- 

 lished in the Journal de Physique, on the tails of comets, in 

 which he examines in detail the various hypotheses that have 

 been proposed to account for them, but conceives them all to be 

 inadequate. After taking a short review of the opinions enter- 

 tained on this subject by the ancients, and the earlier of the 

 moderns, he examines more particularly those of Kepler and 

 Descartes, and finally comes to that of Newton. This great 

 philosopher conjectured that the tails of comets were composed 

 of an extremely rare vapour, which proceeded from their nucleus, 

 generated by the great heat which these bodies acquire when 

 they approach the sun. He formed a calculation of the degree 

 of heat which the comet of 1680 would experience in its perihe- 

 lion ; and he estimated it at a temperature 2000 times greater 

 than the heat of red-hot iron. 



To this hypothesis M. Flaugergues objects, that on account 

 of the rapidity of the motion of comets, it is very doubtful whether 

 they can acquire a degree of heat nearly equal to that assigned 

 to them by Newton. Besides, it is remarked that the tails of 

 comets are by no means in proportion to their proximity to the 

 sun ; some comets which have approached very near the sun 

 having had very little of this appearance, while others have had 

 large tails, although they never came veiy near the sun in any 

 part of their course. Another objection against the hypothesis 

 is, that the centrifugal force which is produced by the motion 

 of the comet in a curve round the sun, being common to the 

 comet and to the vapour which is supposed to form the tail, 

 cannot tend in any degree to detach the comet and the vapour 

 from each other. It is further urged that the greatest part of 

 the matter which composes the tail of a comet ought, after it 

 has passed its perihelion, to follow after the comet in the direc- 

 tion of its motion, and not precede it, as is always the case. 

 Again, the matter which forms the tail of a comet, being sur- 

 rounded with matter which is more dense, and which, conse- 

 quently, ought to reflect light more strongly, the tail ought not 

 to be distinguished by its brilliancy from the other parts of space. 

 As, according to the hypothesis of Newton, the vapour which 

 forms the tail of a comet is elevated from the nucleus because it 

 has less specific gravity than the medium with which it is 

 surrounded, the lateral motion of the tail should be entirely 

 destroyed by the resistance of this medium; the matter of the 



* Abridged from Journ. Pliy«. for Feb. 1818. 



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