248 Dr Lehmann cni tlie Tails of' Comets. 



ing so placed, the diminution of gravitation is much smaller in 

 the parts of the atmosphere turned toward the sun, than in those 

 which are on the opposite side ; the tail must, therefore, be much 

 shorter in this region than in the other. And as the centre of 

 gravity may be supposed extremely near the surface of the 

 nucleus, as has been said above, it may be conceived that here 

 the dilatation of the atmosphere no longer forms a tail, but only 

 a nebulosity. 



Lastly, the tail is ordinarily inflected, so as to turn its conca- 

 vity toward the side from whence the comet comes, and to have 

 the plane of its curvature coinciding with that of the orbit of the 

 star. The reason is this, — the particles of the tail cannot follow 

 the circulating motion round the sun, with the same rapidity 

 as the nucleus, because to the same linear velocity correspond 

 angular velocities, so much the smaller in proportion to the great- 

 ness of the distance from the sun. The radius will be tangent 

 to the curve of the tail in the vicinity of the nucleus, because 

 there the angular velocity of the particles of which it is compos- 

 ed does not differ from that of the nucleus. It is easy also to 

 comprehend, that the tail will appear so much the more strongly 

 inflected, the larger it is ; a result of our hypothesis that agrees 

 with observation. 



According to what has been said above, the formation and 

 change of the tails of comets may be considered as a sort of flux 

 and reflux of the atmosphere of their bodies, perfectly similar to 

 the tides which are caused by the moon in our ocean, and per- 

 haps even in our atmosphere. 



On the Snakes of Southern Africa. By Andrew Smith, M. D. 

 M. W. S. Assistant-Surgeon 98th Regiment, and Superinten- 

 dent of the South African Museum *. Communicated by the 

 Author, 



An no branch of natural history is the want of accurate and 

 perspicuous description more felt than in Ophiology. Such im- 

 perfections have unquestionably tended to retard the advance- 

 ment of the science, to create diffidence and doubt in the mind 

 of the inquirer, and to keep back communications on the sub- 



• Read before the Wernerian Society, 22d April 1826. 



