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.