PERIODIC COMETS. 



403 



the equator pass below him with a velocity of about a thousand miles an hour. 

 At the polos themselves this kind of movement does not exist ; at intermediate 

 latitudes it is less than at the equator. 



The waters of the ocean, although they partake of this rapid motion, do not 

 invade the surrounding country, for in every place the shore has precisely the 

 same velocity as the water, and under all latitudes the continents and the seas 

 that bathe them are in a relative repose. If this state of things were to change ; 

 if the waves at any given point were to continue their original velocity, while 

 that of the adjacent land was suddenly to diminish, the ocean would at the same 

 time overflow its limits. . 



In order to fix our ideas, let us imagine the oblique shock of a comet instan- * 

 taneously to turn the whole solid part of the earth round its diameter at the 

 point of Brest. That city having become the pole, the whole peninsula of 

 Brittany would be in an almost perfect repose ; but the ocean which washes 

 its shores on the west would not be so ; for as we have before observed on 

 the occasion of the movement of translation, it would be only resting on the 

 solid base of which its bed is formed. The waters would precipitate them- 

 selves in mass upon a shore which would no longer run before them with the 

 former velocity of the parallel of Brest. 



Behold, then, extensive parts of the continent inundated, lofty regions buried 

 under the waves by cometary influence. But have the marine deposites which 

 are actually discovered on the mountains been conveyed in this manner ? By 

 no means. These deposites are frequently horizontal, of great breadth, very 

 thick, and very regular. The varied and often very small shells which, com- 

 pose them have preserved their crests, their most delicate points, their most 

 brittle parts, unbroken. Every circumstance, then, dissipates the idea of a 

 violent transposition ; everything shows the deposites to have been formed on 

 the spot. What now remains to complete the' explanation without having re- 

 course to an eruption of the sea ? It must be admitted that the mountains and 

 undulating grounds upon which they are based have risen up from below, 

 like mushrooms ; that they have grown up through the bosom of the waters. 

 In 1694, Halley already cited this hypothesis as a possible explanation of the 

 presence of marine productions upon the sides and on the summits of the 

 highest mountains. This explanation was the true one ; it is at present al- 

 most generally admitted. A comet which should perceptibly alter either the 

 movement of rotation or the progress of translation of the earth would, without 

 any doubt, occasion terrific convulsions in the shell of the globe ; but, it must 

 be repeated, these physical revolutions would differ in a thousand circumstances 

 from those which are at present the objects of geological research. 



The first glance of the matter of the present discourse may perhaps raise a 

 question with some whether all comets must not be periodic ; the difference 

 among them being only that the periods of a few of them have been discovered, 

 and those of the others still remain unascertained. It does not, however, fol- 

 low at all that the comets move periodically round the sun. Newton showed 

 that the law of gravitation would allow a body to move under the sun's attrac- 

 tion in any of those species of curves called conic sections ; and that the par- 

 ticular species in which any body might happen to move would depend alto- 

 gether on the velocity and direction in \Vhich such body might have originally 

 been projected. There are three species of conic sections : the ellipse, the 

 parabola, and the hyperbola. Now it is only the ellipse which would cause 

 a periodical revolution round the sun. A body moving in either of the other 

 curves would enter the system in some determinate direction, and leave it in 

 another never to return to it. 



Although it is not certainly ascertained that any comets have moved in 



