the Formatiofi of the Tails of Cornets. 247 



magnifying power for observing it, because this magnifier de- 

 minishes the intensity of the light. 



So long as the comet approaches the sun, and also when it be- 

 gins to retire from it, the causes which contribute to elongate 

 the tail will gain much upon those which tend to shorten it ; 

 and the greatest length will take place immediately after the 

 perihelion. But when, after the perihelion, the comet begins to 

 retire from the sun a sufficient space, the latter causes will begin 

 in their turn to gain upon the first. The gravitation towards 

 the nucleus will always be less surpassed by the power of expan- 

 sion, which will itself have become smaller and smaller, from 

 the rarefaction of the atmosphere. There will thus be a period 

 at which these two powers will be mutually balanced, and at 

 which, in consequence, the power will vanish, in virtue of which 

 the particles of the atmosphere were propelled from the nucleus 

 on the side opposite the sun. After this there will immediately 

 rise a contrary power, which will draw them towards the nucleus. 

 However, in virtue of the law of inertia, the tail will still grow, 

 but with a slower progress ; after a certain time it will be sta- 

 tionary, and at length it will rapidly diminish, so as to form a 

 new and condensed atmosphere. As thisatmospherecannot entire- 

 ly disappear, the growth of the tail will diminish more and more, 

 till the period when the coniet will be sufficiently removed from 

 the sun for its atmosphere to return to the same state of con- 

 densation which it possessed at the commencement of the phases 

 which we have described. 



Hitherto we have only considered the parts of the atmosphere 

 of a comet which are situated on the side farthest from the sun. 

 We might, by a perfectly similar mode of reasoning, conclude with 

 respect to those which are on the side next the sun, that they 

 ought to retire from the nucleus and stretch toward the sun, as 

 the others extended in the contrary direction. There is no es- 

 sential difference between the two regions of the comet : It must 

 form as long a tail on the one side as on the other. Why, 

 then, do we only see one on the hemisphere farthest from the sun ? 

 Because the centre of gravity of the nucleus does not coincide 

 with its centre of form, but is situated much nearer the surface 

 of the hemisphere turned toward the sun. 



It is demonstrated by calculation, that the centre of gravity be- 



