252 Comets. 



Accordingly, whenever powerful telescopes have been turned 

 on these bodies, they have not failed to dispel the illusion 

 which attributes solidity to that more condensed part of the 

 head, which appears to the naked eye as a nucleus ; though 

 it is true that in some a very minute stellar point has been 

 seen, indicating the existence of a solid body. 



It is in all probability to the feeble coercion of the elastic 

 power of their gaseous parts, by the gravitation of so small 

 a central mass, that we must attribute this extraordinary 

 development of the atmospheres of comets. If the earth, re- 

 taining its present size, were reduced, by any internal change 

 (as by hollowing out its central parts) to one-thousandth 

 part of its actual mass, its coercive power over the atmo- 

 sphere would be diminished in the same proportion, and, in 

 consequence, the latter would expand to a thousand times its 

 actual bulk ; and indeed much more, owing to the still 

 further diminution of gravity, by the recess of the upper 

 parts from the centre.* An atmosphere, howevei', free to 

 expand equally in all directions, would envelope the nucleus 

 spherically, so that it becomes necessary to admit the action 

 of other causes to account for its enormovis extension in the 

 direction of the tail, — a subject to which we shall presently 

 take occasion to recur. 



That the luminous part of a comet is something in the 

 nature of a smoke, fog, or cloud, suspended in a transparent 

 atmosphere is evident from a fact which has been often no- 

 ticed, viz., that the portion of the tail where it conies up and 

 surrounds the head, is yet separate fi'om it by an interval less 

 luminous, as if sustained and kept off from contact by a trans- 

 parent stratum, as we often see one layer of clouds over an- 

 other, with a considerable clear space between. These, and 

 most of the other facts observed in the history of comets. 



* Newton has calculated (Princ. ill., p. 512) that a globe of air of ordinary 

 density at the earth'j surface, of one inch in diameter, if reduced to the density 

 due to the altitude above the surface of one radius of the earth, would occupy 

 a sphere exceeding in radius the oi-bit of Saturn. The tail of a great comet, 

 then, for aught we can tell, may consist of only a very few pounds or even 

 ounces of matter. 



