TAILS OF COMETS. 



sides were well defined and tolerably bright, and the spaces 

 between them were as dark as the other parts of the heavens. 



The tails of comets have frequently appeared, not only of 

 immense real length, but extending over considerable spaces of 

 the heavens. It will be easily understood that the apparent 

 length depends conjointly upon the real length of the tail, and 

 the position in which it is presented to the eye. If the line of 

 vision be at right angles to it, its length will appear as great as 

 it can do at its existing distance ; if it be oblique to the eye, it 

 will be foreshortened, more or less, according to the angle of 

 obliquity. The real length of the tail is easily calculated when 

 the apparent length is observed and the angle of obliquity 

 known. 



In respect of magnitude, the tails are unquestionably the most 

 stupendous objects which the discoveries of the astronomer have 

 ever presented to human contemplation. 



The following are the results of the observation and measure- 

 ment of a few of the more remarkable. 



The magnitude of these prodigious appendages is even less 

 amazing than the brief period in which they sometime emanate 

 from the head. The tail of the comet of 1843, long enough to 

 stretch from the sun to the planetoids, was formed in less than 

 twenty days. 



83. The masses of comets, like those of the planets, would be 

 ascertained if the reciprocal effects of their gravitation and those 

 of any known bodies in the system could be observed. But 

 although the disturbing action of the planets on these bodies is 

 conspicuous, and its effects have been calculated and observed, 

 not the slightest effect of the same kind has ever been ascertained 

 to be produced by them, even upon the smallest bodies in the 

 system, and those to which comets have approached most nearly. 



In fine, notwithstanding the enormous number of comets, 



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