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492 PHYSICAL CONSTITUTION OF COMETS. 



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sun. This principle was hastily generalized, and is even at present too gen- ? 

 erally adopted. It is true that in most cases the tail extends itself from that 

 part of the comet which is most remote from the sun ; but its direction rarely 

 corresponds with the direction of a shadow of the comet. Sometimes it has 

 happened that the tail forms with the line drawn to the sun a considerable an- 

 gle, and cases have occurred when it was actually at right angles to the direc- 

 tion of the sun. 



Another character which has been observed to attach to the tails of comets, 

 which, however, is not invariable, is, that they incline constantly toward the 

 region last quitted by the comet, as if, in its progress through space, it were 

 subject to the action of some resisting medium, so that the nebulous matter with 

 which it is invested, suffering more resistance than the solid nucleus, remains 

 behind it and forms the tail. 



The tail sometimes appears to have a curved form. The comet of 1744 

 formed almost a quadrant. It is supposed that the convexity of the curve, if it 

 exists, is turned in the direction from which the comet moves. It is proper to 

 state, however, that these circumstances regarding the tail have not been clearly 

 and satisfactorily ascertained. 



The tails of comets are not of uniform breadth or diameter ; they appear to 

 diverge from the comet, enlarging in breadth and diminishing in brightness as 

 their distance from the comet increases. The middle of the tail usually pre- 

 sents a dark stripe, which divides it longitudinally into two distinct parts. It 

 was long supposed that this dark stripe was the shadow of the body of the 

 comet, and this explanation might be accepted if the tail was always turned 

 from the sun ; but we find the dark stripe equally exists when the tail, being 

 turned sideward, is exposed to the effect of the sun's light. 



This appearance is usually explained by the supposition that the tail is a 

 hollow, conical shell of vapor, the external surface of which possesses a cer- 

 tain thickness. When we view it, we look through a considerable thickness 

 of vapor at the edges, and through a comparatively small quantity at the mid- 

 dle. Thus, upon the supposition of a hollow cone, the greatest brightness would 

 appear at the sides, and the existence of a dark space in the middle would be 

 perfectly accounted far. 



The tails of comets are not always single ; some have appeared at different 

 times with several separate tails. The comet of 1744, which appeared on the 

 7th or 8th of March, had six tails, each about 4 in breadth, and from 30 to 41 

 in length. Their 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 easi- 

 ly 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 appear oblique to the eye, it will be foreshortened more 

 or less, according to the angle of obliquity. The real length of the tail is easi- 

 ly calculated when the apparent length is observed and the angle of known ob- 

 liquity. The following results of actual observation and calculation have been 

 given by Arago. 



The comet of 1811 exhibited a tail which extended over 23 of the heavens. 

 It was observed by Herschel and Schroeter, the latter of whom deduced from 

 his calculations the following results : That the central globe of light or nucleus 

 was 50,000 miles in diameter, or about six and a half times the diameter of the 

 earth. The nebulosity was extremely rarified in comparison with nucleus, re- 

 sembling a faint, whitish light, scattered in separate portions. It was separated 



