COMETS. 



one of the visits of Halley's comet to the solar system, Pierre 

 Apian observed that the comet generally presented its tail in the 

 direction opposite to that of the sun. This principle was hastily 

 generalised, and is even at present too generally 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 which the shadow of the comet 

 would take. Sometimes it has happened that the tail forms with 

 a line drawn to the sun a considerable angle, and cases have 

 occurred when it was actually at right angles to it. 



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. That of 

 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 presents 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 sideways, 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 vapour, the external surface 

 of which possesses a certain thickness. When we view it, we 

 look through a considerable thickness of vapour at the edges, and 

 through a comparatively small quantity at the middle. 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 for. 



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 44 in length. Their 



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