COMETARY INFLUENCES. 



the smallest stars stars so minute as to be barely visibla by 

 the aid of powerful telescopes, have been distinctly seen, and 

 seen without any perceptible diminution of their lustre, through 

 the very centre of the head of these bodies. It would follow, 

 therefore, that the matter composing them is so attenuated 

 that a thickness of so many thousand miles of it has no sensible 

 imperfection of transparency. 



There is, therefore, the strongest reason to conclude that the 

 material of which comets are composed is vaporous or aeriform, 

 and that it is in the most attenuated state that can well be 

 imagined, being probably some thousand times less dense than 

 our atmosphere. 



It has also been ascertained on satisfactory grounds that this 

 matter is not luminous, but, like the clouds which float in our 

 atmosphere, is illuminated by the sun, and thus rendered visible. 

 Some circumstances attending the variation of the magnitude of 

 the visible material of these bodies also render it probable that 

 they are composed of vapour, which when raised to a certain 

 temperature by their proximity to the sun, becomes absolutely 

 transparent and invisible, and which as the comet recedes from 

 the centre of light and heat is gradually condensed and becomes 

 visible, just as steam issuing from the safety-valve of a boiler is, 

 at the moment of its escape and before its condensation, transparent 

 and invisible, and assumes a greater and greater volume of 

 whitish cloudy matter, as its distance from the valve and its 

 exposure to the condensing effect of the cold air increases. In 

 this way is explained the fact, that comets in general are augmented 

 in their visible volume as they recede from the sun. 



Such then being generally the nature and character of these 

 bodies, so far as observation has enabled astronomers to deter- 

 mine them, it remains to inquire how far there are any grounds 

 for the various effects and influences which have been ascribed 

 to them. 



4. Of all the effects which have been ascribed to comets, that of a 

 collision with the earth is perhaps the least unreasonable. 



That such an event is possible, cannot be denied. It remains, 

 therefore, only to estimate its probability, and the effects it might 

 produce if it occurred. 



That a comet should encounter a planet, two conditions must 

 evidently be fulfilled : 1st, the path of the comet must intersect 

 that of the planet ; and, 2nd, the two bodies must arrive at the 

 same time at this point of intersection. 



Now, of all the known comets there is not one of which the 

 orbit intersects the orbit of any planet. There is, however, one 

 whose orbit passes so near the earth's orbit, that the distance 

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