204 PKOCEEDINGS Or THE AMERICAN ACADEMY 



corroborates the inference from the form of the tail, that the matter is 

 not ejected from the head with any great velocity, but that it passes off 

 before and behind, so as to be but little expanded in a direction per- 

 pendicular to the plane of the orbit. 



" 10. The successive envelopes ascend uniformly towards the sun, 

 at the rate of about thirty-five miles an hour. 



"11. This law of ascent is peculiarly consistent with that of a vapor 

 in an atmosphere. The appearance of the envelope of Halley's comet 

 in 1835, as described by Herschel, was also that of a fine vapor. 



" 12. The uniformity of the law of ascent is iri-econcilable with the 

 hypothesis of a mass rising in a free space under the combined action 

 of accelerating and retarding forces, which emanate from the comet 

 and the sun. The continuance of the ascent by the same law, during 

 the approach to perihelion and the removal from it, is opposed to the 

 ingenious explanation of Herschel, which attributes the apparent in- 

 crease of magnitude, when the comet recedes fi-om the sun, to the de- 

 posit of vapor as the comet cools. The phenomena exhibited by other 

 comets seem to confirm these views. 



" 13. The great extent of atmosphere which this theory, or that of 

 Herschel, requires the comet to possess, involves the necessity of a 

 much greater mass of the comet than is usually supposed, although the 

 mass will not exceed the limits adopted by Laplace in the case of 

 Lexell's comet. While, on the other hand, the small volume of the 

 nucleus into wi4iich this mass is chiefly concentrated will give a density 

 to some of the comets which will exceed that of the planets, and even 

 be metallic in its nature. With such a mass and density, the blow 

 of a comet might be quite disastrous to the inhabitants of a planet. The 

 metallic density of the two comets of 1680 and 1843, which approached 

 so near the sun, seems to be a necessary condition for the preservation 

 of their internal continuity and coherence, when they are melted by 

 the great heat of the sun. This metallic character of the comet is 

 analogous to that of meteoric stone. 



" 14. The nearly spherical outlines of the envelope are simply 

 results of uniform ascent of a vapor ; but they differ sensibly from the 

 level surfaces which would be given either by th§ theory of Roche or 

 by that of Herschel. 



" 15. After a careful examination of the observations of the head of 

 the comet, Pape did not find any established law of vibration, like that 



