OF ARTS AND SCIENCES. 205 



which Bessel demonstrated in the Halley comet of 1835. But from 

 a new examination, upon a different basis, it appears that there was 

 such a vibration, that each new envelope had its own initial period of 

 vibration, that there was one common law of vibration, and that the vi- 

 bration continued as long as the envelope was near the nucleus. The 

 phenomena seemed to be those Avhich would be exhibited if the nucleus 

 were in a state of magnetic oscillation, as suggested by Bessel, or if it 

 were rotating about an axis pointed towards the sun, and if each 

 envelope were projected from a different point of the comet's surface. 



"16. The phenomenon attributed to the resisting medium would 

 be fully as accurately explained in the mode suggested by Herschel. 

 The loss of the repelled matter must involve an apparent augmentation 

 of the weight of the attracted portion without that corresponding in- 

 crease of velocity which is required to preserve the mean distance 

 and time of revolution. Thus, in the equation 



V 



.2 



2 1 



II r a' 



the increase of the denominator of the first member is analytically 

 the same with the decrease of the numerator; or, physically, to in- 

 crease the gravity is the same as to decrease the velocity. The 

 apparent resistance, in the case of Encke's comet, might be accounted 

 for by a loss of about -gu^^^ of its mass at each return. But even 

 without the loss of mass, an approach to the same phenomenon would 

 result from the mere separation of the polar forces of the comet under 

 the solar influence, which would draw one portion towards the sunny 

 side of the comet, and drive the repelled force to the opposite side. 



" 17. The driving off the tail would tend to give rotation to the 

 comet, or rather to return to the comet as rotation part of the force 

 which had been taken out as translation. The opposite phenomenon 

 might be exhibited in the case of a planet, on account of its rotation 

 being in the same direction with the revolution of the planet about the 

 sun. It is possible that this may account for the decrease of mean 

 motion, which is indicated, with some uncertainty, by Leverrier's pro- 

 found investigations of the planet Mercury, so that there may be ex- 

 hibited a transformation of rotation into translation. 



" 18. The formation of the comet's tail is not without analogy in the 

 terrestrial phenomenon of the thunder-cloud. Here, at least, is ex- 

 hibited the phenomenon of a vapor rising in an electrified condition. 



