144 COSMOS 



are very unequally distant from the eaith, it is not possible, 

 according to the laws of the velocity and transmission of light, 

 that we should be able, in so short a period of time, to per- 

 ceive any actual changes m a cosmical body of such vast ex- 

 tent. These considerations in no way exclude the reality of 

 the changes that have been observed in the emanations from 

 the more condensed envelopes around the nucleus of a comet, 

 nor that of the sudden irradiation of the zodiacal light from 

 internal molecular motion, nor of the increased or diminished 

 reflection of light in the cosmical vapor of the luminous ring, 

 but should simply be the means of drawing our attention to 

 the differences existing between that which appertains to the 

 air of heaven (the realms of universal space) and that which 

 belongs to the strata of our terrestrial atmosphere. It is not 

 possible, as well- attested facts prove, perfectly to explain the 

 operations at work in the much-contested upper boundaries of 

 our atmosphere. The extraordinary lightness of whole nights 

 in the year 1831, during which small print might be read at 

 midnight in the latitudes of Italy and the north of Germany, 

 is a fact directly at variance with all that we know, accord- 

 ing to the most recent and acute researches on the crepuscular 

 theory, and of the height of the atmosphere.* The phenom 

 ena of light depend upon conditions still less understood, and 

 their variability at twilight, as well as in the zodiacal light, 

 excite our astonishment. 



We have hitherto considered that which belongs to our solar 

 system that world of material forms governed by the Sun 

 which includes the primary and secondary planets, comets of 

 short and long periods of revolution, meteoric asteroids, which 

 move thronged together in streams, either sporadically or in 

 closed rings, and finally a luminous nebulous ring, that re- 

 volves round the Sun in the vicinity of the Earth, and for 

 which, owing to its position, we may retain the name of zo- 

 diacal light. Every where the law of periodicity governs the 

 motions of these bodies, however different may be the amount 

 of tangential velocity, or the quantity of their agglomerated 

 material parts ; the meteoric asteroids which enter our atmos- 

 phere from the external regions of universal space ai3 alono 

 arrested in the course of their planetary revolution, and re- 

 tained within the sphere of a larger planet. In the solar sys- 

 tem, whose boundaries determine the attractive force of the 

 central body, comets are made to revolve in their elliptical 



* Biot, Trail* fAs'ron. Physique, 36me 6J., 1841, t. i., p. 171, 238. 

 and 312. 



