134 COSMOS. 



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

 to the most recent and acute researches on the crepuscular 

 theory, and of the height of the atmosphere.*' The phenomena 

 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 revolves 

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

 owing to its position, we may retain the name of zodiacal 

 light. Eveiywhere the law of periodicity governs the motions 

 of these bodies, however different may be the amount of tan- 

 gential velocity, or the quantity of their agglomerated material 

 parts ; the meteoric asteroids which enter our atmosphere from 

 the external regions of universal space, are alone arrested in 

 the course of their planetary revolution, and retained within the 

 sphere of a larger planet. In the solar system, whose bound- 

 aries determine the attractive force of the central body, comets 

 are made to revolve in their elliptical orbits at a distance 44 

 times greater than that of Uranus ; nay, in those comets 

 whose nucleus appears to us, from its inconsiderable mass, like 

 a mere passing cosmical cloud, the Sun exercises its attractive 

 force on the outermost parts of the emanations radiating from 

 the tail over a space of many millions of miles. Central 

 forces, therefore, at once constitute and maintain the system. 



Our Sun may be considered as at rest when compared to all 

 the large and small, dense and almost vaporous cosmical bodies, 

 that appertain to and revolve around it ; but it actually rotates 

 round the common centre of gravity of the whole system, 

 which occasionally falls within itself, that is to say, remains 

 within the material circumference of the Sun, whatever 

 changes may be assumed by the positions of the planets. A 

 very different phenomenon is that presented by the translatory 

 motion of the Sun, that is, the progressive motion of the centre 



* Biot, Traits d'Astron. Physique, Seme ed., 1841, t. i. pp. 171, 

 238, and 312. 



