88 COSMOS. 



mechanical clock-work and photographic apparatus, as the 

 result of prolonged observations during the many months of 

 serene weather enjoyed in a tropical climate. The meteor- 

 ological processes at work in the gaseous envelopes of the 

 dark body of the Sun are the causes which produce the phe- 

 nomena termed Sun-spots and conglobate luminous clouds. 

 It is probable that there, as in the meteorology of our own 

 planet, the disturbances of very multifarious and complicated 

 character depend upon such general and local causes, that it 

 can only be by means of prolonged observations, character- 

 ized by completeness, that we can hope to solve even a por- 

 tion of this still obscure problem. 



II. 



THE PLANETS. 



GENERAL comparative considerations of a whole class of 

 cosmical bodies must here precede their individual descrip- 

 tion. These considerations refer to the 22 principal planets 

 and 21 'moons [satellites, or secondary planets] which have 

 been discovered up to the present time, not to the planetary 

 bodies in general, among which the comets whose orbits have 

 been calculated are alone ten-fold more numerous. The 

 planets possess, upon the whole, a feeble scintillation, inas- 

 much as they shine by the reflected light of the Sun, and 

 their planetary light emanates from disks. (Cosmos, vol. iii., 

 p. 76.) In the ash-colored light of the Moon, as well as in 

 the red light of its obscured disk, which is seen with great in- 

 tensity between the tropics, the Sun's light undergoes, in 

 reference to the observer upon the Earth, a twice repeated 

 change in its direction. Attention has been already directed 

 elsewhere* to the fact that the Earth and other planets pos- 

 sess in themselves a feeble power of emitting light, as is 

 specially proved by some remarkable phenomena upon that 

 portion of Venus which is turned away from the Sun. 



We shall consider the planets according to their number, 

 the sequence of their discovery, their volumes compared either 

 with each other or with their distances from the sun ; ac- 

 cording to their relative densities, masses, periods of rotation, 

 degrees of eccentricity, the inclinations of their axes, and 

 characteristic differences within and beyond the zone of the 



* Cosmos, vol. i., p. 201, and note p. 202. 



