502 COSMOS. 



aqueous depositions, and the dispersion of clouds, will be 

 treated of in the last and purely telluric part of the Cosmos* 



MASS. 



The diameter of this planet, notwithstanding its consider- 

 ably greater distance from the Sun, is only 0-519 of the 

 Earth's, or 3,568 geographical miles. The excentricity of his 

 orbit is 0*0932168, next to Mercury, the greatest of all the 

 planetary orbits ; and also on this account, as well as from its 

 proximity to the Earth, the most suitable for Kepler's great 

 discovery of the elliptical form of the planetary orbits. His 

 period of rotation* is, according to Madler and Wilhelm 



heat radiating from a body far below the temperature of 

 ignition, cannot reach the surface of the Earth, since it is 

 absorbed in the upper strata of our atmosphere, where it 

 converts visible clouds into transparent vapour." The phe- 

 nomenon of the rapid dispersion of clouds by the full Moon, 

 when they are not extremely dense, is considered by Sir John. 

 Herschel, " as a meteorological fact, which," he adds, " is 

 confirmed by Humboldt's own experience and the universal 

 belief of the Spanish sailors in the tropical seas of America." 

 See Report of the Fifteenth Meeting of the British Association 

 for the Advancement of Science, 1846, Notices, p. 5; and 

 Outlines, p. 201. 



53 Beer and Madler, Beit-rage zur Phys. Kenntniss des Son- 

 nensy stems, 1841, p. 113, from observations in 1830 and 

 1832; Madler, Astronomw, 1849, p. 206. The first consi- 

 derable improvement in the data for the period of rotation, 

 which Dominique Cassini found 24h. 40m., was the result of 

 laborious observations by William Herschel (between 1777 

 and 1781) which gave 24h. 39m. 21'7s. Kunowsky found, in 

 1821, 24h. 36m. 40s., very near to Madler's result. Cassini's 

 oldest observation of the rotation of a spot upon Mars 

 (Delambre, Hist, de VAstron. Mod. torn. ii. p. 694), appears 

 to have been soon after the year 1670; but in the very rare 



