110 COSMOS. 



it 231i. 20m., while Bianchini^^ of E,ome, 1726, assumed che 

 slow rotation of 24 ^ days. More accurate observations by De 

 Vico, from 1840 to 1842, afford, by means of a great number 

 of spots upon Venus, as the mean value of her period of ro- 

 tation, 23h. 21' 21"-93. 



These spots are not very distinct, and are mostly variable ; 

 they seldom appear at the boundary of the separation be- 

 tween light and shadow in the crescent-shaped phase of the 

 planet, and both the Herschels, father and son, are conse- 

 quently of opinion that they do not belong to the solid sur- 

 lace of the planet, but more probably to an atmosphere. f 

 The changeable form of the horns of the crescent, especially 

 the southern, has been taken advantage of by La Hire, 

 Schroter, and Miidler, partly for the estimation of the height 

 of mountains, partly and more especially for the determina- 

 tion of the rotation. The phenomena of this changeability 

 are of such a nature that they do not require for their ex- 

 planation the assumption of the existence of mountain- 

 peaks, twenty geographical miles in height (121,520 feet'' 

 as Schroter of Lilientnai stated, but merely elevations liKe 

 those which our planet presents in both continents. $ With 

 the little that we know with certainty of the appearance of 

 the surfaces of the planets near the Sun, Mercury, and Ve- 

 nus, and their physical constitution, the phenomenon of an 

 ash-colored light, sometimes observed in the dark parts, and 



* Delambre, Hist, de V Astron. au dixliuitieme siecle, p. 256-258. The 

 result obtained by Bianchini was supported by Hassey and Flaugergues; 

 Hansen also, whose aulliority is justly so great, considered it to be the 

 more probable until 1836. (Schumacher's Jahrbvck for 1837, p. 90.) 



t Arago, on the remarkable observation at Lilienthal on the 12th of 

 August, 1700, in the Annuaire for 1842, p. 539. "Ce qui favorise aussi 

 la probabilite de I'existence d'une atmosphere qui enveloppe Venus 

 c'est le resultat optique obtenu par I'emploi d'une lunette prismatique. 

 L'intensite de la lumiere de I'interieur du croissant est sensiblement 

 plus faible que celle des points situes dans la partie circulaire du disque 

 de la planete." — Arago, Mainiscripts of 1847. "That circumstance 

 which also favors the probability of the existence of an atmosphere 

 surrounding Venus is the optical result obtained by employing a pris- 

 matic telescope. The intensity of the light of the interior of the cres. 

 cent is sensibly weaker than that of the points situated in the circular 

 part of the planet's disk." 



X ^Vilhelm Beer and Miidler, Beitrdge zur Physischen Kenntnist dgr 

 Himmlischeit Koi-per, p. 148. The so-called moon of Venus, which 

 Fontana, Dominique Cassini, and Short declared that they had seen, for 

 which Lambert calculated tables, and which was said to have been 

 seen in the center of the Sun's disk, full three hours after the egress of 

 Venus, belongs to the astronomical myths of an uncritical age. 



