140 COSM08. 



it 23h. 20m., while Bianchini* of Home. 1726, assumed the 

 slow rotation of 24i 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 irer 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- 

 face 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 Madler, 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 Lilienthal 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 

 a^h-colored light, sometimes observed in the dark parts, and 



* Delambre, Hist, de VAstron. au dixhuitieme siecle, p. 256-258. The 

 result obtained by Bianchini was supported by Hussey aud Flaugergues; 

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

 more probable until 1836. (Schumacher's Jahrbuch 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 probability de 1'existence d'une atmosphere qui enveloppe Venus 

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

 L'intensite de la lumiere de l'interieur dii croissant est serrsiblement 

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

 de la planete." — Arago, Manuscripts 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." 



$ Wilhelm Beer and Madler, Beitruge zur Physischen Kenntniss der 

 Himmlischen Korper, 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. 



