TENUS. 475 



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 rotation, 23h. 21' 21"'93. 



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

 they seldom appear at the boundary of the separation between 

 light and shadow in the crescent-shaped phase of the planet, 

 and both the Herschels, father and son, are, consequently, of 

 opinion that they do not belong to the solid surface of the 

 planet, but more probably to an atmosphere. 15 The change- 

 able 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 determination of the rotation. 

 The phenomena of this changeability are of such a nature 

 that they do not require for their explanation 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 



authority is justly so great, considered it to be the more 

 probable until 1836. (Schumacher's Jahrbuch for 1837, 

 p. 90.) 



15 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 Texistence d'une 

 atmosphere qui enveloppe Venus, c'est le resultat optique 

 obtenu par 1'emploi d'une linaette 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, Manuscripts of 1847. 

 " That circumstance which also favours the probability of 

 the existence of an atmosphere surrounding Venus, is the 

 optical result obtained by employing a prismatic telescope. 

 The intensity of the light of the interior of the crescent is 

 sensibly weaker than that of the points situated in the cir- 

 cular part of the planet's disc." 



