These spots are not very distinct, and are mostly variable ;
Ihey seldom appear at the boundary of the separation be-
tween light and shadow in the crescent-shaped phase of the
planet, and both the Hersohels, father and son, are conse-
quently of opinion that they do not belong to the isolid sur-
face of the planet, but more probably to an atmosphere. t
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. J "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 VAstron. au dixhuitiime siiele, p. 256-258. The
result obtained by Bianchini was supported by Hussey and 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 Annnaire for 1842, p. 539. "Oe qui favorise aussi
la probabilite de I'existence d'une atmosphere qui enveloppe Vfenus
c'est le resultat optique obtenu par I'emploi d'une lunette prismatique.
