r hand, went altogether wide of the truth as
regards Mars. He held that the surface visible to us is a mere shell of
drifting cloud, deriving a certain amount of apparent stability from the
influence on evaporation and condensation of subjacent but unseen
areographical features;[970] and his opinion prevailed with his
contemporaries. It was, however, rejected by Kunowsky in 1822, and
finally overthrown by Beer and Maedler's careful studies during five
consecutive oppositions, 1830-39. They identified at each the same dark
spots, frequently blurred with mists, especially when the local winter
prevailed, but fundamentally unchanged.[971] In 1862 Lockyer established
a "marvellous agreement" with Beer and Maedler's results of 1830, leaving
no doubt as to the complete fixity of the main features, amid "daily,
nay, hourly," variations of detail through transits of clouds.[972] On
seventeen nights of the same opposition, F. Kaiser of Leyden obtained
drawings in which nearly all the markings noted in 1830 at Berlin
reappeared, besides spots frequently seen respectively by Arago in 1813,
by Herschel in 1783, and one sketched by Huygens in 1672 with a
writing-pen in his diary.[973] From these data the Leyden observer
arrived at a period of rotation of 24h. 37m. 22.62s., being just one
second shorter than that deduced, exclusively from their own
observations, by Beer and Maedler. The exactness of this result was
practically confirmed by the inquiries of Professor Bakhuyzen of
Leyden.[974] Using for a middle term of comparison the disinterred
observations of Schroeter, with those of Huygens at one, and of
Schiaparelli at the other end of an interval of 220 years, he was
enabled to show, with something like certainty, that the time of
rotation (24h. 37m. 22.735s.) ascribed to Mars by Mr. Proctor[975] in
reliance on a drawing executed by Hooke in 1666, was too long by _nearly
one-tenth of a second_. The minuteness of the correction indicates the
nicety of care employed. Nor employed vainly; for, owing to the
comparative antiquity of the records available in this case, an almost
infinitesimal error becomes so multiplied by frequent repetition as to
produce palpable discrepancies in the positions of the markings at
distant dates. Hence Bakhuyzen's period of 24h. 37m. 22.66s. is
undoubtedly of a precision unapproached as regards any other heavenly
body save the earth itself.

Two facts bearing on the state of things at the surface of Ma