Schroter attributed the other markings on Mars to drifting clouds. But
Beer and Madler, in 1830-39, identified the same dark spots as being
always in the same place, though sometimes blurred by mist in the
local winter. A spot sketched by Huyghens in 1672, one frequently seen
by W. Herschel in 1783, another by Arago in 1813, and nearly all the
markings recorded by Beer and Madler in 1830, were seen and drawn by
F. Kaiser in Leyden during seventeen nights of the opposition of 1862
(_Ast. Nacht._, No. 1,468), whence he deduced the period of rotation
to be 24h. 37m. 22s.,62--or one-tenth of a second less than the period
deduced by R. A. Proctor from a drawing by Hooke in 1666.

It must be noted that, if the periods of rotation both of Mercury and
Venus be about twenty-four hours, as seems probable, all the four
planets nearest to the sun rotate in the same period, while the great
planets rotate in about ten hours (Uranus and Neptune being still
indeterminate).

The general surface of Mars is a deep yellow; but there are dark grey
or greenish patches. Sir John Herschel was the first to attribute the
ruddy colour of Mars to its soil rather than to its atmosphere.

The observations of that keen-sighted observer Dawes led to the first
good map of Mars, in 1869. In the 1877 opposition Schiaparelli revived
interest in the planet by the discovery of canals, uniformly about
sixty miles wide, running generally on great circles, some of them
being three or four thousand miles long. During the opposition of
1881-2 the same observer re-observed the canals, and in twenty of them
he found the canals duplicated,[4] the second canal being always 200
to 400 miles distant from its fellow.