T. Fig. 51
was taken on the 30th of July, 1894, and Fig. 52 on the 16th of August,
1894.

The southern polar cap on Mars, as seen by Professor William H.
Pickering at Lowell Observatory on the 1st of July, 1894, is represented
in Fig. 54.[17] The remarkable black mark intruding into the polar area
will be noticed. In Fig. 53 are shown a series of unusually marked
elevations and depressions upon the "terminator" of the planet, drawn as
accurately as possible to scale by the same skilful hand on the 24th of
August, 1894.

In making an examination of the planet it is to be observed that it does
not, like the moon, always present the same face towards the observer.
Mars rotates upon an axis in exactly the same manner as the earth. It is
not a little remarkable that the period required by Mars for the
completion of one rotation should be only about half an hour greater
than the period of rotation of the earth. The exact period is 24 hours,
37 minutes, 22-3/4 seconds. It therefore follows that the aspect of the
planet changes from hour to hour. The western side gradually sinks from
view, the eastern side gradually assumes prominence. In twelve hours the
aspect of the planet is completely changed. These changes, together with
the inevitable effects of foreshortening, render it often difficult to
correlate the objects on the planet with those on the maps. The latter,
it must be confessed, fall short of the maps of the moon in definiteness
and in certainty; yet there is no doubt that the main features of the
planet are to be regarded as thoroughly established, and some
astronomers have given names to all the prominent objects.

The markings on the surface of Mars are of two classes. Some of them are
of an iron-grey hue verging on green, while the others are generally
dark yellow or orange, occasionally verging on white. The former have
usually been supposed to represent the tracts of ocean, the latter the
continental masses on the ruddy planet. We possess a great number of
drawings of Mars, the earliest being taken in the middle of the
seventeenth century. Though these early sketches are very rough, and are
not of much value for the solution of questions of topography, they have
been found very useful in aiding us to fix the period of rotation of the
planet on its axis by comparison with our modern drawings.

Early observers had already noticed that each of the poles of Mars is
distinguished by a white spot. It is, however,