d
not well exist upon such a sphere.

Next beyond Mercury is Venus, a sphere only a little less in diameter
than the earth. Of this sphere we know more than we do of Mercury, for
the reason that it is farther from the sun and so appears in the
darkened sky. Most astronomers hold that the surface of this planet
apparently is almost completely and continually hidden from us by what
appears to be a dense cloud envelope, through which from time to time
certain spots appear of a dark colour. These, it is claimed, retain
their place in a permanent way; it is, indeed, by observing them that
the rotation period of the planet has, according to some observers,
been determined. It therefore seems likely that these spots are the
summits of mountains, which, like many of our own earth, rise above
the cloud level.

Recent observations on Venus made by Mr. Percival Lowell appear to
show that the previous determinations of the rotation of that planet,
as well as regards its cloud wrap, are in error. According to these
observations, the sphere moves about the sun, always keeping the same
side turned toward the solar centre, just as the moon does in its
motion around the earth. Moreover, Mr. Lowell has failed to discover
any traces of clouds upon the surface of the planet. As yet these
results have not been verified by the work of other astronomers;
resting, however, as they do on studies made with an excellent
telescope and in the very translucent and steady air of the Flagstaff
Station, they are more likely to be correct than those obtained by
other students. If it be true that Venus does not turn upon its axis,
such is likely to be the case also with the planet Mercury.

Next in the series of the planets is our own earth. As the details of
this planet are to occupy us during nearly all the remainder of this
work, we shall for the present pass it by.

Beyond the earth we pass first to the planet Mars, a sphere which has
already revealed to us much concerning its peculiarities of form and
physical state, and which is likely in the future to give more
information than we shall obtain from any other of our companions in
space, except perhaps the moon. Mars is not only nearer to us than any
other planet, but it is so placed that it receives the light of the
sun under favourable conditions for our vision. Moreover, its sky
appears to be generally almost cloudless, so that when in its orbital
course the sphere is nearest our earth it is