density is 73 per cent of the earth's density. Gravity on its surface is
only 38 per cent of terrestrial gravity--i.e., a one hundred-pound
weight removed from the earth to Mars would there weigh but thirty-eight
pounds. Mars evidently has an atmosphere, the details of which we shall
discuss later.

The poles of the planet are inclined from a perpendicular to the plane
of its orbit at very nearly the same angle as that of the earth's poles,
viz., 24 deg. 50 min. Its rotation on its axis is also effected in
almost the same period as the earth's, viz., 24 hours, 37 minutes.

When in opposition to the sun, Mars may be only about 35,000,000 miles
from the earth, but its average distance when in that position is more
than 48,000,000 miles, and may be more than 60,000,000. These
differences arise from the eccentricities of the orbits of the two
planets. When on the farther side of the sun--i.e., in conjunction with
the sun as seen from the earth--Mars's average distance from us is about
235,000,000 miles. In consequence of these great changes in its
distance, Mars is sometimes a very conspicuous object in the sky, and at
other times inconspicuous.

The similarity in the inclination of the axis of the two planets results
in a close resemblance between the seasons on Mars and on the earth,
although, owing to the greater length of its year, Mars's seasons are
much longer than ours. Winter and summer visit in succession its
northern and southern hemispheres just as occurs on the planet that we
inhabit, and the torrid, temperate, and frigid zones on its surface have
nearly the same angular width as on the earth. In this respect Mars is
the first of the foreign planets we have studied to resemble the earth.

Around each of its poles appears a circular white patch, which visibly
expands when winter prevails upon it, and rapidly contracts, sometimes
almost completely disappearing, under a summer sun. From the time of
Sir William Herschel the almost universal belief among astronomers has
been that these gleaming polar patches on Mars are composed of snow and
ice, like the similar glacial caps of the earth, and no one can look at
them with a telescope and not feel the liveliest interest in the planet
to which they belong, for they impart to it an appearance of likeness to
our globe which at first glance is all but irresistible.

To watch one of them apparently melting, becoming perceptibly smaller
week after week, while the gener