ves only half as much heat and light as at the opposite
period; and, in consequence, his distance from the Earth varies
considerably.

[Illustration: FIG. 34.--Mercury near quadrature.]

This globe exhibits _phases_, discovered in the seventeenth century by
Galileo, which recall those of the Moon. They are due to the motions of
the planet round the Sun, and are invisible to the unaided eye, but with
even a small instrument, one can follow the gradations and study Mercury
under every aspect. Sometimes, again, he passes exactly in front of the
Sun, and his disk is projected like a black point upon the luminous
surface of the flaming orb. This occurred, notably, on May 10, 1891, and
November 10, 1894; and the phenomenon will recur on November 12, 1907,
and November 6, 1914.

Mercury is the least of all the worlds in our system (with the exception
of the cosmic fragments that circulate between the orbit of Mars and
that of Jupiter). His volume equals only 5/100 that of the Earth. His
diameter, in comparison with that of our planet, is in the ratio of 373
to 1,000 (a little more than 1/3) and measures 4,750 kilometers (2,946
miles). His density is the highest of all the worlds in the great solar
family, and exceeds that of our Earth by about 1/3; but weight there is
less by almost 1/2.

Mercury is enveloped in a very dense, thick atmosphere, which doubtless
sensibly tempers the solar heat, for the Sun exhibits to the Mercurians
a luminous disk about seven times more extensive than that with which we
are familiar on the Earth, and when Mercury is at perihelion (that is,
nearest to the Sun), his inhabitants receive ten times more light and
heat than we obtain at midsummer. In all probability, it would be
impossible for us to set foot on this planet without being shattered by
a sunstroke.

Yet we may well imagine that Nature's fecundity can have engendered
beings there of an organization different from our own, adapted to an
existence in the proximity of fire. What magnificent landscapes may
there be adorned with the luxuriant vegetation that develops rapidly
under an ardent and generous sun?

[Illustration: FIG. 35.--The Earth viewed from Mercury.]

Observations of Mercury are taken under great difficulties, just because
of the immediate proximity of the solar furnace; yet some have detected
patches that might be seas. In any case, these observations are
contradictory and uncertain.

Up to the present it has been imp