e heat of the sun in the middle of a summer's day increased
six or seven fold! If there were no mitigating influences, the face of
the earth would shrivel as in the blast of a furnace, the very stones
would become incandescent, and the oceans would turn into steam.

Still, notwithstanding the tremendous heat poured upon Mercury as
compared with that which our planet receives, we can possibly, and for
the sake of a clearer understanding of the effects of the varying
distance, which is the object of our present inquiry, find a loophole to
admit the chance that yet there may be living beings there. We might,
for instance, suppose that, owing to the rarity of its atmosphere, the
excessive heat was quickly radiated away, or that there was something in
the constitution of the atmosphere that greatly modified the effective
temperature of the sun's rays. But, having satisfied our imagination on
this point, and placed our supposititious inhabitants in the hot world
of Mercury, how are we going to meet the conditions imposed by the
rapid changes of distance--the swift fall of the planet toward the sun,
followed by the equally swift rush away from it? For change of distance
implies change of heat and temperature.

It is true that we have a slight effect of this kind on the earth.
Between midsummer (of the northern hemisphere) and midwinter our planet
draws 3,000,000 miles nearer the sun, but the change occupies six
months, and, at the earth's great average distance, the effect of this
change is too slight to be ordinarily observable, and only the
astronomer is aware of the consequent increase in the apparent size of
the sun. It is not to this variation of the sun's distance, but rather
to the changes of the seasons, depending on the inclination of the
earth's axis, that we owe the differences of temperature that we
experience. In other words, the total supply of heat from the sun is not
far from uniform at all times of the year, and the variations of
temperature depend upon the distribution of that supply between the
northern and southern hemispheres, which are alternately inclined
sunward.

But on Mercury the supply of solar heat is itself variable to an
enormous extent. In six weeks, as we have seen, Mercury diminishes its
distance from the sun about one third, which is proportionally ten times
as great a change of distance as the earth experiences in six months.
The inhabitants of Mercury in those six pregnant weeks see the s