e situated--and the exposed mountain sides. It will be seen that
this lower rate would bring the temperature of Mars at the equator down
to 20 deg. F. below the freezing point of water from this cause alone.

[Footnote 17: A four inches barometer is equivalent to a height of
40,000 feet above sea-level with us.]

But all enquirers have admitted, that if conditions as to atmosphere
were the same as on the earth, its greater distance from the sun would
reduce the temperature to-31 deg. F., equal to 63 deg. below the freezing
point. It is therefore certain that the combined effect of both causes
must bring the temperature of Mars down to at least 70 deg. or 80 deg.below the
freezing point.

The cause of this absolute dependence of terrestrial temperatures upon
density of the air-envelope is seldom discussed in text-books either of
geography or of physics, and there seems to be still some uncertainty
about it. Some impute it wholly to the thinner air being unable to
absorb and retain so much heat as that which is more dense; but if this
were the case the soil at great altitudes not having so much of its heat
taken up by the air should be warmer than below, since it undoubtedly
_receives_ more heat owing to the greater transparency of the air above
it; but it certainly does not become warmer. The more correct view seems
to be that the loss of heat by radiation is increased so much through
the rarity of the air above it as to _more_ than counterbalance the
increased insolation, so that though the surface of the earth at a given
altitude may receive 10 per cent. more direct sun-heat it loses by
direct radiation, combined with diminished air and cloud-radiation,
perhaps 20 or 25 per cent. more, whence there is a resultant cooling
effect of 10 or 15 per cent. This acts by day as well as by night, so
that the greater heat received at high altitudes does not warm the soil
so much as a less amount of heat with a denser atmosphere.

This effect is further intensified by the fact that a less dense cannot
absorb and transmit so much heat as a more dense atmosphere. Here then
we have an absolute law of nature to be observed operating everywhere on
the earth, and the mode of action of which is fairly well understood.
This law is, that reduced atmospheric pressure increases radiation, or
loss of heat, _more rapidly_ than it increases insolation or gain of
heat, so that the result is _always_ a considerable _lowering_ of
temperature. W