entirely neutralises
the effects of increase of sun-heat, however great, when these
cumulative agencies are not present.[12]

[Footnote 12: The effects of this 'cumulative' power of a dense
atmosphere are further discussed and illustrated in the last chapter of
this book, where I show that the universal fact of steadily diminishing
temperatures at high altitudes is due solely to the diminution of this
cumulative power of our atmosphere, and that from this cause alone the
temperature of Mars must be that which would be found on a lofty plateau
about 18,000 feet higher than the average of the peaks of the Andes!]

_Temperature on Polar Regions of Mars._

There is also a further consideration which I think Mr. Lowell has
altogether omitted to discuss. Whatever may be the _mean_ temperature
of Mars, we must take account of the long nights in its polar and
high-temperate latitudes, lasting nearly twice as long as ours, with the
resulting lowering of temperature by radiation into a constantly clear
sky. Even in Siberia, in Lat. 67-1/2 deg.N. a cold of-88 deg.F. has been
attained; while over a large portion of N. Asia and America above 60 deg.
Lat. the _mean_ January temperature is from-30 deg.F. to-60 deg.F., and the
whole subsoil is permanently frozen from a depth of 6 or 7 feet to
several hundreds. But the winter temperatures, _over the same latitudes_
in Mars, must be very much lower; and it must require a proportionally
larger amount of its feeble sun-heat to raise the surface even to the
freezing-point, and an additional very large amount to melt any
considerable depth of snow. But this identical area, from a little below
60 deg. to the pole, is that occupied by the snow-caps of Mars, and over the
whole of it the winter temperature must be far lower than the
earth-minimum of-88 deg.F. Then, as the Martian summer comes on, there is
less than half the sun-heat available to raise this low temperature
after a winter nearly double the length of ours. And when the summer
does come with its scanty sun-heat, that heat is not accumulated as it
is by our dense and moisture-laden atmosphere, the marvellous effects of
which we have already shown. Yet with all these adverse conditions, each
assisting the other to produce a climate approximating to that which the
earth would have if it had no atmosphere (but retaining our superiority
over Mars in receiving double the amount of sun-heat), we are asked to
accept a mean temperature fo