hat this lowering is can be seen in the universal fact,
that even within the tropics perpetual snow covers the higher mountain
summits, while on the high plains of the Andes, at 15,000 or 16,000 feet
altitude, where there is very little or no snow, travellers are often
frozen to death when delayed by storms; yet at this elevation the
atmosphere has much more than double the density of that of Mars!

The error in Mr. Lowell's argument is, that he claims for the scanty
atmosphere of Mars that it allows more sun-heat to reach the surface;
but he omits to take account of the enormously increased loss of heat by
direct radiation, as well as by the diminution of air-radiation, which
together necessarily produce a great reduction of temperature.

It is this great principle of the prepotency of radiation over
absorption with a diminishing atmosphere that explains the excessively
low temperature of the moon's surface, a fact which also serves to
indicate a very low temperature for Mars, as I have shown in Chapter VI.
These two independent arguments--from alpine temperatures and from those
of the moon--support and enforce each other, and afford a conclusive
proof (as against anything advanced by Mr. Lowell) that the temperature
of Mars must be far too low to support animal life.

A third independent argument leading to the same result is Dr. Johnstone
Stoney's proof that aqueous vapour cannot exist on Mars; and this fact
Mr. Lowell does not attempt to controvert.

To put the whole case in the fewest possible words:

All physicists are agreed that, owing to the distance of Mars from the
sun, it would have a mean temperature of about-35 deg. F. (= 456 deg. F. abs.)
even if it had an atmosphere as dense as ours.

(2) But the very low temperatures on the earth under the equator, at a
height where the barometer stands at about three times as high as on
Mars, proves, that from scantiness of atmosphere alone Mars cannot
possibly have a temperature as high as the freezing point of water; and
this proof is supported by Langley's determination of the low _maximum_
temperature of the full moon.

The combination of these two results must bring down the temperature of
Mars to a degree wholly incompatible with the existence of animal life.

(3) The quite independent proof that water-vapour cannot exist on Mars,
and that therefore, the first essential of organic life--water--is
non-existent.

The conclusion from these three independent