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.

What 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 proofs, which enforce each other in the multiple ratio of their respective weights, is therefore irresistible--that animal life, especially in its higher forms, cannot exist on the planet.
Mars, therefore, is not only uninhabited by intelligent beings such as Mr.Lowell postulates, but is absolutely UNINHABITABLE..