Only a _dense_ population with _ample_ means of subsistence could possibly have constructed such gigantic works; but, given these two conditions, no adequate motive existed for the conception and execution of them--even if they were likely to be of any use, which I have shown they could not be.
_Further Considerations on the Climate of Mars._ Recurring now to the question of climate, which is all-important, Mr.
Lowell never even discusses the essential point--the temperature that must _necessarily_ result from an atmospheric envelope one-twelfth (or at most one-seventh) the density of our own; in either case corresponding to an altitude far greater than that of our highest mountains.[17] Surely this phenomenon, everywhere manifested on the earth even under the equator, of a regular decrease of temperature with altitude, the only cause of which is a less dense atmosphere, should have been fairly grappled with, and some attempt made to show why it should not apply to Mars, except the weak remark that on a level surface it will not have the same effect as on exposed mountain heights.

But it _does_ have the same effect, or very nearly so, on our lofty plateaux often hundreds of miles in extent, in proportion to their altitude.
Quito, at 9350 ft.

above the sea, has a mean temperature of about 57 deg.
F., giving a lowering of 23 deg.

from that of Manaos at the mouth of the Rio Negro.

This is about a degree for each 400 feet, while the general fall for isolated mountains is about one degree in 340 feet according to Humboldt, who notes the above difference between the rate of cooling for altitude of the plains--or more usually sheltered valleys in which the towns are situated--and the exposed mountain sides.