644 LOWELL—MARS ON GLACIAL EPOCHS. [Noy. 16, 
‘‘Compared with the great body of the air, the aqueous vapor it con- 
tains is of almost infinitesimal amount, 9914 out of every 100 parts of 
the atmosphere being composed of oxygenand nitrogen. In the absence 
of experiment, we should never think of ascribing to this scant and 
varying constituent any important influence on terrestrial radiation ; 
yet its influence is far more potent than that of the great body of the 
air. To say that ona day of average humidity in England the atmos- 
pheric vapor exerts one hundred times the action of the air itself, would 
certainly be an understatement ofthe fact;. . . . and] am not prepared 
to say that the absorption by this substance is not two hundred times 
that of the air in which it is diffused.” 
And below he goes on: 
‘Probably a column of ordinary air ten feet long would intercept from 
ten to fifteen per cent. of the heat radiated from an obscure source, and 
I think it certain that the larger of these numbers fails to express the 
absorption of the terrestrial rays effected within ten feet of the earth’s 
surface.” 
But England has a very moist climate. If, then, the trace of 
vapor there be but the ,+,5 part of the whole, much less must it be 
elsewhere. . If we call it one <5 part of the main body of the air, 
in the drier regions of the earth, and consider the Martian atmos- 
phere at the surface of the planet to be one-seventh of our own at 
sea level, the vapor tension might be as great as ours and yet the 
total amount of vapor present but =, of the whole atmosphere. In 
which case it would render that air as effective a covering as our 
own. Iam far from saying that this is the case; the more so that, 
as we shall presently see, there are local conditions which, in the 
event of its being as copious as, would render it much more effec- 
tive than, ourown. But it is worth noting how little we need go 
out of our way in possibilities to furnish Mars with sufficient cov- 
ering. 
7. While we are on the subject of carbonic acid, we may note 
that that gas shows, unlike water vapor, remarkable exclusiveness 
in the absorption of heat. Angstrom finds its absorption belt in 
the spectrum very circumscribed, and in the lecture quoted above 
Tyndall tells us that practically it absorbs no heat but what 
radiates from carbonic acid itself. Such domesticity limits its 
absorptive efficiency in the world at large. The result of which is 
that there would be a tendency to equalize its deposit over the 
whole planet ; for the surface covered by carbonic acid snow would 
