26 S. Newcomb—Some points in Climatology. 
glacial epoch at a comparatively moderate height in the atmo- 
sphere and on the tops of most high ranges of mountains far 
removed from the equator. It is evident that if, at any former 
epoch, the state of things at the surface of the ground was the 
same that it now is at the height of two or three miles in the 
atmosphere, there must have been a glacial epoch. To what 
cause are we to attribute the cold of the upper regions of the 
air? There are two known causes but we cannot assign an 
exact quantitative effect to each. 
I. The passage of air from the lower to the upper regions is- 
accompanied by expansion, and the reverse motion by com- 
pression, which would naturally result in the upper regions be- 
ing colder than the lower: the exact amount of cooling, sup- 
posing no disturbing cause to come into play, is readily 
computed, and has, I think, been assigned by Professor Sir 
William Thompson and others, but I need not now refer to the 
results. é 
II. Researches on radiant heat seem to show that the atmo- 
sphere absorbs the extreme rays of the spectrum, especially 
those of greatest wave length, more powerfully than the rays of 
mean wave length. The rays radiated by the earth are of 
longer wave length than the great mass of those received by 
the sun. The natural result of this selective absorption would 
be to make the temperature of the earth higher than if there 
were no atmosphere, or if the atmosphere exercised no selec- 
tive absorption on heat rays. It seems probable that this selec- 
tive absorption is due, very largely if not entirely, to aqueous. 
vapor in the air. If this be so, an epoch of dry air would be 
a glacial one. 
A crude test of the efficacy of the first cause might be de- 
vised. In order that it may act it is essential that there shall 
be a continuous interchange of air between low and high alti- 
tudes. Now if there are any high table lands so extended that 
in their central portions the air has not during several days an 
ee to be replenished from lower regions, such air 
should be warmer than that at an equal height on isolated 
mountains. Probably the conditions for such an observation 
do not exist on the earth’s surface. 
n conclusion I may be allowed to express my regret at not 
being able to make a contribution of positive value to the in- 
vestigation of this subject. The state of the question is about 
this: A well founded theory of terrestrial temperature can be 
built only upon an accurate knowledge of the laws of emission 
and absorption of radiant energy of different wave lengths, 
especially in the atmosphere, and the result will appear as a 
numerical calculation, more or less exact, of the temperature 
resulting from assigned conditions, and not as the conclusion of 
an argument to show one thing or another. 
