ABSORBING AND EMITTING SURFACE. 167 
hills near the town has proved very prejudicial to this valu- 
able tree. 
Marchand informs us that, since the felling of the woods, 
late spring frosts are more frequent in many localities north 
of the Alps; that fruit-trees thrive no longer, and that it is 
difficult even to raise young fruit-trees.* 
Influence of the Forest, considered as Inorganic Matter, on 
Temperature. 
The evaporation of fluids, and the condensation and expan- 
sion of vapors and gases, are attended with changes of temper- 
ature; and the quantity of moisture which the air is capable of 
containing, and of course, other things being equal, the evapo- 
ration, rise and fall with the thermometer. The hygroscopical 
and the thermoscopical conditions of the atmosphere are, there- 
fore, inseparably connected as reciprocally dependent quantities, 
and neither can be fully discussed without taking notice of 
the other. The leaves of living trees exhale enormous quan- 
tities of gas and of aqueous vapor, and they largely absorb 
gases, and, under certain conditions, probably also water. 
Hence they affect more or less powerfully the temperature as 
well as the humidity of the air. But the forest, regarded 
purely as inorganic matter, and without reference to its living 
processes of absorption and exhalation of gases and of water, 
has, as an absorbent, a radiator and a conductor of heat, and as 
a mere covering of the ground, an influence on the temperature 
of the air and the earth, which may be considered by itself. 
Absorbing and Emitting Surface. 
A given area of ground, as estimated by the every-day rule 
of measurement in yards or acres, presents always the same 
* Ueber die Entwaldung der Gebirge, p. 28. 
Interesting facts and observations on this point will be found in the valuable 
Report on the Effects of the Destruction of the Forests in Wisconsin, by LAPHAM 
and others, pp. 6, 18, 20. 
