ABSORBING AND EMITTING SUEFACE. 165 



and the quantity of moisture wliicli tlie air is capable of contain- 

 ing, and, of course, other things being equal, the evaporation, rise 

 and fall with the thermometer. The hygroscopical and the ther- 

 moscopical conditions of the atmosphere are, therefore, insepa- 

 rably connected as reciprocally dependent quantities, and neither 

 can be fully discussed without taking notice of the other. The 

 leaves of living trees exhale enormous quantities 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 con- 

 ductor of heat, and as a mere covering of the ground, an influ- 

 ence on the temperature of the air and the earth, which may be 

 considered by itself. 



Absorhing a/nd Emittrng Surface. 



A given area of ground, as estimated by the every-day rule of 

 measurement in yards or acres, presents always the same apparent 

 quantity of absorbing, radiating and reflecting surface ; but the 

 real extent of that surface is very variable, depending, as it does, 

 upon its configuration and the bulk and form of the adventitious 

 objects it bears upon it ; and, besides, the true superflcies remain- 

 ing the same, its power of absorption, radiation, reflection and 

 conduction of heat will be much affected by its consistence, its 

 greater or less humidity, and its color, as well as by its inclination 

 of plane and exposure. An acre of clay, rolled hard and smooth, 

 would have great reflecting power, but its radiation would be 

 much increased by breaking it up into clods, because the actually 

 exposed surface would be greater, though the outline of the field 

 remained the same. The inequalities, natural or artificial, which 

 always occur in the surface of ordinary earth, affect in the same 

 way its quantity of superficies acting upon the temperature of the 

 atmosphere and acted on by it, though the amount of this action 

 and reaction is not susceptible of measurement. 



Analogous effects are produced by other objects, of whatevei 

 form or character, standing or lying upon the earth, and no solid 



