57 



Mr. Marsh says that, concerning the influence of the forest, considered as inorganic 

 matter on temperature : — " The evaporation of fluids and the condensation and expansion 

 of vapours and gases are attended with changes of temperature ; and the quantity of mois- 

 ture which the air is capable of containing ; and of course, other things being equal, the 

 evaporation rises and falls with the thermometer. The hygroscopical and the thermoscopi- 

 <;al conditions of the atmosphere are therefore inseparably connected as reciprocally de- 

 pendent 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 vapour, 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 pro- 

 cesses 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 tem- 

 perature of the air and the earth, which may be considered by itself. 



"Balance of Conflioting Influences of Forest on Atmospheric Heat and Humidity. 



" We have shown that the forest, considered as dead matter, tends to diminish the 

 moisture of the air, by preventing the sun's rays from reaching the ground and evapora- 

 ting the water that falls upon the surface, and also by spreading over the earth a spongy 

 mantle which sucks up and retains the humidity it receives from the atmosphere ; while, 

 at the same time, this covering acts in the contrary direction by accumulating in a reser- 

 "voir not wholly inaccessible to vaporizing influences, the water of precipitation which 

 might otherwise suddenly sink deep into the bowels of the earth, or flow by superficial 

 channels to other climatic regions. We now see that, as a living organism, it tends, on 

 the one hand, to diminish the humidity of the air, by sometimes absorbing moisture from 

 it, and, on the other, to increase that humidity by pouring out into the atmosphere, in a 

 vaporous form, the water it draws up through its roots. This last operation, at the same 

 time, lowers the temperature of the air in contact with or proximity to the wood, by the 

 same law as in other cases of the conversion of water into vapour. 



" As I have repeatedly said, we cannot measure the value of any one of these ele- 

 ments of climatic disturbance, raising or lowering of temperature, increase or diminution 

 of humidity ; nor can we say that in any one season, any one year, or any one fixed cycle, 

 however long or short, they balance and compensate each other. They are sometimes, but 

 certainly not always contemporaneous in their action, whether their tendency is in the 

 same or in opposite directions, and, therefore, their influence is sometimes cumulative, 

 sometimes conflicting, but, upon the whole, their general eifect is to mitigate extremes of 

 atmospheric heat and cold, moisture and drought. They serve as equalizers of tempera- 

 ture and humidity, and it is highly probable that in analogy with most other works and 

 workings of nature, they, at certain or uncertain periods restore the equilibrium, which, 

 whether as lifeless masses 6r as living organisms they may have temporarily disturbed. 



" When, therefore, man destroys these natural harmonizers of climatic discords, he 

 sacrifices an important conservative power, though it is far from certain that he has 

 thereby affected the mean, however much he may have exaggerated the extremes of at- 

 mospheric temperature and humidity, or, in other words, may have increased the range 

 and lengthened the scale of thermometric and hygrometric variations. 



" Special Influence of Woods on Precipitation. 



" With the question of the action of forests upon temperature and upon atmospheric 

 humidity is intimately connected that of their influence upon precipitation, which they 

 may affect by increasing or diminishing the warmth of the air and by absorbing or ex- 

 haling uncombined gas and aqueous vapour. The forest being a natural arrangement, the 

 presumption is that it exercises a conservative action, or at least a compensating one, and 



• consequently that its destruction must tend to produce pluviometrical disturbances as well 

 as thermometrical variations. And this is the opinion of perhaps the greatest number of 

 observers. Indeed, it is almost impossible to suppose that, under certain conditions of 

 time and place, the quantity and the periods of rain should not depend, more or less, upon 



'the presence or absence of forests ; and without insisting that the removal of forests has 



