1 882.] Forests— their Influence upon Climate and Rainfall 27 



have dried up, evaporation from the soil outside the woods has 

 almost ceased, transpiration continues unremittingly to furnish 

 atmospheric moisture in order to keep as nearly as possible a uni- 

 form proportion of this important substance in the air. What an 

 harmonious adaptation of means to an end does nature exhibit 

 here— plant life atomizing tons and tons of watery vapor into the 

 surrounding medium, even during time of drought, and this same 

 vapor in turn protecting luxuriant vegetation from the evil conse- 

 quences of terrestrial radiation. Moist air during winter tends to 

 moderate extreme cold, during the summer, on the contrary, it 

 tends to cool the draughts, hence forests by moistening the air in 

 summer give us cool and delightful breezes ; another means by 

 which forests affect extremes of temperature. 



This brings us face to face with the old question, do forests, 

 apart from their mechanical action, to any extent affect the rain- 

 fall? Be it remembered that the total annual evaporation and 

 rainfall bear a constant relation. We do not claim for forests that 

 they influence in any degree the general course of storms, for the 

 latter are governed by other and more general forces. May not 

 forests, however, influence the local distribution of rains and dews, 

 and within certain limits and periods of time, the amount of pre- 

 cipitation ? We have seen that during the spring and summer 

 the amount of water yielded to the atmosphere is very nearly 

 equivalent to half the rainfall, even at Philadelphia. Now, grant- 

 ing that our premises are correct, it will be conceded that a part, 

 at least, of the water atomized to the atmosphere by a wood, is 

 most likely returned to the surrounding country in the form of 

 rain or heavy mists. Where is this moisture given to the air by 

 trees condensed into rain, and how produced ? It has already 

 been stated that rain is usually formed from one to two miles 

 above the surface of the earth, hence it follows that forests located 

 on mountain ridges, besides strongly favoring the ascent of vapor- 

 laden currents by a mechanical effect as already pointed out, may 

 also have their own moisture readily condensed, owing to their 

 altitude as well as in the manner to be presently described. It 

 will also be remembered that in considering the mechanical action 

 of forests, it has been stated that when not situated upon mountain 

 ridges they are incapable of raising the vapor-laden currents suffi- 

 ciently high to be condensed into rain, and this is true, but there 

 is a notable exception to the rule that rain is produced at so 

 great an elevation as above indicated. 



