202 APPENDIX TO MEMOIR OF PELTIER. 



ancewitli the sensation experienced by living beings. Often, in fact, inclividnals 

 realize a much sharper cold or intense heat than the thermometer seems to verify ; 

 this depends evidently on the hygrometric condition of the air. In its natural 

 state the body of man is always covered with a film of humidity, an insensible 

 transpiration. If the air is calm and saturated with humidity there will be no 

 evaporation on the surface, and complaint will be made of oppressive heat ; if, 

 on the contrary, the air is dry and agitated by the wind, the evaporation will b6 

 considerable, and a disagreeable sensation of cold will be com})lained of, alto- 

 gether disproportioned to the thermometric indications. 



Before concluding this section a word should be said of the effect of denuding 

 mountains of their trees on the annual mean quantity of rain, on its distribution, 

 and consequently on the climate. This influence, long denied, is now every- 

 where admitted — facts speak loudly enough for that. As to the explanation, 

 we shall endeavor to give it. 



The clouds which are in the middle region of the atmosphere are almost always 

 resinous. As long as their electric tension is moderate and inferior to the tension 

 of the earth, this latter repels them and keeps them at a greater height than 

 comports with their specific gravity. When these clouds pass above naked 

 and woodless mountains, inasmuch as the mountains more nearly ajjproach 

 them, the action in question is more efficacious, and the clouds are forced to 

 ascend somewhat higher in consequence of the energetic repulsion exerted by 

 the mountains. In this case the clouds pass without a discharge of rain. If, on 

 the contrary', the clouds have a considerable electric tension, this tension is more 

 powerful than that of the earth. When, therefore, these clouds pass over moun- 

 tains destitute of trees, their resinous electricit}' represses the resinous electricity 

 of the mountains into the interior of the soil, decomposes a portion of their 

 natural electricity, and attracts the vitreous to the surface. The phenomena of 

 repulsion are then changed into the phenomena of attraction, and the cloud is 

 wholly precipitated, and that with violence, upon the mountain. 



When the country is mountainous and wooded, the occurrence is quite different. 

 I have already said that vapors, transparent or opaque, were kept at distance by 

 two forces, heat and electricity; that all the phenomena which diminished by 

 their action one or the other of these two forces, induced indirectly the con- 

 densation of the vapors, and consequently the precipitation of a part of them. 

 These principles are directly applicable to the question with which we are 

 engaged. When masses of transparent or opaque vapors, charged with resin- 

 ous electricity, pass above wooded mountains, the vitreous electricity developed 

 by influence in the soil flows off" by the trees, which furnish thousands of points, 

 and neutralizes a part of the resinous electricity of the super-jacent masses of 

 vapor. The vapors, being less repelled, draw together and are condensed, the 

 transparent vapors into opaque vapors, and these into drops of rain which fall in 

 a regular manner and in measure proportionate to their formation. 



In sum, then, the cloud, in the case of mountains naked and divested of wood, 

 either passes without discharge or is precipitated in its entire mass ; the result is 

 an incessant oscillation from great drought to deluges of rain ; in wooded moun- 

 tains, on the contrary, the rains are gentle and continuous. From this we may 

 see that to denude mountains of their woods does not perhaps diminish the 

 annual quantity of rain, but that it modifies the distribution of the rain, or, to 

 speak with more exactness, its mode of precipitation. 



