B. TERRESTRIAL PHYSICS AND METEOROLOGY. 25 



spouts. Third, the cooling of the mass of air by coming into 

 contact with a cooler mass of air than itself; this forms cir- 

 rus. 12 A, November 10, 1871, 28. 



DIRECT CONDENSATION OF WATERY VAPOR. 



Professor Forel, of Lausanne, after long-continued observa- 

 tion, has determined the quantity of water passing the Rhone 

 below the Lake of Geneva, and finds that to furnish this 

 amount it would require an atmospheric precipitation in the 

 basin above of nearly 45 inches. The actual precipitation, 

 however, amounts to but 27-J inches, and the question arises, 

 therefore, whence comes the surplus water? Professor Du- 

 four finds its origin in the direct condensation of the atmos- 

 pheric vapor on the ice, the cold rocks, and the snow-fields 

 of the Alps. The following experiment may serve to eluci- 

 date the principle involved: A vessel containing a cooling 

 mixture of 672 grams weight, on being exposed for an hour 

 in the calm, open air, increased five grams in weight from the 

 vapor condensed on its exterior. Direct measurements at 

 suitable points would be interesting for the purpose of ascer- 

 taining approximately what quantity of water is thus actu- 

 ally carried to the river. F. (7., 1871, 179. 



INFLUENCE OF TREES ON CLIMATE. 



The subject of the influence of " foresting," or the planting 

 of trees, uj^on the climate of a country, and of " deforesting," 

 or destroying the forest growth, continues to excite much in- 

 terest throughout the world, as it is now well established that 

 the climate of many localities has been materially altered by 

 one or the other of these processes. Systematic efforts have 

 been made, in different parts of the world, for introducing a 

 growth of trees where these had either disappeared or had 

 never been known, from which important results have fol- 

 lowed in many instances. "We well know the effect upon the 

 climate of India of planting extensive forests of different spe- 

 cies; and we are informed that, as the consequence of a sim- 

 ilar experiment, Egypt, which formerly had only about six 

 rainy days every year, since being replanted on a large scale, 

 has already attained to twenty-four. Among the enlightened 

 measures of the administration of the French government, 

 %ne which is especially noteworthy is that of planting ira- 



B 



