-1859] WRITINGS OF JOSEPH HENRY. 57 



world. It however in its turn will probably require the 

 consideration of modifying conditions in its applications; 

 and while at present the data are known with sufficient pre- 

 cision to warrant the assumption of the evolution of the im- 

 mense force we have mentioned, they are not in all cases 

 sufficiently well determined to enable us to predict, with 

 numerical accuracy, the results which have been shown to 

 proceed from them. The same principle of condensation of 

 vapor and evolution of heat is fertile in the explanation of 

 the approximate cause of rain: for example, so long as the 

 wind blows over a surface of uniform height and tempera- 

 ture, there is no cause to induce it to precipitate its vapor; 

 but if in its course it should meet a mountain, the slope of 

 which it is obliged to ascend, the vapor will be condensed 

 on the windward side by the cold due to the increased ver- 

 tical height. The latent heat will be evolved, the circum- 

 ambient air will be abnormall}^ heated, and an upward 

 motion will ensue, towards which air will flow with increas- 

 ing velocity to restore the equilibrium of the ascending 

 column. In this way Mr. Espy explains very satisfactorily 

 the fact that the wind blows over the desert of Sahara to 

 supply the diminished pressure occasioned by the rains over 

 the windward side of the Himalaya mountains. The same 

 principle is immediately applicable to the explanation of 

 the rainless districts in South America, Mexico, and other 

 portions of the earth. The air, as it ascends on the wind- 

 ward side of the mountains, deposits its moisture ; and if the 

 elevation is sufficiently high, it will pass over in a desiccated 

 condition. 



The idea that mountains attract vapor is not founded on 

 any well established principle of science. Molecular attrac- 

 tion extends only to imperceptible distances, and the attrac- 

 tion of gravitation is too feeble a force to produce results of 

 this kind. The evaporation of water, and the transfer and 

 subsequent condensation of the vapor in other parts of the 

 earth, is undoubtedly the most active cause which produces 

 the continual and apparently fitful changes of the weather. 



We have stated that within the torrid zone there exists a 



