RECLAMATION OF DESERTS. X51 



trees, in imparting warmth to the surrounding air. During the 

 winter season the temperature of trees, which are protected by an 

 impervious covering, does not fall so low as the temperature of 

 the surrounding air ; consequently a small amount of heat is im- 

 parted to the air directly from the trees, which serve as conden- 

 tors of heat from the ground. Another cause that was noticed is 

 the friction resulting from the moving of winds through forests. 

 A third cause, and one which perhaps exercises a greater influence 

 than either or both of the others, is the condensation on the side 

 where the compression takes place. You are all aware of the fact, 

 that when air is compressed to any great extent a large amount of 

 heat is evolved. When winds sweep down upon a forest, conden- 

 sation of air takes place, by which an amount of heat is evolved 

 or the temperature of the air elevated, and this heated air is swept 

 along through the forest, and may be regarded a third cause of the 

 elevation of temperature in the central part of the forest. 



Mr. Chamberlain. There is another question which I would 

 like to ask, and that is, if he did not omit to state an important 

 physical fact in connection with his remark on the engineering 

 project of letting water upon the Desert of Sahara ? 



Prof. Fernald. That this can be done at all is due to the 

 fact that the central portions of the desert are lower than the level 

 of the Red Sea, and consequently, when the barrier around the 

 desert can be cut through, the water will flow in of itself. If this 

 shall ever be done, the water of the Nile will undoubtedly be 

 changed in its course, and be made to contribute its fertilizing de- 

 posits to the desert. 



Mr. Chamberlain. A remark was made by the lecturer on the 

 possibility of reclaiming present desert places through human 

 agency. I will ask him if he is able to state what has already 

 been accomplished on the border of this same desert by French 

 engineering in Algeria ? 



Prof. Fernald. The French in Algeria have already carried on 

 this process of reclamation, to a certain extent, by boring artesian 

 wells. It is found that they do not have to bore to a very great 

 depth before they come to an impervious layer in the earth, be- 

 neath which is a pervious stratum, and beneath this is still another 

 impervious layer. They have but to penetrate this upper imper- 

 vious stratum, and the waters supplied from the higher lands 

 north of the desert to this pervious stratum, gush out. This pro- 

 cess of irrigation has been carried on to a considerable extent by 



