Intertropical Springs and Bhers. 101 



wells in the deserts of Egypt, like those of Ajmir and the western 

 deserts of India, are frequently of great depth, lying under strata of 

 sand, gravel, and calcareous sandstone, on an argillaceous or marly 

 bed, sometimes at a depth of 300 feet below the surface of the sur- 

 rounding country. In the granitic districts of Upper Egypt,— in 

 the Thebaid districts, — however, I have observed springs rising 

 through the almost vertical strata to the surface. 



In India, most of the wells marked as occurring in granite, trap, 

 limestone, and sandstone, result from springs, and are consequently 

 not so much influenced in temperature by the monsoon rains as thosd 

 in lateritic rocks, which,' from their porous structure, admit of tho 

 percolation of rain water to a considerable extent. 



The temperature was generally taken at about 10 a.m., a time 

 when I found it to approach nearest the diurnal mean ; and, when- 

 ever practicable, at the depth of about ten feet from the surface. 



The following are the general results of many hundred observa- 

 tions : — 



\stj In low latitudes the temperature of the deepest wells and 

 springs is a little higher than the mean temperature of the air. Ex- 

 ceptions occur; for example, the temperature of a deep well at 

 Gadiganur, on the banks of the Toombuddra, between the 16th and 

 IGtli parallels of nortb latitude, at an elevation of about 1200 feet 

 from the sea, was so low as 72°*5 (the temperature of the air in the 

 shade, at the time of observation, 80°- 5), while that of the springs 

 and river in the vicinity was from 77° to 79°*5. Ranges of hills, 

 attaining an altitude of 1500 feet above the plain, rose at no great 

 distance, a circumstance suggesting the probability that the cold 

 spring had its source at an elevation having a mean temperature 

 lower than that of the plain where the water appears on the surface. 



2d, The temperature of strongly saline and sulphureous springs 

 is, on the average, higher than those of pure water. 



3g?, Both saline and cold springs are seen to occur within a few 

 feet from thermal and fresh water springs ; a fact to be ascribed 

 probably to their rising through different seams of the subjacent 

 strata (often highly inclined), and to the different depths and heights 

 from which the supply of water is derived. 



4ith, The temperature of wells, particularly those with a small 

 area much used for purposes of irrigation, is thereby artificially in- 

 creased. 



bth, The temperature of shallow exposed wells, springs, and 

 rivers, especially such as have sandy beds, is subject to great diurnal 

 fluctuations, conforming, though to a less extent, to that of the su- 

 perincumbent atmosphere. The surface water of deep wells par- 

 takes of this fluctuation, to a depth varying according to the trans- 

 parency of the water, extent of surface, degree of exposure, and 

 clearness of the sky. In muddy water, the surface is heated to 9 



