1857.] Report on the Progress of the Magnetic Survey. 119 



ture of the water below the cover is considerably raised by insola- 

 tion during the day, whilst it loses little heat by nocturnal radiation, 

 (a phenomenon of accumulated heat analogous to that of Saussure's 

 and Sir John Herschel's Helio thermometers). In the salt pools the 

 loss of heat by noctural radiation is still more lessened by a circum- 

 stance peculiar to perfectly concentrated saline solutions, namely, 

 that all the little openings in the superficial salt cover which exist 

 during the day, are closed by a deposit of solid salt as soon as the 

 temperature begins to fall in the evening. In the lower strata of 

 the water contained in those conical reservoirs the temperature 

 decreases very rapidly, a phenomenon which seems perfectly to 

 agree with the explanation given above, but to exclude the supposi- 

 tion that the temperature is raised by heat having its origin in 

 the lower strata of the ground, as is the case in hot springs. 

 Besides, artificial salt pools were constructed immediately after 

 our return to Leh, and we used the materials, (salt and clay) 

 which we had brought for this purpose from the salt pools in 

 the Karakash Valley. The variation of temperature in these 

 artificial salt pools was observed from hour to hour, and these 

 observations equally showed that the excess of temperature only 

 begins after the formation of a solid stratum of salt, which more 

 or less closes up the surface of the pools. 



Meteorology. 



The snow line (the highest summer limit of perpetual snow), was 

 found to attain the greatest height in the environs of Karakorum, 

 its height exceeding 18,600 feet. The snow line sinks very little 

 towards Leh, but considerably on the northern slopes of the 

 mountains towards Khotan. Near Oitash, above Bushia, we found 

 its height not exceeding 16,000 feet. 



Bas-Neves completely hidden under detritus, a sort of subterra- 

 nean glaciers, but generally speaking of limited extent, which we 

 had seen first on the slopes descending from the Parang Pass, oc- 

 curred again in the Kuenluen on the northern slopes of the Sassar 

 mountains. In this group, which we crossed twice, we had an 

 opportunity of determining the variation of the snow line. In the 

 middle of September it had already sensibly descended, whilst the 



