Origin of Springs, 293 



this rock from other higher districts, often issue with a much 

 lower temperature than is usually found in springs flowing out 

 at such a level. 4. The altitude at which the last springs 

 can occur depends on the general elevation of the mountain 

 mass ; their distance from the mean altitude of the summits 

 and crests is greater in limestone ranges than in those of 

 crystalline slates of equal height. In Alpine ranges of si- 

 milar geological formations this distance becomes far greater, 

 where they rise far above 9000 feet (French), where, owing to 

 the formation of steep precipices and summits, and of 

 massive snow-beds and glaciers, the depression of the limit 

 of springs is, comparatively, very considerable. 5. The di- 

 minution of the temperature, in proportion to the altitude, 

 does not take place in an equal arithmetical or geometrical 

 progression. In the valleys it progresses more slowly than 

 at the declivities and summits ; and cmteris paribus ad- 

 vances more rapidly at higher elevations. 6. Almost the 

 same temperature is found at the limits of the growth of 

 trees in the different Alpine ranges, although the altitude of 

 this limit may itself somewhat vary. We may take 3*5° C. 

 as the mean temperature. Immediately above the limit of 

 arboreal growth, we remark the most sudden diminution of 

 the ground temperature, and the most marked differences 

 between the various springs. 7. The springs in valleys are, 

 at equal heights, warmer than those on the declivities and 

 summits, and this is strikingly perceptible in the higher re- 

 gions. In like manner, owing to the greater radiation from 

 isolated rock masses, a remarkable depression of the ground 

 temperature takes place in the limestone Alps, on the free 

 declivities towards the north. 8. The minimum tempera- 

 ture of the highest springs in the Alps appears to be 0"8° C. 

 9. The height of the mountain ranges has considerable in- 

 fluence on the ground temperature. We find at equal alti- 

 tudes above the sea level the warmer springs where the 

 mean elevation is greater; the isogeothermal lines are thereby 

 subjected to curvatures analogous to those of the lines of 

 elevation in the district. These curvatures are shown in a 

 diagram representing a section of the Alps. The numerical 

 results of the numerous observations (given in a tabulated 

 form at pp. 269-273) made with reference to the temperature 



