Hot and Thermal Springs. B69 



by the same cause, namely, the difference of density between 

 the air within and the atmosphere.* 



If the space through which a glacier stream flows, between 

 the under surface of a glacier and the soil on which it rests, may 

 be considered as one continued channel from the cleft B, (in Fig. 

 ], Plate 1, of next, or Vol. XXIV. of this Journal) to the 

 ice-cavern A ; then in summer, when the external temperature 

 is above 32% the temperature within being 32°, an uninter- 

 rupted current of air will issue from A. Hence the continual 

 draught of cold air from the ice caverns, which is greater the 

 higher the temperature of the atmosphere, and is consequently 

 found to be more violent in the afternoon, growing weaker in 

 the night, and almost ceasing towards break of day. Let us 

 suppose, for example, as is the case with the lower glacier at 

 Grindelwald, that B C =2140 feet, let the temperature of the 

 atmosphere be 86°.0 t, and that of the air in the channel beneath 

 the glacier 32°, then the density of the column of air A G will be 

 to the density of the column A B as 10 to 11. The velocity of 

 the motion of the air in the channel depends on the difference 

 of weight between these two columns of air, both supposed to 

 be perpendicular. If we express the weights of the two co- 

 lumns of air of 2140 feet high, by the heights of two columns of 

 water of respectively equal weights, we shall find — 



For BC (column of cold air), . . 399.9 lines. 



For A G (column of warm air), . . 363.6 lines. 



Difference, 36.3 lines. 



For one square foot of the section of the channel, or of the co- 

 lumn of air, this difference would be equal to the weight of a 

 quarter of a cubic foot of water, equal to 16,5 lb. Supposing the 

 channel there to have no outlet from A to B, and to be wider 

 than its lower opening in the same proportion that the tube of 

 a large bellows bears to its contrated aperture ; then, according 



* I am indebted for the following calculation to the kindness of my friend 

 and fellow-traveller Mr Althans, architect to the smelting works at Sayii, 

 with whom I held frequent consultations, during our visits to several glaciers, 

 on the various circumstances related above, and the results of which I have 

 made use in preparing this chapter. 



t Khel, vol i. p. 6. 



