that the surfaces over which any currents there may be are constrained to pass, present an abundance of moisture to refrigerate the currents; and it is not unreasonable to suppose that the large number of evaporating surfaces, which currents passing through heaps of debris—­such as the basaltic stones described on page 261—­come in contact with, are the main cause of the specially low temperature observed under such circumstances.

Pictet’s theory, however, did not convince all those into whose hands his paper fell, and M.J.  Deluc wrote against it in the Annales de Chimie et de Physique of the same year, 1822.[192] Deluc had not seen any glaciere, but he was enabled to decide against the cold-current theory by a perusal of Pictet’s own details, and of one of the accounts of the cave near Besancon.  He objected, that in many cases the ice is found to melt in summer, instead of forming then; and also, that in the Glaciere of S. Georges, which Pictet had described, there was no current whatever.  Further, in all the cases of cold currents investigated or mentioned by De Saussure, the presence of summer ice was never even hinted at, and the lowest temperatures observed by him were considerably above the freezing point.  I may add, from my own experience, that on the only occasions on which I found a decided current in a glaciere—­viz., in the Glaciere of Monthezy, and that of Chappet-sur-Villaz,—­there was marked thaw in connection with the current.  In the latter case, the channel from which the current came was filled with water; and in the former, water stood on the surface of the ice.

The view which Deluc adopted was one which I have myself independently formed; and he would probably have written with more force if he had been acquainted with various small details relating to the position and surroundings of many of the caves.  The heavy cold air of winter sinks down into the glacieres, and the lighter warm air of summer cannot on ordinary principles of gravitation dislodge it, so that heat is very slowly spread in the caves; and even when some amount of heat does reach the ice, the latter melts but slowly, for ice absorbs 60 deg.  C. of heat in melting; and thus, when ice is once formed, it becomes a material guarantee for the permanence of cold in the cave.

For this explanation to hold good, it is necessary that the level at which the ice is found should be below the level of the entrance to the cave; otherwise the mere weight of the cold air would cause it to leave its prison as soon as the spring warmth arrived.  In every single case that has come under my observation, this condition has been emphatically fulfilled.  It is necessary, also, that the cave should be protected from direct radiation, as the gravitation of cold air has nothing to do with resistance to that powerful means of introducing heat.  This condition, also, is fulfilled by nature in all the glacieres I have visited, excepting that of S. Georges; and there art