Time of Formation of Ice.—Everything I have seen points to the fact that ice begins to form in a cave as soon as the temperature of the cave has sunk below freezing point, whenever, from any cause, water gets into the cave. The cold may begin to penetrate caves as soon as outside frosts have occurred, that is in the fall months, about November; and as soon as the temperature inside sinks below freezing point, ice will begin to form, provided also that water gets into the cave, from rains or springs or any other source.
In the mid-winter months, although there is then plenty of cold, the water supply is generally lacking, as the outside moisture is mostly frozen up and the result is that the winter months are not those when the ice is mainly formed. Some is undoubtedly formed in certain caves whenever during the course of the winter a surface thaw outside furnishes water to the cave,[66] but in other cases this is not so and the ice does not appear before the spring. In all cases it is in the spring, before the cave has parted with its store of cold, and when both the air and the rock walls are chilled below freezing point[67] that the ice forms fastest. Then plenty of water is furnished by the melting of the snows and the unlocking of the brooks, and also by early spring rains. All this surface water runs through the fissures into the still freezing cave and there becomes ice. Not only the air, but also the rock walls are chilled below freezing point, and as the rocks part slowly with the cold stored in them, this cold helps to freeze the water pouring in.
[66] See Part III.: Chaux-les-Passavant, [page 203]; Saint-Georges, [page 220].
[67] See Part IV.: Townson, [page 275]; Thury, [page 285]; Trouillet, [page 297]; Schwalbe, [page 298]; Terlanday, [page 301]; Kovarik, [page 307].
The natural law in relation to time seems to be this: Ice may be formed in caves as soon as the outside temperature sinks below freezing point. In some caves it forms intermittently all through the cold months because there is a water supply. In other caves it only forms in the spring, because there is no water supply in the winter months. In all cases, however, the end of winter is the time when most of the ice is formed.
[PART III.]
LIST OF GLACIÈRES.