Christoph pit, it is found that the mean temperature, at 15 fathoms below the surface, is only slightly above 42 deg.  F.; while at the Morgenroether cross-cut the same mean temperature is found at a depth of 46 fathoms.  The annual change of temperature is very small in these mines, and the maximum and minimum are reached very late; so that, if a point could be found with a mean temperature of 32 deg.  F., ice would increase there up to June or even July, and then diminish until December or January; in which case the phenomenon so often said to be observed in connection with subterranean ice—­the melting in winter and forming in summer—­would really be presented.

The ice on the Sauberg is frequently found to commence at a depth of 3 or 4 fathoms, and in the years 1811 and 1813 it extended to 24 fathoms below the surface:  this depth, however, was exceptionally great, and as a rule the limit is reached at about 14 fathoms.[137] The ice is usually not very firm, and can be broken by stout blows with a stick; but between the years 1790 and 1800, when it was found at a depth of from 3 to 9 fathoms, it was so hard that blasting became necessary, and at that time the miners were with difficulty protected from the effects of the severe cold.  The greatest quantity of ice is found in the interstices of the rubbish-beds of old workings, and here it assumes a crystalline form, the rocks being covered with a ‘fibrous’ structure, arranged perpendicularly to their surface.

Reich reports the universal presence of cold currents of air in these shafts and mines, and, in consequence, takes the opportunity of contradicting a statement in Horner’s Physik.  Woerterbuch,[138] that the absence of all current of air is essential to the formation of subterranean ice.  He quotes the case of the cheese-caves of Roquefort as a further confirmation of his own observations with regard to the connection between ice in caves and cold currents of air; but of the many accounts which I have met with of the curious caves referred to, both in books and from the lips of those who have visited them, not one has made any mention of ice.[139] He states, too, that when the strength of the current is diminished, its temperature is increased; a fact which all observations of the cold currents in caves, especially those made with so much care by M. Saussure, abundantly establish.

In the way of explanation, Reich mentions the possibility of rocks of peculiar formation possessing actually a low degree of temperature;[140] but he rejects this suggestion, preferring to believe that in some cases the cold resulting from evaporation is the cause of ice, and in others the greater specific gravity of cold as compared with warmer air.

In the Bulletin des Sciences Naturelles,[141] it is stated that a large quantity of ice is found in one of the recesses of the grotto of Antiparos—­a fact which I have not seen mentioned elsewhere.  After penetrating a long way through difficult fissures, a square chamber is at length reached, measuring 300 feet in length and breadth, with a height of about 80 feet.  The walls and roof and floor are beautifully decorated with ice, and reflect all the colours of the rainbow.  There are groups of pyramidal and round columns, and in some parts of the cave screens or curtains of ice 10 or 12 feet broad hang down to the floor.