There are also some mechanical difficulties in the way. Mr. John Ritchie[53] touches them when he says: "If the passage through which the water flows down is at all tubular the column will be subjected to the usual hydrostatic pressure." The word _tubular_ is the hard one to answer. Limestone rock fissures are certainly not tubular. They have all sorts of shapes and angles and corners, every one of which would interfere with anything like a regular pressure. [53] _Boston Transcript_, January 2d, 1897. This latter objection would not apply to borings in mines. I have been assured that in some borings in Western mines ice has been formed by pressure, and there may be truth in this, although I doubt it, as I have yet to hear of ice in any mines in warm latitudes. Mr. John Ritchie[54] has suggested, also, that if compressed air does not perhaps act strongly enough to form ice, yet it may help in keeping the temperature low and aid in the formation of draughts in caves and boulder heaps. At present, however, I can see no reason to think that the ice in caves is due to compressed air.[55] [54] _The Happy Thought._ Boston, January 23d, 1897. [55] See Part I.: page 89. III. I have already said that I believe that the cold of winter is the cause of the ice in caves. To make this clearer, I may say that I look on glacières as the last outcrop, the outside edge, so to speak, of the area of low temperatures, which has its culminating point in the Northern Hemisphere in the Arctic Ocean, Greenland and Siberia, and in the Southern Hemisphere in the Antarctic; and which is manifested to us in the snows of mountain peaks, and immediately round us in frozen ponds and rivers and snowy blizzards; and which, as it disappears each summer, leaves its last traces in our latitudes in sequestered gorges and convenient caverns. In every case, it seems to me, glacières are simply refrigerators, which preserve the ice and snow accumulated in them during the winter. They all follow the same general laws as to the origin of their contents, modified only in slight degree according to the varying natural local conditions, such as the water supply, or the protection from sun and wind, or the thickness of the overhead rock, or the altitude or latitude. I cannot see that there is anything remarkable about the fact that the cold of winter is able to penetrate and make itself felt sometimes for a slight depth in the earth's crust; a depth, so far as yet known, never exceeding one hundred and fifty meters. It seems to me that glacières only emphasize a law of nature, which has doubtless been formulated many times in connection with springs and phreatic waters, and that is, that where we find cold waters underground, we may be sure that they have penetrated from the outside.