[25] See Part IV.: Butler, page 308.
From the few winter observations we have, there can be no doubt that in winter the movements of the atmosphere are lively, the break in the air column occurring as soon as the outside temperature is lower than that within, when the outer air immediately begins to sink into the cave.[26]
[26] See Part III.: Chaux-les-Passavant, [page 203]; Saint-Georges, [page 220].
If I have doubts as to the existence of absolutely static caves, it is different about dynamic caves. When a subterranean hollow goes through rocks, with one opening higher than the other, there will surely be distinct draughts. These dynamic caves exist in many parts of the world under such names as cold current caves or blowing caves or windholes. Sometimes they are fissures in broken limestone. Often they are the cracks between piles of boulders. A cool air generally pours from the lower opening in summer while the cold air pours into it in winter, the draught being then reversed. At the upper opening the operation takes place in the opposite way, the hot air being sucked in in summer, and given out in winter. Sometimes, however, changes take place, according to the differences in the outside temperature, in the direction of the air current in the course of a single day.
The causes of the movements of air in these windholes are exceedingly simple. The movements of air depend on the fact that in summer the air in the tube becomes colder from contact with the rocks and, therefore, heavier than the air outside, and by gravity the heavy inside air displaces the lighter outside air and comes rushing out at the lower opening. This leaves a vacuum, which is filled by the warmer air dropping into the tube from above. In winter on the contrary, the air within the tube is warmed by contact with the rocks and becomes lighter than the air outside. It, therefore, rises and streams out from the upper opening, and the vacuum is filled by the heavy cold air pushing in at the lower opening.
Fig. 11. Vertical Section of a Windhole.
G. F. Parrot’s[27] explanation is so satisfactory that I give it with one or two changes. He considers the air movements an ordinary statical phenomenon of the air, in caves which have two openings at different altitudes. Let E G D represent the section of such a cave with the openings A and B. Let us think that there are over C and B two vertical air columns and from B to C a horizontal air column B C; then the two air columns over B and C are at all times of the year equal in weight. Not so the air columns A C and A E G D B, because their temperatures are different. Assume the temperature in the cave G is +12° the whole year round. If in summer the air column A C is at a temperature of +25°, then the heavy air in the cave G pours out through A and is replaced by air flowing in through B. If in winter the air column A C is at a temperature of -1°, then the air pours with equal inverse velocity at A into the cave, and out at B. The velocity of the current in both cases depends on the difference of temperature within and without.