Page:Popular Science Monthly Volume 82.djvu/291

Rh spring of the year, as the snow on the hillside begins to melt and the frost comes out of the ground, water naturally begins to trickle down the sides of the shaft, where, strange as it may seem, it is frozen in the form of small icicles. This freezing process continues, until by July the sides of the pit are completely covered with a coating of ice a foot or more in thickness. In the early fall the process stops and the formation of ice gradually melts. The sides of the shaft are of loose shale, in which there are numerous crevices extending back and up into the hill, the rock strata being rather sharply inclined. A draught of cold air, which at some places is strong enough to extinguish the flame of a small taper, issues from these fissures in the summer time. This draught is variable, being stronger on hot than on cool days. A heavy mist may also be seen rising out of the pit and floating off down the hill close to the ground. The temperature of the pit during the past summer varied between 25 and 32 degrees Fahrenheit.

The explanation of this phenomenon appears to lie in the cold currents of air issuing from the crevices of the rocks along the sides of the shaft. The air must gain access to these fissures at some other point, which must be at a higher altitude than that of the pit, as will be seen from the following discussion.

This being true, it is evident that in the winter time the column of air directly over the pit is cooler and consequently heavier than that in the rock passages. Therefore, it forces its way down into the pit and up through the rock strata, chilling the rocks to a great depth and storing up a vast quantity of "cold." We see, then, that the amount of "cold" which is stored up, or the depth to which the rocks are chilled at the beginning of warm weather in the spring, depends upon the length and severity of the winter.

As the warm weather comes on the column of air over the pit becomes heated and is displaced by the cold, heavy air flowing down out of the passages. This cold current of air freezes any surface water which flows over the edges of the pit and maintains a freezing temperature as long as the supply of "cold" in the hill lasts, after which the circulation of air ceases and the ice formation melts.