"Clay is not only nearly impervious to air and water, but it is one of the worst conductors of heat in nature. (Note.--To test the question whether clay was a poor conductor of heat or not, I took two basins of equal size, and in one put a coating of clay one-half inch thick, into which I put water of a temperature of 52° Fahrenheit. Into the other dish, which was clean, I put water of the same temperature, and subjected the two basins to equal amounts of heat; and in five minutes the water in the clean dish indicated a temperature of 70° while that of the one coated with clay was raised only to 56°.) If we can rely upon the statements of those who dug out the frozen earth, it rested upon a stratum of clay that lay upon the bed of pebbles in which the water was found, for it was described as being a very sticky kind of hard pan. "This being the case, if the water contained in the pebbly mass had a temperature above the freezing point, the heat would be but imperfectly transmitted to the frost, through the clay, provided there was no other way for its escape. But we have seen that the stratum of clay that overlays the bed of pebbles in the side of the gravel pit was not horizontal, but inclined towards the well at an angle of 25°. Now if this drip was continued to the well, and existed there (which is highly probable), it will be seen that the ascending current of heated air, in the pebbly bed, would be checked upon meeting the overlying barrier of clay and be deflected out of its upward course. The tendency of heated air is to rise, hence it would continue its course along the under side of the clay, through the interstices in the bed of pebbles, till it found a place of escape at the surface, which in this case may have been at the gravel pit before named."