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pares with the size of the globe only as the skin of an apple does to the body of the fruit itself.

I shall first invite the reader's attention to some facts about weight, pressure, and gravity in the earth. Let us consider a cubic foot of soil forming part of the outer surface of the earth. This upper cubic foot presses upon its bottom with its own weight, perhaps one hundred and fifty pounds. The cubic foot below it weighs an equal amount, and therefore presses on its bottom with a force equal to its own weight with the weight of the other foot added to it. This continual increase of pressure goes on as we descend. Every square foot in the earth's interior sustains a pressure equal to the weight of a column of the earth a foot square extending to the surface. Not many yards below the surface this pressure will be measured in tons; at the depth of a mile it may be thirty or forty tons; at the depth of one hundred miles, thousands of tons; continually increasing to the centre. Under this enormous pressure the matter composing the inner portion of the earth is compressed to the density of a metal. By a process which we will hereafter describe, the mean density of the earth is known to be five and one half times that of water, while the superficial density is only two or three times that of water.

One of the most remarkable facts about the earth is that the temperature continually increases as we penetrate below the surface in deep mines. The rate of increase is different in different latitudes and regions. The general average is one degree Fahrenheit in fifty or sixty feet.