The known facts as to the earth and its structure, beyond the knowledge derived from what is visible on its surface, are comparatively few. Its general shape has been determined and its principal dimensions and the proportion of flattening measured. The mean density has been computed. Increasing temperature below the surface has been observed. The interior of the earth is estimated to be more rigid than steel, but it yields to great forces. Its elasticity is observed in earth tides and in the transmission of earthquake shocks. Volcanic craters, geysers, and other outlets give information as to subsurface activities and materials. Earthquakes afford some measure of the more violent adjustments of the crust. The earth is a great magnet.

The greatest depth to which man has penetrated is but 7,579 feet, about l/2700th part of the earth’s radius. This limitation and the meager knowledge of the planet most important . . .

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