Without taking into consideration the increase of density in water at 2 miles deep, at that depth we may suppose we have come to solid matter, the specific gravity of which could not be less than twice that of water, on account of the pressure of that depth of water upon it. If we now go down 2-1/2 miles further we shall have the solid matter subjected to a pressure proportioned to that depth; and if we take its weight per cubic foot at an average between granite (at 163 lb.) and earth (at 77 lb.), or 120 lb., the pressure at 2-1/2 miles deep of solid matter alone will be about 700 tons per square foot, or just about the crushing strain of our strongest granites, and therefore, the density of the matter under it must be equal to that of granite, or 2·5 times that of water. We do not add the pressure of the water, at present, because that may be looked upon by some people as of the same nature as of the atmosphere upon a human body, which neither increases the pressure upon it nor adds to its weight; but we see that at that depth the solid matter must have a density equal to the average between water on its surface and 2·5--that of granite; and if we choose to take the average between 2 miles of water and 2-1/2 miles of solid matter, we shall have 1·82 as the average density of the outer 4-1/2 miles in thickness of the crust of the earth. For our purposes, however, and for obvious reasons, we shall consider the average density of the 2-1/2 miles alone of solid matter to be 2·25 times that of water. We shall now go down to 9 miles deep, because the diameter of 7918 miles we have adopted for the earth will there be reduced to 7900 miles, which will be convenient for our further operations. At that depth we shall have a superincumbent pressure at the very least as follows:-- Tons. At 2 miles deep, 2 miles of sea at 150 tons per mile 300 2-1/2 " 2-1/2 " solid matter at spec. grav. 2·25 equal to 331·25 tons per mile 828 9 " 4-1/2 " of rock at 163 lb. per cubic foot 1730 ---- Total pressure per square foot 2858 or just about 4 times the crushing strain of our best granites. Then, as when crushing takes place compression begins, it will, we believe, be far below the mark to estimate the general specific gravity of the earth at 9 miles deep to be 3 times that of water.