feet for the latter; and as both these are far within the limits of surface inequality of level, the method, which assumes the surface as a plane, introduces no error in depth worth consideration; especially when it is remembered, that two contrary and mutually corrective errors are produced; the direct, effect of sphericity becoming evanescent being, to depress the observed depth of focus; but as the angle e is always measured from the horizon of the station of observation, so also sphericity will throw up, the line of the wave-path along the seismic vertical.
Referring now to the Diagram, No. 2; it will be found, that, out of twenty-six separate wave-paths, twenty-three start from the seismic vertical, at a depth of above 7⅛ geographical miles, or of 43·284 feet. The maximum depth is 8⅛ geographical miles, or 49·359 feet, and the minimum depth is 2¾ geographical miles, or 16·705 feet.
Eighteen of the wave-paths, start from the seismic vertical, within a vertical range in depth of 12,000 feet, and having a mean focal depth of 5¾ geographical miles, or of 34·930 feet, which may be taken as the depth of the focus; all these measurements being from the level of the sea. The extreme vertical range, between maximum and minimum depth, is 32·654 feet. On examining the Diagram, however, having regard to the points in the seismic vertical, whence the wave-paths start thickest, it will be apparent that the probable vertical depth of the focal cavity itself does not exceed 3 geographical miles, or 18·225 feet, at the outside.
