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Three Vertical Posts

Three vertical posts are placed at intervals of one mile along a straight canal, each rising to the same height above the surface of the water. The line of vision joining the tops of the two extreme posts cuts the middle post at a point eight inches below the top; find to the nearest mile the radius of the earth.

A Treatise on Plane Trigonometry, Ernest Hobson, 1891

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"Three Vertical Posts," Convergence (March 2005)