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Three Vertical Posts

Three vertical posts are placed at an interval 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 8" below the top; find to the nearest mile the radius of the earth. (Hobson, Plane Trigonometry, 1891) Click here to reveal the answer

"Three Vertical Posts," Convergence (March 2005)