On a clear day an object one foot above a level plain may be seen at a distance of 1.31 miles; one ten feet high, 4.15 miles;one twenty feet high, 5.86 miles; one100 feet high, 13.1 miles; one a mile high, as the top of a mountain, 95.23 miles. This allows seven inches; or, to be exact, 6.99 inches from the curvature of the earth, and assumes that the size and illumination of the object are sufficient to produce an
image.