Limits of Natural Vision

The limits of nautical vision vary with elevation, conditions of the atmosphere, intensity of illumination, and other modifying
elements in different cases. In a clear day an object one foot above a level plain may be seen at the distance of 1.31 miles; one ten feet high, 4.15 miles; one twenty feet high, 5.86 miles; one 100 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) for the curvature of the earth, and assumes that the size and illumination of the object are sufficient to produce an image. Five miles may be taken
as the extreme limit at which a man is visible on a flat plain to an observer on the same level.