Some distance off plant a stick, a yard high, A B; one yard from this we plant another and longer one, C D. Measure exactly the distance, B F, and applying the eye at A, we aim at the summit of the tower, E; mark on the stick, C D, the point where the visual ray meets the stick, i.e., point G.

Then, by measuring the distance, D G, and subtracting one yard you get G I, and may be expressed in the following statement:

A H : A I :: E H : G I

In the given example let us suppose that A H = 150 yards, A I will, of course, be equal to one yard; G I =, say four fifths of a yard; the problem will be: 150 yards : 1 yard :: x : four fifths of a yard. Work the sum out, and the value of x is 120 yards.

Having taken our lease, A H, at one yard from the ground, we must add one yard to 120, making 121 yards, which is the height of the tower wanted.