Suppose you are standing at the bank of a river at the point A and wish to know the width across to B. As you have no means of crossing the river, what is the easiest way of finding its width?
Measure any convenient distance along the bank from A. to C, say 40 yards. Then measure any distance perpendicularly to D, say 12 yards. Now sight along DB and find the point E. You can then measure the distance from A to E, which will here be 24 yards, and from E to C, which will be 16 yards. Now AB:DC = AE:EC, from which it is evident that AB, the width of the river, must be 18 yards. 
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