c h f a b - e n g f - d

When you wish to measure a river in its width, move away from its bank somewhat more than the width of the river itself, and sight some known mark on the opposite bank of that river. Thus, let the width of the river be a b and the distance you move back from that river be a c, which is somewhat more than the width of the river. Besides this, at the end of that distance raise a perpendicular line of whatever length you please, and let it be the line c d; and from that d sight once again the mark b, which you noted beyond the river, and have a mark set on the bank at the point f, which lies on the same line d b. Once you have done this, divide the perpendicular c d in half at the point e, and from that e raise another perpendicular straight on, and where it cuts the line d f place a mark, from which raise the third perpendicular g f. And thus you will have made the quadrilateral c f e g, of which you know that its side c f is equal to f b, because just as e is the point at the middle of the line c d, so f is the point at the middle of the other line c b. Therefore subtract from a f the bank segment f c, that is f h, and there will remain to you h c, equal to the width of the aforesaid river, etc.