Note 209 — II. AN ESSAY ON THE MEANS OF IMPROVING THE HEALTH AND INCREASING THE COMFORT OF THE LABOURING CLASSES.

“To give an idea of the principle of contour lines, we may suppose a hill, or any elevation of land covered with water, and that we want to trace the course of all the levels at every 4 feet of vertical height; suppose the water to subside 4 feet at a time, and that at each subsidence the line of the water’s edge is marked on the hill; when all the water is withdrawn, supposing the hill to be 24 feet high, it will be marked with a set of six lines, denoting the contours of each of the levels, exactly 4 feet above each other.”—Mr. Butler Williams’s evidence before the Health of Towns Commission.