He showed that an imaginary spheroidal shell, concentric with the earth and cutting the slope between the elevated and depressed areas at the contour-line of 1700 fathoms, would not only leave above it a volume of the crust equal to the volume of the hollow left below it, but would also divide the surface of the earth so that the area of the elevated region was equal to that of the depressed region.6 A similar observation was made almost simultaneously by Romieux, 7 who further speculated on the equilibrium between the weight of the elevated land mass and that of the total Areas of waters of the ocean, and deduced some interesting relathe cru st tions between them.
The top of Broad Law on the confines of Peeblesshire and Selkirkshire, for example, is a level moor comprising between 300 and 400 acres above the contour line of 2500 ft.
These points lie, therefore, upon a horizontal plane at a given elevation passing through the land shown on the map, and the contour-line is the intersection of that horizontal plane with the surface of the ground.
The contour-line of o, or datum level, is the coastal boundary of any land form.
