To sink a well by underpinning, an excavation is first made to such a depth as the strata will allow without falling in. At the bottom of the excavation is laid a curb, that is, a flat ring, whose internal diameter is equal to the intended clear diameter of the well, and its breadth equal to the thickness of the brickwork. It is made of oak or elm planks 3 or 4 inches thick, either in one layer fished at the joints with iron, or in two layers breaking joint, and spiked or screwed together. On this, to line the first division of the well, a cylinder of brickwork, technically called steining, is built in mortar or cement. In the centre of the floor is dug a small pit, at the bottom of which is laid a small platform of boards; then, by cutting notches in the side of the pit, raking props are inserted, their lower ends abutting against a foot block, and their upper ends against the lowest setting, so as to give temporary support to the curb with its load of brickwork. The pit is enlarged to the diameter of the shaft above; on the bottom of the excavation is laid a new curb, on which is built a new division of the brickwork, giving permanent support to the upper curb; the raking props and their foot-blocks are removed; a new pit is dug, and so on as before. Care should be taken that the earth is firmly packed behind the steining. A common modification of this method consists in excavating to such a depth as the strata will admit without falling in. A wooden curb is laid at the bottom of the excavation, the brick steining laid upon it and carried to the surface. The earth is then excavated flush with the interior sides of the well, so that the earth underneath the curb supports the brickwork above. When the excavation has been carried on as far as convenient, recesses are made in the earth under the previous steining, and in these recesses the steining is carried up to the previous work. When thus supported the intermediate portions of earth between the sections of brickwork carried up are cut away and the steining completed. In sinking with a drum curb, the curb, which may be either of wood or iron, consists of a flat ring for supporting the steining, and of a vertical hollow cylinder or drum of the same outside diameter as the steining, supporting the ring within it and bevelled to a sharp edge below. The rings, or ribs, of a wooden curb are formed of two thicknesses of elm plank, 1-1/2 inch thick by 9 inches wide, giving a total thickness of 3 inches. [Illustration: Iron Drum Curb Plan. Fig. 19.] [Illustration: Iron Curb Enlarged Segment. Fig. 20.]