When the boring tool has detached a sufficient quantity of material, the boring rod and tool are drawn up by means of the rope O, Fig. 158, winding upon the drum Q, which is driven by straps and gearing from the steam-engine T. A shell is then lowered into the bore-hole by the wire-rope U, from the other drum V, and is afterwards drawn up again with the excavated material. A friction break is applied to the drum Q, for regulating the rate of lowering the boring rod down the well. The shell shown in Figs. 186, 187, consists of a riveted iron cylinder, with a handle at the top, which can either be screwed to the boring rod or attached to the wire-rope; and the bottom is closed by a large valve, opening inwards. Two different forms of valve are used, either a pair of flap-valves, as shown in Fig. 186, or a single-cone valve, Fig. 187; and the bottom ring of the cylinder, forming the seating of the valve, is forged solid, and steeled on the lower edge. On lowering this cylinder to the bottom of the bore-hole, the valve opens, and the loose material enters the cylinder, where it is retained by the closing of the valve, whilst the shell is drawn up again to the surface. In boring through chalk, as in the case of the deep wells in the Paris basin, the hole is first made of about half the final diameter for 60 to 90 feet depth, and it is then enlarged to the full diameter by using a larger tool. This is done for convenience of working; for if the whole area were acted upon at once, it would involve crushing all the flints in the chalk; but, by putting a shell in the advanced hole, the flints that are detached during the working of the second larger tool are received in the shell and removed by it, without getting broken by the tool. [Illustration: Shells. Figs. 185-187.] The resistance experienced in boring through different strata is various; and some rocks passed through are so hard, that with 12,000 blows a day of a boring tool weighing nearly 10 cwt., with 19 inches height of fall, the bore-hole was advanced only 3 to 4 inches a day. As the opposite case, strata of running sand have been met with so wet, that a slight movement of the rod at the bottom of the hole was sufficient to make the sand rise 30 to 40 feet in the bore-hole. In these cases Dru has adopted the Chinese method of effecting a speedy clearance, by means of a shell closed by a large ball-clack at the bottom, as shown in Fig. 186, and suspended by a rope, to which a vertical movement is given; each time the shell falls upon the sand a portion of this is forced up into the cylinder, and retained there by the ball-valve.