In the construction of this machine it will be seen that the great desideratum of all earth boring has been well kept in view; namely, to bore-holes of large diameter to great depths with rapidity and safety. The object is to keep either the boring-head or the shell-pump constantly at work at the bottom of the bore-hole, where the actual work has to be done; to lose as little time as possible in raising, lowering, and changing the tools; to expedite all the operations at the surface; and to economize manual labour in every particular. With this machine, one man standing on a platform at the side of the percussion cylinder performs all the operations of raising and lowering by the winding engine, changing the boring-head and shell-pump, regulating the percussive action, and clamping or unclamping the rope: all the handles for the various steam valves are close to his hand, and the break for lowering is worked by his foot. Two labourers attend to changing the cutters and clearing the pump. Duplicate boring-heads and pumps are slung to the overhead suspension bar Q, Fig. 189, ready for use, thus avoiding all delay when any change is requisite. As is well known by those who have charge of such operations, in well boring innumerable accidents and stoppages occur from causes which cannot be prevented, with however much vigilance and skill the operations may be conducted. Hard and soft strata intermingled, highly-inclined rocks, running sands, and fissures and dislocations are fruitful sources of annoyance and delay, and sometimes of complete failure; and it will therefore be interesting to notice a few of the ordinary difficulties arising out of these circumstances. In all the bore-holes yet executed by this system, the various special instruments used under any circumstances of accident or complicated strata are fully shown in Figs. 199 to 207. [Illustration: Grapnel for Stiff Clay. Fig. 207.] [Illustration: Straightening Plug for Tubes. Fig. 205.] [Illustration: Grapnel for Cores. Fig. 206.]