The large trepan, Figs. 128 to 130, weighs 16-1/2 tons, is forged in one solid piece, and has twenty-eight teeth. A projection of iron forms the centre of this trepan, and fits loosely into the hole made by the small trepan, acting as a guide for the tool. At a distance of 7 feet 6 inches above the teeth, a guide is sometimes fixed on the frame, but is not furnished with teeth. At a distance of 13 feet 3 inches from the teeth are two other guides at right-angles to each other. These guides are let down the pit with the boring tool, the hinged part of the guides being raised whilst passing through the beams at the top of the pit, which are only 6 feet 7 inches apart. When the tool is ready to work, the two arms are let down against the side of the pit, and are hung in the shaft by ropes, thus acting as a guide for the trepan, which moves through them. To provide against a shock to the spears when the trepan strikes the rock on the down stroke, at the upper part of the frame a slot motion is arranged, the play of which amounts to about half an inch. The teeth of the large trepan are not horizontal, but are deeper towards the inside of the pit, the face of the inside tooth being 3-3/4 inches lower than the outside. The object of this is to cause the _débris_ to drop at once into the small hole, by the face of the rock at the bottom of the pit being somewhat inclined. The teeth used, Figs. 131 to 134, are the same both for the large and the small trepan, and weigh about 72 lb. each. As a rule, only one set of teeth is kept in use, this set working for twelve hours, the alternate twelve hours being employed in raising the _débris_. This time is divided in about the following proportions;--Boring, twelve hours; drawing the rods, one hour to five hours, according to depth; raising the _débris_, two hours; and lowering the rods one hour to five hours. The maximum speed of the larger trepan may be taken at about 3 feet a day. The ordinary distance sunk is not more than 2 feet a day, and in flint and other hard rocks the boring has proceeded as slowly as 3 inches a day. [Illustration: Tools used in Boring. Figs. 135, 136.] [Illustration: Epicycloidal Hook. Figs. 137, 138.] [Illustration: Rod Disconnection Key. Figs. 139, 140.] [Illustration: Connections to the Trepan and Spears or Rods. Fig. 141, 142.]