[Illustration: The Small Trepan. Fig. 124.] [Illustration: Plan of Guide B. Fig. 125.] [Illustration: Swivelled Ring Front Elevation. Fig. 126.] [Illustration: Swivelled Ring Side Elevation. Fig. 127.] [Illustration: Large Trepan Front Elevation. Fig. 128.] [Illustration: Large Trepan Side Elevation. Fig. 129.] [Illustration: Large Trepan Component, Fig. 130.] [Illustration: Trepan Teeth. Figs. 131-134.] After the working platform is fixed, the first boring tool applied is the small trepan, Figs. 122 to 125. This tool is attached to the wooden beam by the same arrangement shown by Fig. 109. The boring tools can be lowered at pleasure by means of an adjusting screw. Next in order comes the handle for boring. This is worked by four men on the platform, and is turned by the aid of a swivel. Attached to the handle-piece are wooden rods, made from Riga pitch pine. These rods are 59 feet in length and 7-3/4 inches square. A swivelled ring, Figs. 126, 127, is attached to the rope when raising and lowering the boring rods. The small trepan cuts a hole 4 feet 8-3/4 inches in diameter, and has fourteen teeth, fitted in cylindrical holes and secured by pins entering through circular slots. The teeth are steeled. At a distance of 4 feet 4 inches above the main teeth of the trepan there is an arm, with a tooth at each end. This piece answers the purpose of a guide, and at the same time removes irregularities from the sides of the hole. At a distance of 13 feet 6 inches above the main teeth are the actual guides, consisting of two strong arms of iron fixed on the tool, and placed at right-angles to each other. The hole made by the small trepan is not kept at any fixed distance in advance of the full-sized pit, but the distance generally varies from 10 to 30 yards. With the small trepan, which weighs 8 tons, the progress varies from 6 to 10 feet a day.