In order to commence operations, and avoid too much clay going down to the bottom of the hole, a straw-plug was firmly fixed in the lias portion of the hole. The lower portion of the new tubes was then screwed around the old ones by means of powerful clamps, attached to the exterior in such a manner as to avoid injuring the surface; and when they could be screwed no farther, the knife or cutter, Figs. 92 to 94, was introduced inside the old tubing. Some force was needed to get this knife down into the tubing, but the spring a giving so as to accommodate itself to the hole, permitted its descent to the distance required; this being effected, it was turned round so that the steel cutter, shown at _b_, being forced against the sides of the tube, cut it through in the course of ten minutes or a quarter of an hour's turning. See section at _b_, _c_, Fig. 93. [Illustration: The Action of the Knife or Spring-cutter. Prepared for Cutting. Fig. 92.] [Illustration: The Action of the Knife or Cutter. Ready for Removal after Cutting. Fig. 93.] [Illustration: Back View of Knife or Cutter. Fig. 94.] The old tubes being three-ply, three of these knives or cutters were required to cut out the three tubes, the inner one being detached first, and then the two exterior ones; and so soon as these latter were cut out as far as they had been forced into the clay, the work became simplified into following down the interior tubing by the new tubes, as shown by the dotted lines. From _d_ at the lower end, it was found that the old inner tube had been so damaged or torn, either by the putting in or hammering it down, as to leave a vent or fissure for the sand to descend, and thus spoil the whole of the work for all future success in the boring, to say nothing of the very great cost of lifting the sand out, and subsequent most arduous labour to put the hole right.