In small borings, from 6 to 12 inches diameter, the tubes are inserted into the bore-hole by means of screw-jacks, by the simple and inexpensive method shown in Figs. 210, 211. The boring machine foundation A A, which is of timber, is weighted at B B by stones, pig iron, or any available material; and two screw-jacks C C, each of about 10 tons power, are secured with the screws downwards, underneath the beams D D crossing the shallow well E, which is always excavated at the top of the bore-hole. A tube F having been lowered into the mouth of the bore-hole by the winding engine, a pair of deep clamps G are screwed tightly round it, and the screw-jacks acting upon these clamps force the tube down into the ground. The boring is then resumed, and as it proceeds the jacks are occasionally worked, so as to force the tube if possible even ahead of the boring tool. The clamps are then slackened and shifted up the tubes, to suit the length of the screws of the jacks; two men work the jacks, and couple the lengths of tubes as they are successively added. The actual boring is carried on simultaneously within the tubes, and is not in the least impeded by their insertion, which simply involves the labour of an additional man or two. [Illustration: Tube-forcing Apparatus with Screw-jacks. Fig. 210, 211.] A more perfect and powerful tube-forcing apparatus is adopted where tubes of from 18 to 24 inches diameter have to be inserted to a great depth, an illustration of which is afforded by an extensive piece of work at the Horse Fort, standing in the channel at Gosport. This fort is a huge round tower, as shown in Fig. 212; and to supply the garrison with fresh water, a bore-hole is sunk into the chalk. A cast-iron well A, consisting of cylinders 6 feet diameter and 5 feet long, has been sunk 90 feet into the bed of the channel in the centre of the fort, and from the bottom of this well an 18-inch bore-hole B is now in progress. The present depth is 400 feet, and the bore-hole is tubed the whole distance with cast-iron tubes 1 inch thick, coupled as before described. [Illustration: _Horse Fort, Spithead._ _Section shewing Bore-hole._ Fig. 212.] [Illustration: Tube-forcing Apparatus with Hydraulic Presses. Fig. 213.]