A running sand in soft clay is, however, the most serious difficulty met with in well boring. Under such circumstances the bore-hole has to be tubed from top to bottom, which greatly increases the expense of the undertaking, not only by the cost of the tubes, but also by the time and labour expended in inserting them. When a permanent water supply is the main object of the boring, the additional expense of tubing the bore-hole is not of much consequence, as the tubed hole is more durable, and the surface water is thereby excluded; but in exploring for mineral it is a serious matter, as the final result of the bore-hole is then by no means certain. The mode of inserting tubes has become a question of great importance in connection with this system of boring, and much time and thought having been spent in perfecting the method now adopted, its value has been proved by the repeated success with which it has been carried out. The tubes used by Mather and Platt are of cast-iron, varying in thickness from 5/8 to 1 inch according to their diameter, and are all 9 feet in length. The successive lengths are connected together by means of wrought-iron covering hoops 9 inches long, made of the same outside diameter as the tube, so as to be flush with it. These hoops are from 1/4 to 3/8 inch thick, and the ends of each tube are reduced in diameter by turning down for 4-1/2 inches from the end, to fit inside the hoops, as shown in Fig. 209. A hoop is shrunk fast on one end of each tube, leaving 4-1/2 inches of socket projecting to receive the end of the next tube to be connected. Four or six rows of screws with countersunk heads, placed at equal distances round the hoop, are screwed through into the tubes to couple the two lengths securely together. Thus a flush joint is obtained both inside and outside the tubes. The lowest tube is provided at the bottom with a steel shoe, having a sharp edge for penetrating the ground more readily.