an inch of the surface of the rock by means of the feed screw. As the rock is penetrated the stroke is lengthened, and the force of the blow increased so as to attain the full drilling capacity of the machine; the length of stroke varies in different rock drills, that of the 'Ingersoll-Sergeant' being from 6 to 7 inches. In tappet-actuated valves the stroke may be shortened to about 4 inches by working on a cushion of steam or compressed air in the cylinder, the full stroke of the drill, the valve of which is operated by this means, being 7 inches. The variable stroke alone constitutes a very great advantage, and a rock drill possessing this property will start a bore-hole in half the time required by one working on a longer stroke. Again, a perfectly straight bore-hole is also easily made with a machine having a variable stroke. Another advantage possessed by a rock drill with a variable stroke is that the stroke can be shortened should a bit become accidentally fixed. Many of the different types of rock drills work at a pressure of 40 lbs. to the square inch, but the 'Ingersoll-Sergeant' drill, when working at a pressure of from 70 to 80 lbs. per square inch, will perform a greater amount of boring in proportion to the difference of the pressure stated. The oiling of the valves and piston is effected after the drill has been at work some little time, and when the condensed steam is got rid of, the oil being applied in small quantities at frequent intervals. The piston should be kept thoroughly lubricated, in order that the friction in the stuffing-box may be reduced to a minimum. The feed or forward movement is regulated by the feed screw, and depends on the rapidity with which the cutting edge of the bit penetrates the rock. It is essential to expeditious boring that the drill-bit, except in starting a bore-hole, should be fed just fast enough to give the piston its full stroke. When fed too slowly it will strike the front head, and if too fast, the useful effect of the cutting tool is reduced, and the power expended on the machine itself instead of on the rock. The depth to which a bore-hole should be drilled will depend greatly on the size of the rock drill employed and the class of work undertaken. For holes 20 feet deep, experience shows that the best results are obtained with a machine having a cylinder 3 inches in diameter. The hole is commenced with a drill-bit having a cutting edge 3 inches wide, and is finished at the depth stated with a 12 inch bit. For general handiness, where manual labour only is available for handling the machine, weight being an influencing factor for expeditious work, a drill having a cylinder diameter of 31 inches, and capable of boring holes to a depth of 14 or 16 feet, is to be recommended. With this size of machine a bore-hole is commenced with a diameter of 2 inches, and finished at the depth of 14 feet with a drill-bit 1 inches across. This depth and diameter |