rotations per minute. of the water. when freed, goes through the hollow bit, keeping it cool and clearing the hole of ()Ark. A water pressure of from 300 to 450 pound. per square inch is required to operate these drills. Rock drills are extensively used in quarrying. tunnel ing,. mining, and rock excavation of all kinds, Rotary drills differ from percussion drills in the fact that the drill makes its way into the rock by a boring action produced by the axial rotation of the drill-rod. The most common form of rotary drill is the diamond drill. This machine consists of a hollow cylindrical bit hav ing a eutting edge of diamonds (Fig. 3), which is revolved at the rate of from '?Of to 400 revolutions per minute, by suitable machinery operated by steam or com pressed air. The diamonds are set in the cutting edge of the bit. so as to project outward from its annular face and also slightly- inside and outside its eylindrieal sides. When the drill-rod with the bit attached is rotated and thrust forward, the bit cuts an annular hole in the rock, the drillings beimg removed from the hole by a con stant stream of water. which is forced down the
hollow drill-rod and emerges carrying the di.bris with it up through the narrow space between the outside of the drill-rod and the walls of the hole. Owing to the hollow bit, the nucleus of the hole bored is taken out as a solid core. This possi bility of the diamond drill makes it a valuable tool for prospecting for minerals, or for deter mining the character of the material which will be met in sinking deep foundat ions or ill driving tunnels. _An other form of rotary drill which is employed in Eu rope for tunnel work is the Brandt . hydraulic drill.
The cutting edge Of this drill consists of hardened steel teeth (Fig. 4). The bit is pressed against the rock by hydraulic pressure, and usually makes from seven to eight and for prospecting for minerals. For a concise history of the development of rock drills, consult Drinker, Tunneling, Explosive Compounds, and Rock Drills (New York, 1901).