The form of rotary drill used chiefly for prospecting purposes is the diamond drill. This machine consists of a hollow cylindrical bit having a cutting-edge of diamonds, which is revolved at the rate of from two hundred to four hundred revolutions per minute by suitable machinery operated by steam or compressed 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 of its cylindrical sides (Fig. 13). When the drill rod with the bit attached is rotated and fed forward the bit cuts an annular hole into the rock; the drillings being removed from the hole by a constant stream of water which is forced down through the hollow drill rod and emerges, carrying the débris with it, up through the narrow space between the outside of the bit and the walls of the hole. There are various makes of diamond drills, but they all operate in essentially the same manner. The rotary drill principally employed in Europe in tunneling is the Brandt. The cutting-edge of the Brandt drill consists of hardened steel teeth. The bit is pressed against the rock by hydraulic pressure, and usually makes from seven to eight revolutions per minute. Some of the water when freed goes through the hollow bit, keeping it cool, and cleaning the hole of débris. A water pressure of from 300 to 450 lbs. per square inch is required to operate these drills. Rotary rock-drills may be mounted either on carriages or on columns for tunnel work. Several machines have recently been constructed for the purpose of breaking the rock in tunnels without blasting, but they did not meet the approval of tunnel engineers. One of these machines is provided with numerous electric torches, which are applied to the rock at the front. By suddenly chilling the rock with a stream of cold water the stone will crumble away. Another machine was tested, with little success, in the excavation for the New Grand Central Depot in New York. On the face of this machine there is a multitude of chipping drills revolving on four arms and driven by air pressure. They attack the rock and chip it into fragments, which are carried away by an endless band. EXPLOSIVES AND BLASTING. When the holes are once drilled, either by hand or power drills, they are charged with explosives. The principal explosives employed in tunneling are gunpowder, nitroglycerine, and dynamite.