A jack hammer drill is shown in Fig. 102. In its lightest form the drill weighs about 20 pounds and is capable of drilling ⅞-inch holes to a depth of 4 feet. Heavier machines are available for drilling larger and deeper holes. The same machine can be adapted to the use of steam or compressed air. When in use the point of the drill is placed against the rock and a pressure on the handle opens a valve admitting air or steam. The piston is caused to reciprocate in the cylinder, striking the head of the drill at each stroke. The drill is revolved in the hole by hand or by a mechanism in the machine. A hollow drill can be used by means of which the operator admits air or steam to the hole, thus blowing it out and keeping it clean. These machines have the advantage of small size, portability and simplicity. They can be easily and quickly set up and the drills can be changed rapidly. Their undesirable features are the vibration transmitted to the operator and the dust raised in the trench. [Illustration: FIG. 102.—Jack Hammer Rock Drill. ] [Illustration: FIG. 103.—Tripod Drill. ] A type of drill heavier and larger than the jack hammer drill is shown in Fig. 103. It requires some form of support such as a tripod, or in tunnel work it can be braced against the roof or sides. Some data on steam and air drills are given in Table 56. The effect of the length of the transmission pipe, temperature of the outside air, pressure at the boiler or compressor, etc., will have a marked effect on the amount of steam or air to be delivered to the drill. Compressed air is affected more than steam by these outside factors, but it has an advantage in that as it loses in pressure it increases in volume so that the loss of power is not so marked. Gillette states: We may assume that a cubic foot of steam will do practically the same work in a drill as a cubic foot of compressed air at the same pressure, because neither the steam nor the air acts expansively to any great extent in a drill cylinder, due to the late cut-off. This being so ... one pound of steam is equivalent to nearly 30 cubic feet of free air ... all at the same pressure of 75 pounds per square inch. If a drill consumes at the rate of 100 cubic feet of free air per minute ... it would therefore consume 240 pounds of steam (at 75 pounds pressure) per hour.... Where not more than three or four drills are to be operated, probably no power can equal compressed air generated by gasoline. It will require 12 horse-power to compress air for each drill, hence 1½ gallons of gasoline will be required per hour per drill while actually drilling. TABLE 56 DATA ON ROCK DRILLS