Since gasoline air compressors are self regulating, when the drill is not using air very little gasoline is burned by the gasoline engine driving the compressor. A gasoline compressor possesses other very important economic advantages over a small steam-driven plant. First, there is the saving in wages of firemen and second, there is the saving in hauling and pumping of water and the hauling of fuel. The cost of gasoline is often less than the cost of coal for operating a small plant. An electric drill[89] operated on the principle of the solenoid does away with motor, valves, pipes, vapor, freezing, and other difficulties attendant on the use of steam or air. The rates of drilling in different classes of rock are shown in Table 57. Frequent changes of drills and relocation of tripods will materially reduce the performance of a drill, for as much as 45 minutes may be lost in making a new set up. In this the jack hammer drills show their advantage as no time is lost in a set up. TABLE 57 RATES OF ROCK DRILLING Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep. (From Gillette) Hard Adirondack granite 48 Maine and Massachusetts granite 45–50 Mica-schist of New York City. Possible 60–70 Mica-schist of New York City. Average 40–50 Hard, Hudson River trap rock 40 Soft red sand stone of Northern New Jersey 90 Hard limestone near Rochester, N. Y 70 Limestone of Chicago Drainage Canal 70–80 Douglass, Indiana, syenite. Difficult set ups 36 Canadian granite on Grand Trunk R. R 30 Windmill point, Ontario limestone: 3⅝-inch drills 75 2¾-inch drills 60 2¼-inch drills 37 =145. Steam or Air for Power=.—The choice between steam or air is dependent on the conditions of the work. Steam is undesirable in tunnels on account of the heat produced. In open cut work it is at a disadvantage because of the loss of power due to radiation from the hose or pipe. The life of the hose is not so long as when air is used, escaping steam causes clouds of vapor which obscure the work, and serious burns may occur due to hot water thrown from the exhaust. It is advantageous since leaks may be easily discovered and remedied, it requires less machinery than air, and it is sometimes less expensive. With compressed air, gasoline or electric motors can be used for operating the compressors. TABLE 58 ROCK BLASTING