"Drilling an oil well is commonly done by means of a heavy string of tools, suspended at the end of a cable and given a churning motion by a walking beam rocked by a steam engine.... The steel tools, falling under their own weight, literally punch their way to the depth desired." The usual custom is for four men working two twelve-hour shifts to sink a well. Their work is to keep the engine that operates the drill running, to watch the operation, and to stoke the engine. The only danger is when the time has come to "shoot" the well. This is done only if the oil does not flow naturally when the oil-bearing strata are reached. The "shooting" consists in dropping a "go devil" upon canisters of nitroglycerine to blow out a cavity at the bottom in which oil may collect. If the charge explodes before it reaches the bottom of the well, it may blow back and wreck the derrick, and the timbers and "bull-wheel" may fall upon the men. This danger, which can be obviated by the simple process of the men going away until after the charge has exploded, is practically the only one connected with drilling for petroleum, except for the incidental danger which all handling of high explosives necessarily involves. A well costs only about one-tenth as much per foot of descent as a mine shaft; and it can be put down in much quicker time. When it is once in contact with the oil, operating expenses are trifling, for in many cases the oil reaches the surface under natural gas pressure, and in others it has merely to be pumped. There are not the expenses for breaking rock, timbering, or haulage, which are common in coal mining. So that in addition to being a better fuel than coal to burn under a steam boiler, petroleum mining is as healthful and safe an occupation as can be well found in contrast with the extremely dangerous work of the coal miner, and it can be far more quickly and more cheaply got out of the ground. It has other advantages. Coal, bituminous coal especially, is so bulky and so liable to spontaneous combustion that it is difficult to store. Petroleum, on the contrary, can be easily and satisfactorily stored in iron tanks which, standing about like great cheese boxes, are characteristic of the oil-producing country. Added to the advantage of easy storage is the much greater one of easy transportation. Unlike coal, the crude petroleum supply of the country makes no demand upon the railroads. Only under exceptional circumstances and for short distances is it hauled about in tank cars. It has its own independent system of pipe lines, after the manner of a great water supply. These systems connect the oil wells with the refineries, the markets, and the seaports.