In rare cases, the rock through which the shield has been forced is strong enough to support itself; in most works the engineers prefer to line the bore, with brick and concrete, as a rule. This lining is set in the path of the cutting-shield before its protection is entirely withdrawn; and so the heavy roof-timbering which was formerly a trade-mark of the successful tunnel engineer is no longer used. Tunnel-boring becomes doubly difficult when the railroad is to be carried under a river or some broad arm of the sea. Men work in an unnatural environment when they work below the surface of great waters, and the record of such work is a record of many tragedies. At any instant firm rock may cease, silt or sand or an underground stream may make its appearance and the helpless workmen find a ready grave. In work where there is even the slightest expectation of such a contingency the air-lock, with its artificial pressure to hold back the soft earth and moisture is brought into use. In another chapter we shall see how the caisson is operated. Suffice it to say now that the necessity of "working under the air," brings no comfort to any one. It vastly hinders and complicates the work of construction, and adds greatly to the expense. Moreover, it has its own record of tragedies. Still it remains, to the infinite credit of a national persistence, that there is no record in the annals of American engineering where the workers have finally given up a tunnel job. Lives have been sacrificed, good-sized fortunes swept away, but in the end the resistless railroad has always found its underground path. The tunnel-workers can tell you of the accident when the subway was being driven under the East River from Manhattan to Brooklyn, three years ago. The cutting-shield, which was advancing from the Brooklyn side, suddenly slipped out from the rock into the unprotected soft mud of the river bottom. The heavily compressed air shot a geyser straight up to the surface of the river some fifty feet above. A workman shot through the geyser, pirouetted gayly for a fraction of a second above the river, then dropped, to be picked up by the crew of a passing ferryboat. In a week he was back at work again inside the cutting-shield. His fortune was the opposite of that which generally awaits a man caught in a tunnel accident. "It ain't as bad as it used to be," one of them informs you. "When I first got into this profession, they didn't have the electricity for lights or moving the cars or nothing. We used to try and get along with safety lamps an' near choke to death. It was more like hell then than it is now." [Illustration: "SOMETIMES THE CONSTRUCTION ENGINEER ... BRINGS HIS LINE FACE TO FACE WITH A MOUNTAIN"] [Illustration: FINISHING THE LINING OF A TUNNEL] [Illustration: THE BUSIEST TUNNEL POINT IN THE WORLD--AT THE WEST PORTALS OF THE BERGEN TUNNELS, SIX ERIE TRACKS BELOW, FOUR LACKAWANNA ABOVE]