Under the coarse glacial bed of Manhattan's murky East River, sand hogs for the last year have been boring the two tubes of a midtown vehicular tunnel intended by 1940 to connect Manhattan Island with Long Island. Each 31 feet in diameter, the tubes are bored by great circular "shields." Like the mouth of a great pipe, the shield is forced ahead by hydraulic pressure, cutting two feet eight inches at each thrust into sub-bottom deposit. Between forward thrusts, workmen remove the muck within the shield, line each new section with cylindrical cast-iron...
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