Morning journal and courier., 1880-06-29
Difficult Engineering Work. From the New York Tribune.l


 What is considered the most delicate and propably the most difficult piece of engineer ing that the excavations for the North river tunnel have made necessary has just been successfully accomplished in the completion of the first twenty feet of the second or south tunnel
 The first tunnel has already been driven 2S0 feet from the working shaft in Jersey City, making POO feet under the river. The shell of the tunnel is composed of two feet of brick, laid inside of plates of boiler iron. These plates are supported, at first, at the top of the tunnel forming what is called an lover -hang," and the weight of the mud is largely resisted by the use of compressed air. In the first tunnel there is plenty of room, so that the upper iron plates can be readily bolted to the plates already completed and the work proceeds simply and easily. The opening of the second tunnel is perhaps twenty feet from the entrance through the air lock. Here tunnel separates into two, which will be united again on nearing the New York side.
 In beginning the second tunnel, there was nothing permanent to work upon. The tena- cious slit in which the excavations are being made becomes hardened under the influence of compressed air and cracks, falling in masses if entirely unsupported, and allowing the air to escape. The iron plates are insert ed at first on the top, and it was necessary to devise some means of supporting them, as, of
 
 the work had been pushed far enough ahead to allow sufficient room. Special arrange ments were therefore made for holding these plates in position until the ring could be com pleted A secondary arch of iron plates, called 'sbridle" was thrown across from the top of the north tunnel, the ''bridle' being supported in position by another arch thrown up from the south side of the tunnel already constructed. This formed a temporary roof which, together with the pressure of com pressed air, served to protect the workmen as they proceeded with the construction of the she'll The first of the work was exceedingly slow and tedious. Small excavations were made at the top under the personal supervis- ion of the superintendent, and the slit was passed out from man to man in buckets like the old fashioned lines at fires. As soon as large enough space was cleared away an iron plate was inserted and held up by props.
 The slit in the center of the tunnel, a core of about fifteen feet in diameter, was left un- disturbed, and the miners after long and careful work, excavated 8 passage about four feet wide all around this core, bolting the plates in position as they proceeded, and se- curing them from lateral pressure by means of braces placed against the core. As soon as the iron shell was connected, the masons pro- ceeded to build along and inside of it a brick wall two feet in thickness. In this way the first five feet of the tunnel was built The space between the brick work and the core of silt was then filled in with well competed filing, and the whole was allowed to remain for several days in orker that the silt along the exterior of the iron shell might settle firm ly into position before the miners proceeded with the work.
 A few days ago it was judged safe to begin excavating. The core of silt was dug out, and the miners protected by the iron shell five feet wide, began excavating the silt before them and putting plates of iron in advance. in the north tunnel. At present about twenty feet of the south tunnel are in course of con- struction, but since the completion of the first five feet it has not been found necessary to leave the silt core.