In soft ground and subaqueous soil the "shield" is the chief apparatus used in tunneling. The most up-to-date appliance of this kind was that used in constructing the tunnels connecting New York City with New Jersey under the Hudson River. It consisted of a cylindrical shell of steel of the diameter of the excavation to be made. This was provided with a cutting edge of cast steel made up of assembled segments. Within the shell was arranged a vertical bulkhead provided with a number of doors to permit the passage of workmen, tools and explosives. The shell extended to the rear of the bulkhead forming what was known as the "tail." The lining was erected within this tail and consisted of steel plates lined with masonry. The whole arrangement was in effect a gigantic circular biscuit cutter which was forced through the earth. The tail thus continually enveloped the last constructed portion of this permanent lining. The actual excavation took place in advance of the cutting edge. The method of accomplishing this, varied with conditions. At times the material would be rock for a few feet from the bottom, overlaid with soft earth. In such case the latter would be first excavated and then the roof would be supported by temporary timbers, after which the rock portion would be attacked. When the workmen had excavated the material in front of the shield it was passed through the heavy steel plate diaphragm in center of the shell out to the rear and the shield was then moved forward so as to bring its front again up to the face of the excavation. As the shell was very unwieldy, weighing about eighty tons, and, moreover, as the friction or pressure of the surrounding material on its side had to be overcome it was a very difficult matter to move it forward and a great force had to be expended to do so. This force was exerted by means of hydraulic jacks so devised and placed around the circumference of the diaphragm as to push against the completed steel plate lining of the tunnel. There were sixteen of these jacks employed with cylinders eight inches in diameter and they exerted a pressure of from one thousand to four thousand pounds per square inch. By such means the shield was pushed ahead as soon as room was made in front for another move. The purpose of the shield is to prevent the inrush of water and soft material while excavating is going on; the diaphragm of the shields acts as a bulkhead and the openings in it are so devised as to be quickly closed if necessary. The extension of the shield in front of the diaphragm is designed to prevent the falling or flowing in of the exposed face of the new excavation.