soft soils, mud, gravel or clay, the engineering problems are very different and exceedingly difficult.
By the use of the shield and compressed air most of these difficulties have been overcome. The shield was invented by Sir Marc Brunel in 1825; and has been steadily improved and has changed so greatly that little remains of the original device except the idea. One of the first great improvements in connection with the shield method was the use of cast iron lining by Peter Barlow in 1869 in the tunnel under the Thames, England. Lord Cochrane first suggested the use of compressed air and in 1830 took out a patent for air locks and other appliances to be used in tunnelling. Compressed air without a shield was used in the old Hudson River tunnel in the seventies. The next great step in the art of tunnel building was made in 1887, when a greatly improved shield was devised by J. H. Greathead. This is the one which is now generally used, but in minor details the construction and use of the shield varies considerably. The shield used in the construction of the Hudson tunnels is thus described in a recent number of the "Review of Reviews":—
"This shield, which is one of the greatest inventions in construction machinery of the past half century, resembles in appearance a great drum built of heavy steel plates. In the head of the drum, which is known as the diaphragm, there are doors for the passage of the workmen and the withdrawal of the clay and other excavated material. The upper edge of the drum is a cutting knife which goes through the hardest material when the shield is driven forward by the pressure from hydraulic jacks, holding up the river as it goes with compressed air while the waste material is removed. The upper portion of the drum which extends backward over that portion of the tunnel tube which has been completed, known as the 'tail of the shield,' forms the protection for the men who are setting up the iron castings, ring by ring, and making the tunnel proper. Immediately back of the head is the great crane, or 'erector,' which picks up the castings and holds them in place while they are bolted together. The entire work is carried on under air pressure which is made possible by placing in the mouth of the completed tunnel some distance in the rear of the shield a solid bulkhead in which are fitted and placed air-locks through which workmen and materials pass to the work at the shield. Thus the completed tunnel advances. The tunnels themselves are made up of iron castings bolted together and set in place consecutively as the boring shield opens the way for them."
Many important tunnels were constructed after the introduction of the improvements referred to, but none that equalled the Hudson tunnels in magnitude or in the difficulties encountered.
In 1874 a civil engineer by the name of D. C. Haskins, conceived the idea and organized a company to construct two tunnels from a point near the Palisades, in Jersey City, to a terminus at or near Washington Square, New York. At that time it was his purpose to
