This explains the principles of the shield and compressed air way of making a tunnel. The following describes very shortly how these principles are put to actual use. Most tunnels which are built by shield and compressed air under rivers or arms of the sea are lined with cast iron plates to protect the railway or roadway which is in the tunnel. The tunnel is a circular tube, or shell, and the plates have flanges on all sides which are bolted together. This shell is put into place, plate by plate, by means of the shield which not only protects the workmen and the work under construction, but which helps to build the iron shell. In fact it corresponds to the sea worm which bores through the wood and lines the hole with a shell. In the case of the tunnel the shell is made of iron. The shield itself consists of a steel tube or cylinder slightly bigger in diameter than the tube or tunnel it is intended to build. The front edge of this shield is made up of a ring of sharp edged castings which form what is called the “cutting edge.” Just behind the cutting edge is a bulkhead or wall of steel, in which are openings which may be opened or closed at will. Behind this bulkhead are placed a number of hydraulic jacks or presses arranged around the shield and within it, so that by thrusting against the last erected ring of iron lining the whole shield is pushed forward. The rear end of the shield is a continuation of the cylinder which forms the front end, and this part, called the “tail,” always overlaps the last few feet of the built up iron shell. [Illustration: This is a photograph of a model of the Pennsylvania Tunnels to New York City, made for the Jamestown Tercentenary Exposition of 1907. It is given because it illustrates, as no photograph of actual work could do, the relationship between the shield, the tunnel itself and the air lock. This view shows the rear part of the shield on the extreme left, with the erector picking up an iron plate. It shows a man bringing a car with two of the iron plates up to the shield. Behind this man comes the bulkhead wall with the emergency air lock in the top and the ordinary air lock for passing in and out at the bottom. It also shows the upper platform to the emergency lock along which the men can get to the emergency lock in case of an accident.] [Illustration: This is another view of the same model, but showing the front view of the shield. The doors on the air locks are clearly shown.]