[Illustration: This is a photograph taken in one of the Pennsylvania tunnels under the Hudson River. It shows the soft mud, through which the tunnel is being built, flowing in a thick stream through one of the doors of the shield. The mud under the Hudson, where these tunnels are, is so soft that often the shield was pushed through the mud with all the doors shut, so that no mud came into the tunnel and no digging had to be done, but the shield pushed its way bodily through the mud, the rings of iron lining being built up behind as usual. Generally, however, a certain amount of mud was brought in and had to be removed. This photograph shows how it looked.] ~HOW THE SHIELD CUTS THROUGH THE GROUND~ The diagram, Fig. 1, shows more clearly what is meant. From an inspection of Figure 1 it is clear that, when the openings in the shield bulkhead are closed, the tunnel is protected from an inrush of either water or earth; the openings in the bulkhead may be so regulated that control is maintained over the material passed through. After a ring of iron lining has been erected within the tail of the shield, the shield doors are opened and men go through them and dig out enough earth for the shield to go ahead. The rams are then thrust out thus pushing the shield ahead. Another ring of iron is built up within the tail for which purpose an hydraulic swinging arm, called the “erector,” is mounted on the shield face. This erector picks up the plates and puts them into position, one by one, while the men bolt them together. Excavation is then carried on again and the whole round of work repeated, gaining every time the jacks are rammed or thrust out a length equal to the length of one ring of iron lining. In carrying out this work in ground charged with water the shield is assisted by introducing compressed air as described before. To use the compressed air thick bulkhead walls of masonry are built across the tunnel behind the shield and into the space between the shield and the bulkhead wall air is pumped, compressed to the same pressure as that of the water in the ground, or in other words the pressure of the air in pounds per square inch is about half the number of feet the tunnel is below the water surface. This dries the ground and simplifies enormously the difficulty of working in it. The diagram, (Fig. 1) shows a bulkhead wall across the tunnel. In order to pass from the ordinary air outside the bulkhead into the compressed air inside it, all the men and the materials have to pass through the “air locks” which are built into the wall. They are called air locks because they are like the locks on a canal which raise the water from a lower to a higher level or lower it from a higher to a lower level as the case may be. The difference is that an air lock enables one to pass from air at a low pressure to one of a higher, or vice versa. An air lock is made like a large boiler with a door at each end. If we wish to enter the compressed air we