When the shield has been pushed across the entire length of the water way which has to be tunnelled, and the whole of the iron tube or shell is in place, a thick lining of concrete is placed inside the iron shell to protect it and make the tunnel stronger. As an added safeguard wherever the tunnel is in rock, gravel, strong clay or other ground which is not so soft that it does not close tightly in on the outside of the tube, liquid cement is forced by compressed air through holes made in the iron plates for this purpose. This liquid cement enters every pore or crevice in the surrounding ground and when it has set hard it still further protects the iron with a coating of cement. Pieces have been cut out of the iron lining of a tunnel built under the river Thames at London, England, in 1869, which showed that the iron at all places was as good as the day it was first put in forty years before, and iron put in the lining of the Hudson River Tunnel about 1878 when removed after thirty years was in perfect condition. [Illustration: SHIELD AT END OF JOURNEY Sometimes, however, shields are not driven to meet one another, but end their journey at some shaft or in some other tunnel previously built, after having gone through thousands of feet of all kinds of ground, from the hardest rock, which had to be blasted out foot by foot before the shield could advance, through hard pan, gravel, boulders, piles, rip-rap, made ground and mud so soft that it flows like melted butter. Naturally, after an experience like this a shield does not look as spick and span as when it started in life. This photograph shows one of the shields of the Hudson and Manhattan Railroad in New York just reaching the end of its journey, battered and bent but still in the ring.] [Illustration: This shows a piece of curved tunnel near Morton Street, on the Hudson and Manhattan Railroad, and is given because of the clear showing it gives of the iron lining. The track and floor are only the temporary roads for use during construction.] [Illustration: Sometimes it is necessary to make borings of the ground below the tunnels. In some of these bore holes vast quantities of water are found at a much higher pressure than the tunnel compressed air. This picture shows a spouting bore hole in one of the Pennsylvania tunnels during construction.] [Illustration: The last thing to do before laying the track is to put the concrete inside the iron lining. This picture shows this work going on and the wooden forms or ribs for holding up the concrete while it is setting.] [Illustration: THE LAND END OF A GREAT TUNNEL UNDER THE HUDSON