[Illustration: Wherever possible, every space and crevice outside the iron lining is filled with cement forced, in a liquid state, through the iron lining by compressed air. This photograph shows the operation of “grouting,” as it is called. The man at the left is in control of the grouting. He has the hose, through which the grout is forced, screwed to a pipe which passes through a hole made for the purpose in the iron lining plates and called a “grout hole.” The two men in the middle of the picture are attending to the “grouting machine” by which the work is done. Water and cement are fed into the small boiler-like tank, the tank closed and compressed air admitted thus blowing the liquid cement through the hose and behind the iron lining. When no more grout can be forced behind the iron lining all the space has been filled. The man on the right is the engineers’ inspector taking note of how much grouting is done, and seeing that the work is properly carried out.] [Illustration: This shows the process by which the iron lining is made perfectly water-tight, so that, when the compressed air is taken off, no water at all can get into the tunnel. Two operations are shown here. One is called “grommetting the bolts,” the other is called “caulking the joints.” The two men on the left, hanging on to the wrench, are tightening up the bolts as tight as they can after having put on, underneath the washers at the head and nut of each bolt, a ring of spun yarn dipped in red lead and oil or tar or some such water-proof material. A few of these “grommets” may be seen at the feet of the third man from the left. The other four men are caulking the joints between the iron plates by driving into the joints a mixture of sal ammoniac and iron borings. This sets as hard as iron and if properly done makes a perfectly water-tight joint.] [Illustration: THE REMARKABLE ACCURACY OF ENGINEERING Usually when crossing, with a tunnel, a wide river or estuary the tunnel is started from each shore and the shields are pushed through the ground until they meet somewhere about the middle of the river. This shows two of the Pennsylvania tunnel shields which have met far below the Hudson River. The white arrow shows where each shield ends. The platform of one shield on which the man stands corresponds exactly with the platform of the other shield. As may be imagined, it takes very careful and skillful engineering and surveying work, both before the work is begun and while it is being carried out, to enable tunnel shields to meet like this. This part of the art of tunnelling would take an article to itself.]