This shows the front of one of the shields used on the Pennsylvania Railroad tunnels crossing the North River at New York. The cutting edge is clearly seen and the various compartments, each with its door, which divide up the front of the shield. These shields weighed about 200 tons each.] HOW TUNNELS ARE BUILT. These notes describe very generally the way in which tunnels are built through mud and gravel under parts of the sea or large rivers in such a way that the men who build them are protected and as safe as the carpenter who is building a house. The way these tunnels are built is called the “shield” way because the machine used is called a shield. It is given this name because it shields the tunnel builders from the water and the mud which are ready at every moment to overwhelm them and kill them. The shield was invented in 1818 by a great Engineer, Marc Isambard Brunel, who was a Frenchman living in England. The idea of the shield came to him as he saw how the sea worm which attacks the wooden piles of docks along the shore bores the holes it makes in the wood. The head of this worm is very hard and can bite its way through the hardest woods. As it goes through the wood its body makes a hard shelly coating which lines the holes which its head has made and prevents the hole from getting filled up. This is the general idea of a tunnel built by a shield. The first shield was used by Mr. Brunel to make a tunnel across the Thames River at London, England. This is still the biggest tunnel ever built by a shield, although not the longest, and is still used by railroad trains. This tunnel was begun in 1825 and was finished in 1843, and provides a history of almost unexampled and not-to-be-excelled courage in attacking difficulties and skill in defeating them. Since the days of Brunel many great improvements have been made in the shield and in the way of working it but the same idea is still there. [Illustration: HOW THE SHIELD IS PUSHED FORWARD This shows the rear end or tail end of one of the smaller shields, used on the Hudson and Manhattan Railroad tunnels under the North or Hudson River at New York. It shows the skin, the hydraulic jacks within the skin and the piping and valves for working them. It also shows the doors leading to the front or “face.” The erector is not shown, but the circular hole in the middle shows where it would be attached.] [Illustration: This shows one side of an air lock bulkhead wall with the air lock in place. The boiler-like appearance of the lock is clearly visible, as well as the door and the pressure gauge to tell the air pressure inside the lock.]