We have not yet reached the point at which hand labor may be dispensed with in tunnel work, but there are machines which will do what formerly could only be accomplished with the manually wielded pick and shovel. We have already (in Chapter VII) described the pneumatic tunnel shield and explained how it is sometimes driven forward through soft silt by means of hydraulic jacks without any excavation of material. This method of tunneling which is analogous to driving a punch through soft metal, has a very limited field of application. It is impossible to force the shield in this manner through sand or through any but very soft silt. TUNNELING BY MACHINE The city of Cleveland takes its drinking water from Lake Erie. In order to obtain water that is not contaminated by the refuse of the city, tunnels are carried out under the bottom of the lake about two miles from the shore where they terminate in water intakes far enough below the surface to avoid floating impurities and far enough above the bottom to avoid impurities that have settled to the lake bed. The material through which the tunnels pass is a stiff clay, that cuts like cheese. So uniform is this material that a special machine was built to bore the tunnel through it. This machine is somewhat similar to a boring mill. It has an arm that revolves against the face of the tunnel heading and carries a cutter that travels along the arm so that it cuts a continuous spiral ribbon of clay. The clay ribbon passes back through the machine and is loaded into a train of dump cars. For boring tunnels in hard rock many different machines have been invented, but not one of them as yet has proved an unqualified success. One very interesting machine, which was tried on the New York subway excavation at 42d Street and Lexington Avenue, consisted of a series of chipping hammers which, by means of pneumatic mechanism, were made to hammer and pulverize the face of the rock with repeated blows. The hammers were arranged in a circle and were revolved as they hammered so that the whole surface of the heading was attacked and a circular tunnel was cut through the rock. A serious disadvantage under which the machine labored was the fact that the rock had to be crushed to a powder or into small chips before it could be removed.