Cutters are used also for loosening packed hard bottoms and some of them will dig into hard pan and even soft rock. A pressure of 100 to 150 pounds per square inch is maintained in a discharge pipe twenty to twenty-four inches in diameter, which is enough to carry along heavy bowlders dug up by the cutter.

THE DEEPEST MINE SHAFT

So far we have dealt only with surface conditions, but man has not been content to stay on the surface of this planet. True, we have scarcely begun to explore the crust of the earth. The deepest mine in the world is the Morro Velho in the province of Minas Geraes, Brazil. Here the earth has been penetrated to a depth of 6,426 feet in quest of gold. This is a depth of less than a mile and a quarter and it shrinks into insignificance when we reflect that we must go more than 3,000 times that distance to reach the center of the earth. However, we have made material progress in shaft-sinking in recent years, and it is quite likely that the lure of scientific research may prove even more powerful than that of gold and that some day we shall be induced to dig many miles into the crust of the earth just to learn something more about this globe that whirls us through space.

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.

In ordinary rock excavation holes are bored into the heading and the rock is then blown out by means of dynamite or some other explosive. Large fragments of rock are then broken up into pieces small enough to be handled readily, but no energy is wasted in reducing the material to a powder. The percussion drills with which the rock is bored have already been briefly described. When boring holes for a blast in soft material, such as bituminous coal, a drill is used which resembles in many respects an ordinary twisted auger bit except that it is many feet in length. It is driven by hand into the coal by the use of a common bit brace. Percussion drills or punches are also used. These are driven either by pneumatic or electric power.

COAL-CUTTING MACHINERY

When excavating coal the heading is first undercut, that is, a deep slot is cut in the wall of coal along the floor line. Then blast holes are bored into the coal above this cut so that when the charges are fired the coal will be broken downward. To undercut the coal special machines are used, driven by compressed air or electricity. These have endless chains fitted with chisel or pick points that bite into and cut the coal. After the material has been shattered and reduced to fragments of convenient size special machinery may be employed to shovel it away from the heading and into dump cars. One electrically driven shoveling machine which was tried out on some of the Catskill aqueduct excavation had a broad open shovel which could be driven into the pile of rock fragments under the control of an operator and would scoop up the material, delivering it upon a traveling belt conveyor which carried it on to the train cars. Many types of mechanical shoveling and loading machines have been built for use in mines. They are driven either by pneumatic or electric power.