The fact that compressed air on expanding is cooled, makes it an ideal power for use in mining machinery. Steam is inconvenient because of the difficulty of transmitting it to the machines without loss of heat, because it would heat the machines so that they cannot be handled readily and because the exhaust steam would fill the mine with an impenetrable fog; electricity is dangerous in mines that are apt to contain explosive gases, because the wires are liable to be broken and cause sparks by short circuiting, and because sparks are likely to form between the brushes and commutators of the motors; but compressed air has none of these objections, and, furthermore, the discharge from the machines furnishes the operators with ample supplies of fresh cool air which drives out disagreeable and dangerous gases. AIR-DRIVEN HAMMERS The ordinary rock drill is really a pneumatic hammer. The tool, which is chisel-shaped, is used as a hammer to pound a hole in the rock. The tool is driven up and down or in and out by compressed air bearing alternately on opposite sides of a piston. The elasticity of the compressed air acts as a cushion to relieve the machine from shock. The drill is mounted upon a tripod in such a way that it may be operated in any direction. The pneumatic riveter is another form of compressed-air hammer. The machine is held in the hands, and the tool, which is rapidly reciprocated by air pressure, pounds the red-hot shank of a rivet. While this is being done an assistant holds a hammer against the head of the rivet. However, there is a type of riveter which has a U-shaped frame. One arm of the frame carries the reciprocating tool while the other reaches around and bears against the head of the rivet. All sorts of tools have been built in which a small air-driven motor furnishes the motive power. There are hand drills in which the tool is revolved by a set of pistons, shears for shearing sheep which operate somewhat on the principle of hair clippers, and pneumatic chisels used for chipping stone. Pneumatic motors are widely used in operating cranes and air hoists. They consist of a simple cylinder and piston, and are used for short, direct lifts of all sorts. STOPPING TRAINS WITH AIR Air has played a most important part in transportation. When the air brake was first introduced its purpose was to prevent collisions and provide greater safety of operation. It was not generally realized that efficient air brakes are not only a safety precaution, but a means of increasing schedule speeds. The more quickly a train can stop, the better speed it may make, particularly on a schedule that calls for frequent stops.