Of the various agencies employed to produce the requisite pressure, four may be named: (r) mechanical means derived from the rotation of the axles themselves; (2) hydraulic pressure conveyed through pipes joined between each two cars and applied at the engine; (3) air-pressure similarly conveyed through pipes and put on at the engine; and (4) the so-called " vacuum " acting through a continuous pipe system and ope rated at the engine. Of all these, the hydraulic system seems to have failed of general or even wide adoption. The mechanical brake, applied Nvlien the engine is reversed or when, from any other cause, the bumpers of two cars approach each other with force, has its advocates and has met with success, but is not widely (if at all) employed for passenger-service. The "vacuum" system, in which there is produced in the pipes and on one side of a system of diaphragms connected with the brake-beams a vacuum made by a jet of steam escaping through a concentric pipe, is the only one suitable for such service as that of the elevated railroads in New York and Brooklyn, where there is not sufficient time between stops to enable an air-brake to be pumped up. It permits of continuous applica tion as long as the steam jet is " on," and can be put on and off in rapid succession about as fast as the manipulating lever can be operated.
But the system which has practically the monopoly for passenger-ser vice, and which is being extensively introduced in freight-service, is the air-brake, which is all the more efficient in that in its most recent adapta tions it is automatic—that is, not only can it be applied by the engineer at will or by the trainmen at any point in the train-length, but it is also auto matically applied in case the train parts or a break, disconnection, or leak occurs in the air-pipe system.
The IVestinghouse consists of the following essential parts: (i) the steam-engine and pump which compress the air, the steam-sup ply being regulated by the pump-governor; (2) the main reservoir, in which the compressed air is stored; (3) the engineer's brake-valve, which regulates the flow of air from the main reservoir into the brake-pipe for releasing the brakes, and from the brake-pipe to the atmosphere for apply ing the brakes; (4) the main brake-pipe, which leads from the main res ervoir to the engineer's brake-valve and thence along the train, supplying the apparatus on each vehicle with air; (5) the auxiliary reservoir, which takes a supply of air from the main reservoir through the brake-pipe and stores it for use on its own vehicle; (6) the brake-cylinder, which has its piston-rod attached to the brake-levers in such a manner that, when the piston is forced out by air-pressure, the brakes are applied; (7) the triple valve, which connects the brake-pipe to the auxiliary reservoir and con nects the latter to the brake-cylinder, and is operated by a sudden varia tion of pressure in the brake-pipe, so as (a) to admit air from the auxiliary reservoir to the brake-cylinder which applies the brakes, at the same time cutting off the communication from the brake-pipe to the auxiliary reser voir, or (b) to restore the supply from the brake-pipe to the auxiliary res ervoir, at the same time letting the air in the brake-cylinder escape, which releases the brake; and (S) the couplings, which are attached to the flex ible hose and connect the brake-pipe from one vehicle to another. These
parts are all shown in the illustration (p/. 103, jig. 4), the arrangement being substantially as used on the train.
In each car there is a " conductor's valve," with a cord running the entire length of the car; and by pulling this cord any trainman can open the valve and let the air escape from the brake-pipe, thus applying the brake. (It is necessary, however, to hold this valve open until the train conies to a stop.) There are, as auxiliaries, a gauge for showing the pres sure in the main reservoir and brake-pipe in the releasing position of the engineer's brake-valve, and the pressure in the brake-pipe alone in the running position of the same valve.
The air-pump is shown in detail in the Figure. The steam from the boiler enters the top cylinder between two pistons forming the main valve, the upper of which is larger than the lower, so that the tendency of the pressure is to raise the valve unless held down by the pressure of a third piston, of still greater diameter, working in a cylinder directly' above the main valve. The pressure upon this third and largest piston is regu lated by a small slide-valve working in the central chamber on the top head, and receiving its motion front a rod (extending into the hollow pis ton) which has a knob at its lower end and a shoulder just below the top head. This valve-chamber in the top head is in constant communication with the steam-space between the two pistons of the main valve. While the steam acts on the third piston and holds the main valve down, steam is let in below the main piston, and as this piston approaches the upper head the reversing valve-rod and its valve are raised until the slide-valve ex hausts the steam from the space above the third or reversing piston, when the main valve is raised by the steam-pressure on the greater area of its upper piston. This movement of the main valve admits steam to the upper end of the main cylinder. When the main valve is moved up to admit steam to the upper end of the cylinder, it opens an exhaust-port at the lower end, just below the lower steam-port, which is closed by the lower piston and the main valve. When the main piston is on its upward stroke the upper exhaust-port is similarly opened.