automatic cock, with a valve inside the plug through which the air passes to and from the train-pipe, so that when the air is being exhausted it is drawn from the reservoir, and the top and bottom of the sack, but when air is admitted to the train-pipe, the valve in the cock closes the connection to the top of the sack and reservoir, but leaves the bottom of the sack open to the train-pipe. While the train is running a continual vacuum is maintained in the train-pipes, reservoirs, and sacks, by means of a small ejector on the engine, and in this condition the brakes are "off," but when it is desired to apply them, air is admitted into the train-pipe by the driver or guard; the valve in the automatic cock moves as previously described, and the pressure of the atmosphere, acting on the under side of the diaphragm, lifts the piston rod and applies the brakes. The same result ensues if the train becomes separated or a coupling breaks, air being thus admitted to the train-pipe, and the brakes being applied automatically throughout the train.
When the brakes have been applied, and it is desired to take them off again, all that is necessary is to renew the vacuum by means of the ejector. The driver's brake valve on the engine is so arranged that both the steam and vacuum brakes can be applied simultaneously by one movement of a lever, but the train-pipe may be used without the steam brake if desired, by means of a separate valve provided for the purpose, both on the engine and in the guard's van. By means of the automatic cock previously referred to, the brake on each vehicle may be released while shunting is going on, and may also be shut off if the brake gear is out of order, leaving the train-pipe intact throughout the train.
