188 CONTINUOUS RAILWAY CRAKES. 



hnuso "automatic brake and tlio "Hardy" automatic vacuum 

 brake, to be presently described. 



The brakes of Class III., whether " simple " or " auto- 

 matic," — but the latter more especially, — have to a very 

 large extent displaced, and will eventually ontiroly super- 

 sede, all other kinds of brakes. 



Two distinct methods are followed for giving motion to 

 the brake-piston : — 



1. That of "differential" pressures, in which the brake- 

 piston, when the brakes are " off," is maintained in a con- 

 dition of unstable equilibrium, and in which the normal 

 unstable equilibrium is destroyed for application of the 

 brakes. 



2. That of using separate " vacuum " or " compressed 

 air" reservoirs, or, in the case of Barker's hydraulic brake, 

 a reservoir in which water is stored under pressure, from 

 which reservoirs, in each instance, communication is opened 

 with the brake cylinders when the brakes are to be applied. 

 When the brakes are " off," the communication between the 

 cylinders and reservoirs is closed, the brake piston being 

 then held in stable equilibrium by air on each side at the 

 normal atmospheric pressure outside the brako-cylindor, the 

 brakes being released by their own weight, or by springs. 



To the class of ^^ Differential" pressm-e brakes belong the 

 " Automatic " vacuum brake, the Steel & McJnnes air- 

 pressure brake, the " Clayton " automatic vacuum brake, the 

 " Wenger " air-pressure brake, and " Aspinall's," " Smith's," 

 and " Saunders's " automatic vacuum brakes. 



To the class of " Separate Reservoir " brakes belong the 

 " Hardy " automatic vacuum brake, the " Westinghouse " 

 automatic air-pressure brake, the " Eames " vacuum auto- 

 matic brake, and the " Barker " hydraulic automatic pres- 

 sure brake. 



