STEAM ENGINES 199 



continuously. An arrangement of this kind is shown in Fig. 2, 

 in which water is led into the rim of the pulley by the pipe j 

 and is scooped out and led away by the pipe k. 



Data for Brake Horsepower. It is necessary to know three 

 factors in order to determine the brake horsepower. These are 



(1) the net pull or pressure exerted at the end of the brake arm, 



(2) the length of the brake arm, and (3) the number of revo- 

 lutions per minute of the brake pulley. The work done by 

 the engine is converted into heat by the friction between the 

 wooden blocks and the face of the pulley, and the resistance 

 offered by the brake to the rotation of the pulley is a meas- 

 ure of the work done. This resistance cannot always be 

 measured conveniently at the surface of the pulley, but it can 

 be measured at the end of the brake arm by the scales or the 

 spring balance. 



Net Pull or Pressure. With the brake shown in Fig. 1, 

 there will be a pull in the spring balance due to the weight of 

 the arm d, when the nuts h and i are slacked and the brake is 

 loose on the pulley. This pull should be observed carefully, 

 and should be subtracted from the pull registered by the spring 

 balance during a test. The difference will be the net pull, 

 from which the brake horsepower is calculated. If the arm d 

 is counterbalanced accurately by adding weights to the arm e, 

 as shown dotted at p, it will be unnecessary to perform this 

 subtraction. The net pull will then be indicated directly by 

 the reading of the spring balance. 



The net pressure must also be calculated if the brake shown 

 in Fig. 2 is used. The nut d should be loosened, and the arms / 

 should be worked up and down, to make sure thnt the blocks 

 are loose on the pulley. The ends of the arms should then be 

 rested on the knife edge g and the reading of the scale beam 

 should be observed. This weight represents the pressure due to 

 the block h and the unbalanced arms /, and it must be sub- 

 tracted from the reading of the scale beam observed during a 

 test in order to determine the net pressure. 



In the case of the rope brake shown in Fig. 3, the net pull 

 is easily determined by subtracting the reading indicated 

 by the spring balance from the weight, in pounds, applied 

 at/. 



