PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 643 



power, amounting to a considerable portion of the power genera- 

 ted. But that will not be the case with compressed air, as there 

 are no valves to suffer the escape of the pressure above the resist- 

 ance of the pistons, and no loss of power can take place, as it 

 must pass through the cylinders and have done work before it 

 can escape at the blow-off passage. The material point, there- 

 fore, will be to ascertain the resistance of the engine and load, 

 and to set the self-acting regulator accordingly. 



" This important but simple operation is effected by a separate 

 cylinder, with- its regulating apparatus placed between the re- 

 ceiver, charged to a very high density, and the driving cylinders. 

 This part of the machine restrains the violent force of the stock 

 of power in the receiver, and at the same time gives a constant 

 pressure on the pistons. Being self-acting after it is set to any 

 given pressure, the engine will work steadily and maintain a uni- 

 formity of speed, upon a level, till the density in the receiver has 

 been gradually reduced to the working pressure ; before which 

 time, if driven properly, the engine will have reached its propo- 

 sed distance, when it can be replenished for the next distance of 

 its journey." 



The apparatus here spoken of, to be placed " under the front 

 plate in large engines," will occupy but little space, as may be 

 seen by illustrative drawings appended to the communication. 

 Though other, and a variety of modes, perhaps equally good for 

 regulating the supply to the cylinders and the pressure on the 

 pistons, may be adopted with equal advantage. 



" As compressed air loses no power from keeping, engines may 

 stand charged and be started at a minute's notice, neither wast- 

 ing power, costing anything, nor doing any damage to the engine, 

 which cannot be the case with steam locomotives." 



The advantages of compressed air power are thus summed up 

 by Mr. Parsey, under the idea of substituting it for steam in 

 moving trains : 



" 1st. A reduction in the prime cost of locomotives of twenty 

 per cent, or more, arising from air reservoirs, &c., costing less 

 than boilers, tubes, fire boxes, &c., for which they are substituted. 



" 2d. A reduction of one-third the present number, as air en- 

 gines will run any length of journey (which steam locomotives 

 will not) without being changed ; and not requiring to be inac- 

 tive in the workshops, so many spare engines will not be wanted 

 as for steam. 



