832 TUANSACTWXS OF THE AMEltWAK ly^STlTUTE, 



pressure, varied accord in gj to its density, upon all sides of the veesels 

 in wliit'li it is inclosed. Tliis pressure or fo?xe is measured in 

 pounds per square inch. 'The elastic force of tlie steam acting upon 

 the engine piston, produces motion, whicli is measured in feet. The 

 combined oifects oi force acting through distance produce mechani- 

 cal worl', wliich is measured in foot-pounds. The number of foot- 

 pounds which an engine is capable of developing in a given time^ 

 expresses tlie power of tlie engine. The unit of the poAver is one 

 horse poicer, the value of which is conventionally 'fixed at 33,000 

 foot pounds per minute. 



In proportioning steam machinery for any particular purpose, the 

 first thing to settle upon is the amount of power required, and this 

 being fixed in all cases, within certain limits, the practiced.- question 

 is to obtain a cei'tain power at ike least possible cost. 



We will first discuss the ways and means used to measure and 

 determine 



The Powke. 



As has been said, the power of an engine depends upon the worh^ 

 done in a given time; and as work implies force and motion^ we must 

 ascertain tliree things before we can calculate the power, vi2. : The 

 viean force and the distance through ^vhich it is exerted ; also the 

 time required t«r the movement. Having these, we first ascertain 

 the distance moved per minute, and this, multiplied by the niean 

 force, gives the number of foot-pounds per minute, which, divided 

 by 33,000, gives the horse poxcer. The distance through which the 

 force is exerted, is usually calculated from the number of i-evolu- 

 tions made per minute by the engine, which can be ascertained 

 approximately,! by actual count, but better by means of a register. 

 The speed of the engine is varied more or less by every change in 

 the load, or in the pressure of steam, even when a governor is used ; 

 for a change in speed must take i)lace l)efore the governor can ope- 

 rate. The variations are small, with seiitiitive regulators, but in a 

 majoritj^ of cases, would materially affect the result. The true plan, 

 then, is to attach a register to the engine, the indications of which 

 should be taken once an hour, to check mistakes, and in the calcula- 

 tions the revolutions per minute should be an average for the whole 

 time through which the ti'ial extends. If the power is to be calcu- 

 lated from the pressure on the piston, the piston movement is also 

 used and ascertained by multiplying the revolutions per minute by 



