THE STEAM-ENGINE. 



519 



at this temperature steam has a pressure of about one pound per square inch. 

 This vapor will continue to fill the cylinder, and will resist the moving power 

 which impels the piston. 



7. Power must be provided for opening and closing the valves or slides, 

 for working the air-pump, hot-water pump, and cold-water pump, and finally 

 to overcome the friction on the journals and centres of the parts of the par- 

 allel motion, the main axle of the beam, the connecting-rod, crank, and fly- 

 wheel axle. 



It will be apparent how very much these sources of resistances must vary 

 in different engines, and how rough an approximation any general estimate ( 

 must be of their gross amount. 



There are many circumstances which obstruct the practical application of 

 any standard of engine-power : the magnitude of furnace, and the extent of 

 heating surface necessary to produce any required rate of evaporation in the 

 boiler, are unascertained ; each engine-maker has his own rule in these mat- 

 ters, and all the rules are equally unsupported by any experimental test enti- 

 tled to respect. Thus the circumstances that govern the rate of evaporation 

 in the boiler may be regarded as almost wholly unknown. But supposing the 

 rate of evaporation to be ascertained, the amount of power absorbed by the 

 condensation of steam on its passage to the cylinder, the imperfect conden- 

 sation of the same steam after it has worked the piston, the friction of 

 the various moving parts of the machinery, and, above all, the difference of 

 effect of these losses of power in engines constructed on different scales of 

 magnitude, are absolutely unknown. We are, therefore, not placed in a con- 

 dition to assign anything more than a general account of what has been the 

 practice of engine-makers in constructing engines which are nominally of a 

 certain power. 



In common low-pressure engines of the larger kind, to which class aloiw 

 we at present refer, it has been usual, with the same fuel and under like 

 circumstances, to allow from 10 to 18 square feet of heating surface in the 

 boiler for every nominal horse-power of the engine. Within these wide limits 

 the practice of engine-makers has varied. It is not, however, to be supposed 

 that the boiler with 18 square feet of surface per horse-power has the same evap- 

 orating power as that which has but 10. This difference, therefore, amounts to 

 nothing more than different manufacturers of steam-engines putting into circu- 

 lation boilers having powers really different while they are nominally the same. 

 The magnitude of the cylinder is regulated by the nominal power of the engine, 

 and it is usual so to regulate the evaporating power of the boiler, that the piston 

 shall move at the average rate of 200 feet per minute. This being assumed, it 

 is customary to allow about 22 square'inches of piston surface for every nomi- 

 nal horse-power of the engine. If this power were in conformity to the stan- 

 dard already defined, this amount of surface moved at 200 ft. per minute would be 

 impelled by a pressure amounting to 7 Ibs. per square inch. The safety-valve 

 of the boiler of such engines is usually loaded at from 4 to 5 Ibs. per square 

 inch and consequently the steam in the boiler will have a pressure of from 19 

 to 20 Ibs. per square inch. If, therefore, the effective pressure on the piston be 

 really only 7* Ibs. per square inch, the pressure expended in overcoming the 

 friction of the engine, and the loss consequent on the partial condensation of 

 steam on one side and its imperfect condensation on the other, would amount to 

 from 12 to 13 tt>s. per square inch, or nearly double the assumed useful effect of 

 the engine. 



Messrs. Maudslay and Field are accustomed to allow an evaporation of 

 ten gallons, or 1-6 cubic feet of water per hour, for each nominal horse- 

 power of the engine. They also allow about 22 square inches of piston- 



