520 THE STEAM-ENGINE. 



surface per nominal horse-power, the piston being supposed to move at the j 

 rate of 200 feet per second.* 



The quantity of grate-surface necessary in proportion to the power of the \ 

 engine, has been equally unascertained, and engine-makers vary in their 

 practice from half a square foot to one square foot per nominal horse- 

 power. 



The proportion which the magnitude of the heating-surface of the boiler, and 

 the fire-surface of the grate bears to the evaporating power of the boiler, has not 

 been determined by experiment, nor, so far as we are informed, by any well- 

 ascertained practical results. 



The estimates or rather conjectures of engine-makers, of the evaporation ne- > 

 cessary to produce one-horse power, vary from one to two cubic feet of water 

 per hour. It has been already shown that the evaporation of 900 cubic inches, h 

 or little more than half a cubic foot per hour, evolves a gross mechanical effect C 

 representing one horse-power ; from which it appears, that if the evaporation 

 of the boilers of steam-engines were what engineers suppose them to be, the 

 gross mechanical power produced in them for every nominal horse power 

 of the engine varies in actual amount from the power of two to that of four 

 horses. 



The above estimates must be understood as referring to double-acting steam- 

 engines above thirty-horse power. The circumstances attending the per- 

 formance of single-acting engines applied to the drainage of mines, have been 

 ascertained with much greater precision. This has been mainly owing to a 

 spirited system of general inspection which has been established in Cornwall, 

 to which we shall hereafter more particularly advert. 



In expressing the duty of engines, it would have been desirable that the duty 

 of the boiler should have been separated from that of the engine. 



The duty of a boiler is estimated by the volume of water evaporated by a , 

 given quantity of fuel, independently of the time which such evaporation may / 

 take. The duty, therefore, will be expressed by the number of cubic feet of 

 water evaporated, divided by the number of bushels of coal necessary for that 

 evaporation, supposing the bushel of coal to be the unit of fuel. It will be ob- 

 served that the duty of an engine or boiler is entirely distinct from, and inde- 

 pendent of, its power. One boiler may be greater than another in power to any 

 extent, while it may be equal to or less than it in duty. A bushel of coals may 

 evaporate the same number of cubic feet of water under two boilers, but may 

 \ take twice as great a time to produce such evaporation under one than under the 

 ' other. In such a case the power of one boiler will be double that of the other, 

 while their duty will be the same. 



In like manner, a bushel of coals consumed in working two engines may pro- 

 duce the same useful effect, but it may produce that useful effect in the one in 

 half the time it takes to produce it in the other. In that case the duty of the 

 engines will be the same, but the power of the one will be double that of the 

 J other. 



In fine, power has reference to time duty, to fuel. The more rapidly the 

 engine produces its mechanical effect, the greater its power will be, whatever 

 may be the fuel consumed in working it. And, on the other hand, the greater 



* If 22 square inches of piston-surface be .allowed to represent a horse-power, the power of 

 an engine may always be computed by dividing the square of the diameter of the piston ex- 

 pressed in inches by 28. And, on the other hand, to find the diameter of piston which would 

 correspond to any given power, multiply the number of horses' power by 28, and take the square 

 root of the product. These rules, however, cannot be applied if the piston be supposed to 

 move with any other velocity : since, in that case, the same amount of piston-surfuce would cease 

 to represent a horse-power, unless the effective pressure on the piston were at the same time 

 changed. 



