45, CORNHILL, E.G., AND 122, REGENT STEEET, W., LONDON. 



559 



HORSE POWER OF STEAM ENGINES. 



When Steam Engines were first introduced, they were commonly applied to work pumps 

 or mills which had been previously wrought by horses. It was therefore convenient to be 

 able to express the performances of these machines by comparison with animal power, to 

 which miners and others had been accustomed. When an Engine was capable of performing 

 the same amount of work in a given time as any given number of horses, such Engine was 

 said to be of so many horses' power. This term having been long in use, it was retained ; it 

 only being requisite to determine upon some standard by which it could be defined. The 

 performance of a horse of average strength working for eight hours a day was therefore 

 selected as a standard or unit of Steam Engine power. 



Smeaton estimated the amount of mechanical effect which the animal could produce at 

 22,916 pounds raised one foot per minute. Desaguliers makes it 27,600 pounds raised through 

 the same height. Messrs. Boulton and Watt caused experiments to be made with the power- 

 ful horses used in London breweries, and from the result of these they assigned 33,000 

 pounds raised one foot per minute as the value of a horse's power ; this estimate is now 

 generally adopted, and when an Engine is said to be of so many horses' power, it means that 

 when in good working order, and properly managed, it is capable of overcoming a resistance, 

 equivalent to so many times 33,000 pounds raised one foot high per minute. Thus an 

 Engine of 10-horse power should be capable of raising 330,000 pounds one foot per minute. 



It being explained that one horse power expresses 33,000 pounds raised one foot high 

 per minute. 1,980,000 pounds raised one foot high per hour, it is required to determine the 

 quantity of water which a boiler must evaporate per hour for each horse power of the 

 Engine which it works. The quantity Of water requisite to produce this result by evapora- 

 tion will be found by considering that one cubic inch of water evaporated will produce a 

 mechanical force equivalent to 2.160 pounds raised one foot high. If we divide 1,980,000 by 

 2,160, it will give the number of cubic inches of water that must be evaporated per hour to 

 produce the mechanical effect expressed by 1 horse power ; the result of this division is 916, 

 which is therefore the number of cubic inches of water per hour whose evaporation is 

 equivalent to one horse power. In actual practice it has been customary for engineers to 

 allow one cubic foot of water per hour for each horse power, a cubic foot being 1,728 cubic 

 inches, or about 11 per cent more than the above estimate. 



Another authority gives the following : 



1 square yard of heating 

 One nominal horse power requires ( 5 gallons of water per hour ) surface. 



approximately . . , ( 15 pounds of coal . . . V 1 square foot of fire bar 



) surface. 



In practice one pound of coal or coke should evaporate about five pounds of water. 

 The nominal horse power of a cylindrical double or single flued boiler may be found 

 approximately by the following rule : The length multiplied by the diameter, and divided 

 by 5. 



Water 

 Pressure. 



Height 



in feet. 



1 



5 



10 

 15 

 20 

 25 



Presure in 



cwt. per 

 square foot. 



55 



2-78 



5-57 



8-36 



11-14 



13-93 



1 Atmosphere equals 14-71 Ibs. per square inch or about 15 Ibs. approximately. 

 Ditto ditto 29 22 inches Mercury. 



Ditto ditto 33-9 feet Water. 



