HORSE POWER OF STEAM ENGINES. 



459 



and depends on conditions which the maker can have no control of, most 

 reputable makers will give no guarantee of the horse-power of their engine, 

 unless they know every condition under which it is to be worked. They 

 very much prefer to sell an engine of a specified size rather than of a specified 

 power. The nominal horse power of small steam engines is found very 

 nearly by squaring half the diameter of the cylinder. Thus an engine with 

 cylinder six inches in diameter would usually be called a nine horse power 

 engine, sometimes a ten horse-power. When the cylinder is seven inches in 

 diameter we have a twelve horse-power engine. When the cylinder is four 

 inches in diameter, we have a four horse-power engine. This rule is a mere 

 accident, and is true only for small engines with cylinders not exceeding 

 eight inches in diameter. This is true simply because the speed of piston 

 per minute for those engines is usually about 260 feet for assumed average 

 pressure of 40 pounds, or 220 where the average pressure is assumed to be 

 50 pounds per square inch of piston. With a mean average pressure on the 

 piston of 50 pounds per square inch, the following values of diameter of 

 cylinder and speed of piston in feet per minute will be found: 



The piston speed in feet per minute can always be readily figured when the 

 length of the stroke and the number of revolutions per minute are known. The 

 strength of any particular horse is an uncertain amount, but the strength or 

 power of an average horse may be known within quite narrow limits. I have 

 already mentioned the investigation of James Watt, who found that the 

 strength of an average horse iised in pulling was 22,000 pounds raised one 

 foot per minute, or two-thirds of the horse-power of the steam engine. This 

 is supposed to represent the strength of a strong, well-fed horse that can be 

 exerted steadily for ten hours each day. 



For shorter periods of time much more strength can be exerted. Trant- 

 wein says that the strength is increased as the day's work is shortened, and 

 a horse working but five hours per day can exert twice as much force per 

 hour as one working ten hours per day. There is no question but this latter 

 principle holds true to a certain extent, and in some cases may account for 

 deficiency in power of an engine when compared with the power of horses. 

 Horses can do more work in some positions or when using certain sets of 

 muscles than in other positions. Thus a man can do about 40 per cent more 

 work in a tread-mill when he uses the muscles of both arms and legs than 

 when he uses his arms alone. 



Nystrom does not, however, find this to hold true with horses in a tread- 



