I employed, as an expansion valve, a rectangular throttle valve, accurately fitting a bored out seat, in which it might be made to revolve, though it did not revolve in working.  This valve was moved by a pin in a pinion, making two revolutions for every revolution of the engine, and the configuration of the seat determined the amount of the expansion.  In 1855 I have again used expansion valves of this construction in engines making one hundred revolutions per minute, and with perfectly satisfactory results.—­ J.B.

CHAPTER IV.

MODES OF ESTIMATING THE POWER AND PERFORMANCE OF ENGINES AND BOILERS.

HORSES POWER.

209. Q.—­What do you understand by a horse power?

A.—­An amount of mechanical force that will raise 33,000 lbs. one foot high in a minute.  This standard was adopted by Mr. Watt, as the average force exerted by the strongest London horses; the object of his investigation being to enable him to determine the relation between the power of a certain size of engine and the power of a horse, so that when it was desired to supersede the use of horses by the erection of an engine, he might, from the number of horses employed, determine the size of engine that would be suitable for the work.

210. Q.—­Then when we talk of an engine of 200 horse power, it is meant that the impelling efficacy is equal to that of 200 horses, each lifting 33,000 lbs. one foot high in a minute?

A.—­No, not now; such was the case in Watt’s engines, but the capacity of cylinder answerable to a horse power has been increased by most engineers since his time, and the pressure on the piston has been increased also, so that what is now called a 200 horse power engine exerts, almost in every case, a greater power than was exerted in Watt’s time, and a horse power, in the popular sense of the term, has become a mere conventional unit for expressing a certain size of engine, without reference to the power exerted.

211. Q.—­Then, each nominal horse power of a modern engine may raise much more than 33,000 lbs. one foot high in a minute?

A.—­Yes; some raise 52,000 lbs., others 60,000 lbs., and others 66,000 lbs., one foot high in a minute by each nominal horse power.  Some engines indeed work as high as five times above the nominal power, and therefore no comparison can be made between the performances of different engines, unless the power actually exerted be first discovered.

212. Q.—­How is the power actually exerted by engines ascertained?