CHAPTER XIX. THE POWER OF STEAM-ENGINES. THE power of engines is universally stated in comparison. with that of horses, and the English standard is the maximum work of which a powerful horse is found to be capable, viz.: 33,000 lbs. raised 1 foot high in 1 minute. As a matter of practice this result is unattainable with ordinary animals for any length of time, therefore a steamengine of given effective power is equal to more than the work of the number of horses stated. An effective horse-power, called also, brake, belt, or actual horse-power, is the real power given by it from the shaft, pulley, or belt. An indicated horse-power is the power developed by the steam in the cylinder, and of course from it has to be deducted the power eaten up in driving the engine itself. The French "force de cheval" is a close approximation to the English, as, I English horse-power 1.01385 force de cheval. Inversely stated I Force de cheval .986337 of an English horse-power. = The rule for finding a French indicated horse-power is as follows: Let D diameter of cylinder in metres. S stroke in metres. Rrevolutions per minute. P = average pressure on piston in kilogrammes per centimetre. French indicated horse-power = 3.49 × D2 × P× Rx S. This rather clumsy definition of a power forms a standard for the power or duty of boilers, which is convenient in the absence of any clearer form of reference. Naturally, with boilers working under widely different conditions, a parity can be established only by proportionate calculations, and the same remark would hold good of steam-engines using more or less than the amount it fixes as a basis. For the purpose of readily ascertaining the power of any given engine cylinder the following table will be found to save much calculation : Initial Pressure TABLE OF MEAN PRESSURES OF STEAM IN CYLINDERS. AVERAGE PRESSURE OF STEAM IN Las. Per Square Inch FOR THE Cut-off at 4. as in very High-class Engines. or in others when Kunning Ligin |