219. Q.—­Can you suggest any eligible method of enabling condensing engines to work satisfactorily at a high rate of speed?

A.—­The most feasible way of enabling condensing engines to work satisfactorily at a high speed, appears to lie in the application of balance weights to the engine, so as to balance the momentum of its moving parts, and the engine must also be made very strong and rigid.  It appears to be advisable to perform the condensation partly in the air pump, instead of altogether in the condenser, as a better vacuum and a superior action of the air pump valves will thus be obtained.  Engines constructed upon this plan may be driven at four times the speed of common engines, whereby an engine of large power may be purchased for a very moderate price, and be capable of being put into a very small compass; while the motion, from being more equable, will be better adapted for most purposes for which a rotary motion is required.  Even for pumping mines and blowing iron furnaces, engines of this kind appear likely to come into use, for they are more suitable than other engines for driving the centrifugal pump, which in many cases appears likely to supersede other kinds of pumps for lifting water; and they are also conveniently applicable to the driving of fans, which, when so arranged that the air condensed by one fan is employed to feed another, and so on through a series of 4 or 5, have succeeded in forcing air into a furnace with a pressure of 2-1/2 lbs. on the square inch, and with a far steadier flow than can be obtained by a blast engine with any conceivable kind of compensating apparatus.  They are equally applicable if blast cylinders be employed.

220. Q.—­Then, if by this modification of the engine you enable it to work at four times the speed, you also enable it to exert four times the power?

A.—­Yes; always supposing it to be fully supplied with steam.  The nominal power of this new species of engine can readily be ascertained by taking into account the speed of the piston, and this is taken into account by the Admiralty rule for power.

221. Q.—­What is the Admiralty rule for determining the power of an engine?

A.—­Square the diameter of the cylinder in inches, which multiply by the speed of the piston in feet per minute, and divide by 6,000; the quotient is the power of the engine by the Admiralty rule.[2]

222. Q.—­The high speed engine does not require so heavy a fly wheel as common engines?

A.—­No; the fly wheel will be lighter, both by virtue of its greater velocity of rotation, and because the impulse communicated by the piston is less in amount and more frequently repeated, so as to approach more nearly to the condition of a uniform pressure.

223. Q.—­Can nominal be transformed into actual horse power?

A.—­No; that is not possible in the case of common condensing engines.  The actual power exerted by an engine cannot be deduced from its nominal power, neither can the nominal power be deduced from the power actually exerted, or from anything else than the dimensions of the cylinder.  The actual horse power being a dynamical unit, and the nominal horse power a measure of capacity of the cylinder, are obviously incomparable things.