216. Q.—­How do you ascertain the power of high pressure engines?

A.—­The actual power is readily ascertained by the indicator, by the same process by which the actual power of low pressure engines is ascertained.  The friction of a locomotive engine when unloaded is found by experiment to be about 1 lb. per square inch on the surface of the pistons, and the additional friction caused by any additional resistance is estimated at about .14 of that resistance; but it will be a sufficiently near approximation to the power consumed by friction in high pressure engines, if we make a deduction of a pound and a half from the pressure on that account, as in the case of low pressure engines.  High pressure engines, it is true, have no air pump to work; but the deduction of a pound and a half of pressure is relatively a much smaller one where the pressure is high, than where it does not much exceed the pressure of the atmosphere.  The rule, therefore, for the actual horse power of a high pressure engine will stand thus:  square the diameter of the cylinder in inches, multiply by the pressure of the steam in the cylinder per square inch less 1-1/2 lb., and by the speed of the piston in feet per minute, and divide by 42,017; the quotient is the actual horse power.

217. Q.—­But how do you ascertain the nominal horse power of high pressure engines?

A.—­The nominal horse power of a high pressure engine has never been defined; but it should obviously hold the same relation to the actual power as that which obtains in the case of condensing engines, so that an engine of a given nominal power may be capable of performing the same work, whether high pressure or condensing.  This relation is maintained in the following rule, which expresses the nominal horse power of high pressure engines:  multiply the square of the diameter of the cylinder in inches by the cube root of the length of stroke in feet, and divide the product by 15.6.  This rule gives the nominal power of a high pressure engine three times greater than that of a low pressure engine of the same dimensions; the average effective pressure being taken at 21 lbs. per square inch instead of 7 lbs., and the speed of the piston in feet per minute being in both rules 128 times the cube root of the length of stroke.[1]

218. Q.—­Is 128 times the cube root of the stroke in feet per minute the ordinary speed of all engines?

A.—­Locomotive engines travel at a quicker speed—­an innovation brought about not by any process of scientific deduction, but by the accidents and exigencies of railway transit.  Most other engines, however, travel at about the speed of 128 times the cube root of the stroke in feet; but some marine condensing engines of recent construction travel at as high a rate as 700 feet per minute.  To mitigate the shock of the air pump valves in cases in which a high speed has been desirable, as in the case of marine engines employed to drive the screw propeller without intermediate gearing, India rubber discs, resting on a perforated metal plate, are now generally adopted; but the India rubber should be very thick, and the guards employed to keep the discs down should be of the same diameter as the discs themselves.