with which the water would enter the condenser.  Now, if a cubic foot of water evaporated per hour be equivalent to an actual horse power, and 28.9 cubic inches of water be requisite for the condensation of a cubic inch of water in the form of steam, 28.9 cubic feet of condensing water per horse power per hour, or 13.905 cubic inches per second, will be necessary for the engine, and the size of the injection orifice must be such that this quantity of water flowing with the velocity of 43.15 ft. per second, or 517.8 inches per second, will gain admission to the condenser.  Dividing, therefore, 13.905, the number of cubic inches to be injected, by 517.8, the velocity of influx in inches per second, we get 0.02685 for the area of the orifice in square inches; but inasmuch as it has been found by experiment that the actual discharge of water through a hole in a thin plate is only six tenths of the theoretical discharge on account of the contracted vein, the area of the orifice must be increased in the proportion of such diminution of effect, or be made 0.04475, or 1/22d of a square inch per horse power.  This, it will be remarked, is the theoretical area required per actual horse power; but as the friction and contractions in the pipe further reduce the discharge, the area is made 1/15th of a square inch per actual horse power, or rather per cubic foot of water evaporated from the boiler.

335. Q.—­Cannot the condensation of the steam be accomplished by any other means than by the admission of cold water into the condenser?

A.—­It may be accomplished by the method of external cold, as it is called, which consists in the application of a large number of thin metallic surfaces to the condenser, on the one side of which the steam circulates, while on the other side there is a constant current of cold water, and the steam is condensed by coming into contact with the cold surfaces, without mingling with the water used for the purpose of refrigeration.  The first kind of condenser employed by Mr. Watt was constructed after this fashion, but he found it in practice to be inconvenient from its size, and to become furred up or incrusted when the water was bad, whereby the conducting power of the metal was impaired.  He therefore reverted to the use of the jet of cold water, as being upon the whole preferable.  The jet entered the condenser instead of the cylinder as was the previous practice, and this method is now the one in common use.  Some few years ago, a good number of steam vessels were fitted with Hall’s condensers, which operated on the principle of external cold, and which consisted of a faggot of small copper tubes surrounded by water; but the use of those condensers has not been persisted in, and most of the vessels fitted with them have returned to the ordinary plan.

336. Q.—­You stated that the capacity of the feed pump was 1/240th of the capacity of the cylinder in the case of condensing engines,—­the engine being double acting and the pump single acting,—­and that in high pressure engines the capacity of the pump should be greater in proportion to the pressure of the steam.  Can you give any rule that will express the proper capacity for the feed pump at all pressures?