A.—­No; it is necessary in practice that the feed pump should be able to supply the boiler with a much larger quantity of water than what is indicated by these proportions, from the risk of leaks, priming, or other disarrangements, and the feed pump is usually made capable of raising 3-1/2 times the water evaporated by the boiler.  About 1/240th of the capacity of the cylinder answers very well for the capacity of the feed pump in the case of low pressure engines, supposing the cylinder to be double acting, and the pump single acting; but it is better to exceed this size.

176. Q.—­Is this rule for the size of the feed pump applicable to the case of high pressure engines?

A.—­Clearly not; for since a cylinder full of high pressure steam, contains more water than the same cylinder full of low pressure steam, the size of the feed must vary in the same proportion as the density of the steam.  In all pumps a good deal of the effect is lost from the imperfect action of the valves; and in engines travelling at a high rate of speed, in particular, a large part of the water is apt to return, through the suction valve of the pump, especially if much lift be permitted to that valve.  In steam vessels moreover, where the boiler is fed with salt water, and where a certain quantity of supersalted water has to be blown out of the boiler from time to time, to prevent the water from reaching too high a degree of concentration, the feed pump requires to be of additional size to supply the extra quantity of water thus rendered necessary.  When the feed water is boiling or very hot, as in some engines is the case, the feed pump will not draw from a depth, and will altogether act less efficiently, so that an extra size of pump has to be provided in consequence.  These and other considerations which might be mentioned, show the propriety of making the feed pump very much larger than theory requires.  The proper proportions of pumps, however, forms part of a subsequent chapter.

[1] A table containing the results arrived at by M. Regnault is given in the Key.

CHAPTER III.

EXPANSION OF STEAM AND ACTION OF THE VALVES.

177. Q.—­What is meant by working engines expansively?

A.—­Adjusting the valves, so that the steam is shut off from the cylinder before the end of the stroke, whereby the residue of the stroke is left to be completed by the expanding steam.

178. Q.—­And what is the benefit of that practice?

A.—­It accomplishes an important saving of steam, or, what is the same thing, of fuel; but it diminishes the power of the engine, while increasing the power of the steam.  A larger engine will be required to do the same work, but the work will be done with a smaller consumption of fuel.  If, for example, the steam be shut off when only half the stroke is completed, there will only be half the quantity of steam used.  But there will be more than half the power exerted; for although the pressure of the steam decreases after the supply entering from the boiler is shut off, yet it imparts, during its expansion, some power, and that power, it is clear, is obtained without any expenditure of steam or fuel whatever.