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?