eaching 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.

179. _Q._--What will be the pressure of the steam, under such
circumstances, at the end of the stroke?

_A._--If the steam be shut off at half stroke, the pressure of the steam,
reckoning the total pressure both below and above the atmosphere, will just
be one-half of what it was at the beginning of the stroke. It is a well
known law of pneumatics, that the pressure of elastic fluids varies
inversely as the spaces into which they are expanded or compressed. For
example, if a cubic foot of air of the atmospheric density be compressed
into the compass of half a cubic foot, its elasticity will be increased
from 15 lbs. on the square inch to 30 lbs. on the square inch; whereas