_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, if
its volume be enlarged to two cubic feet, its elasticity will be reduced to
7-1/2 lbs. on the square inch, being just half its original pressure. The
same law holds in all other proportions, and with all other gases and
vapors, provided their temperature remains unchanged; and if the steam
valve of an engine be closed, when the piston has descended through
one- fourth of the stroke, the steam within the cylinder will, at the end
of the stroke, just exert one-fourth of its initial pressure.