which the plunger
has been drawn up, and the plunger then descends by gravity, forcing out
the water through the cock, the orifice of which has previously been
adjusted, and the plunger in its descent opens the injection valve, which
causes the engine to make a stroke.

250. _Q._--Suppose the cock of the cataract be shut?

_A._--If the cock of the cataract be shut, it is clear that the plunger
cannot descend at all, and as in that case the injection valve cannot be
opened, the engine must stand still; but if the cock be slightly opened,
the plunger will descend slowly, the injection valve will slowly open, and
the engine will make a gradual stroke as it obtains the water necessary for
condensation. The extent to which the cock is open, therefore, will
regulate the speed with which the engine works; so that, by the use of the
cataract, the speed of the engine may be varied to suit the variations in
the quantity of water requiring to be lifted from the mine. In some cases
an air cylinder, and in other cases an oil cylinder, is employed instead of
the apparatus just described; but the principle on which the whole of these
contrivances operate is identical, and the only difference is in the
detail.

251. _Q._--You have now shown that the performance of an engine is
determinable by the indicator; but how do you determine the power of the
boiler?

_A._--By the quantity of water it evaporates. There is, however, no very
convenient instrument for determining the quantity of water supplied to a
boiler, and the consequence is that this element is seldom ascertained.

CHAPTER V.

PROPORTION OF BOILERS.

HEATING AND FIRE GRATE SURFACE.

252. _Q._--What are the considerations which must chiefly be attended to in
settling the proportions of boilers?

_A._--In the first place there must be sufficient grate surface to enable
the quantity of coal requisite for the production of the steam to be
conveniently burnt, taking into account the intensity of the draught; and
in the next place there must be a sufficient flue surface readily to absorb
the heat thus produced, so that there may be no needless waste of heat by
the chimney. The flues, moreover, must have such an area, and the chimney
must be of such dimensions, as will enable a suitable draught through the
fire to be maintained; and finally the boiler must be made capable of
containing such supplies of water and steam as will obviate inconvenient
fluctuations in t