s to prevent the exhausting valve from opening
and to allow the steam valve to open and remain open, otherwise a partial
vacuum may arise in the cylinder, and it may be filled with water from the
injection or from leaks. A single acting engine, when it is in good order,
ought to be capable of going as slow as one stroke in ten minutes, and as
fast as ten strokes in one minute; and if it does not fulfil these
conditions, there is some fault which should be ascertained and remedied.

427. _Q._--Your explanation has reference to the pumping engine as
introduced into Cornwall by Watt: have any modifications been since made
upon it?

_A._--In the modern Cornish engines the steam is used very expansively, and
a high pressure of steam is employed. In some cases a double cylinder
engine is used, in which the steam, after having given motion to a small
piston on the principle of a high pressure engine, passes into a larger
cylinder, where it operates on the principle of a condensing engine; but
there is no superior effect gained by the use of two cylinders, and there
is greater complexity in the apparatus. Instead of the lever walls, cast
iron columns are now frequently used for supporting the main beam in
pumping engines, and the cylinder end of the main beam is generally made
longer than the pump end in engines made in Cornwall, so as to enable the
cylinder to have a long stroke, and the piston to move quickly, without
communicating such a velocity to the pump buckets as will make them work
with such a shock as to wear themselves out quickly. A high pressure of
steam, too, can be employed where the stroke is long, without involving the
necessity of making the working parts of such large dimensions as would
otherwise be necessary; for the strength of the parts of a single acting
engine will require to be much the same, whatever the length of the stroke
may be.

428. _Q._--What kind of pump is mostly used in draining deep mines?

_A._--The pump now universally preferred is the plunger pump, which admits
of being packed or tightened while the engine is at work; but the lowest
lift of a mine is generally supplied with a pump on the suction principle,
both with the view of enabling the lowest pipe to follow the water with
facility as the shaft is sunk deeper, and to obviate the inconvenience of
the valves of the pump being rendered inaccessible by any flooding in the
mine. The pump valves of deep mines are a perpetual source of e