shaft of the mine, or otherwise, the water may be drawn without
inconvenience from any depth. The introduction of the centrifugal pump
would obviously extinguish the single acting engine, as rotative engines
working at a high speed would be the most appropriate form of engine where
the centrifugal pump was employed.

433. _Q._--This would not be a heavy deprivation?

_A._--The single acting engine is a remnant of engineering barbarism which
must now be superseded by more compendious contrivances. The Cornish
engines, though rudely manufactured, are very expensive in production, as a
large engine does but little work; whereas by employing a smaller engine,
moving with a high speed, the dimensions may be so far diminished that the
most refined machinery may be obtained at less than the present cost.

434. _Q._--Are not the Cornish engines more economical in fuel than other
engines?

_A._--It is a mistake to suppose that there is any peculiar virtue in the
existing form of Cornish engine to make it economical in fuel, or that a
less lethargic engine would necessarily be less efficient. The large duty
of the engines in Cornwall is traceable to the large employment of the
principle of expansion, and to a few other causes which may be made of
quite as decisive efficacy in smaller engines working with a quicker speed;
and there is therefore no argument in the performance of the present
engines against the proposed substitution.

VARIOUS FORMS OF MARINE ENGINES.

435. _Q._--What species of paddle engine do you consider to be the best?

_A._--The oscillating engine.

436. _Q._--Will you explain the grounds of that preference?

_A._--The engine occupies little space, consists of few parts, is easily
accessible for repairs, and may be both light and strong at the same time.
In the case of large engines the crank in the intermediate shaft is a
disadvantage, as it is difficult to obtain such a forging quite sound. But
by forging it in three cranked flat bars, which are then laid together and
welded into a square shaft, a sound forging will be more probable, and the
bars should be rounded a little on the sides which are welded to allow the
scoriae to escape during that operation. It is important in so large a
forging not to let the fire be too fierce, else the surface of the iron
will be burnt before the heart is brought to a welding heat. In some cases
in oscillating engines the air pump has been wrought by an eccentric