tion trunnions be immersed in water?

_A._--In some cases a hollow, or lantern brass, about one third or one
fourth the length of the packing space, and supplied with steam or water by
a pipe, is introduced in the middle of the packing, so that if there be any
leakage through the trunnion, it will be a leakage of steam or water, which
will not vitiate the vacuum; but in ordinary cases this device will not be
necessary, and it is not commonly employed. It is clear that there can be
no buckling of the sides of the cylinder by the strain upon the trunnions,
if the cylinder be made strong enough, and in cylinders of the ordinary
thickness such an action has never been experienced; nor is it the fact,
that the intermediate shaft of steam vessels, to which part alone the
motion is communicated by the engine, requires to adapt itself to the
altering forms of the vessel, as the engine and intermediate shaft are
rigidly connected, although the paddle shaft requires to be capable of such
an adaptation. Even if this objection existed, however, it could easily be
met by making the crank pin of the ball and socket fashion, which would
permit the position of the intermediate shaft, relatively with that of the
cylinder, to be slightly changed, without throwing an undue strain upon any
of the working parts.

440. _Q._--Is the trunk engine inferior to the oscillating?

_A._--A very elegant and efficient arrangement of trunk engine suitable for
paddle vessels has latterly been employed by Messrs. Rennie, of which all
the parts resemble those of Penn's oscillating engine except that the
cylinders are stationary instead of being movable; and a round trunk or
pipe set upon the piston, and moving steam tight through the cylinder
cover, enables the connecting rod which is fixed to the piston to vibrate
within it to the requisite extent. But the vice of all trunk engines is
that they are necessarily more wasteful of steam, as the large mass of
metal entering into the composition of the trunk, moving as it does
alternately into the atmosphere and the steam, must cool and condense a
part of the steam. The radiation of heat from the interior of the trunk
will have the same operation, though in vertical trunk engines the loss
from this cause might probably be reduced by filling the trunk with oil, so
far as this could be done without the oil being spilt over the edge.

441. _Q._--What species of screw engine do you consider the best?

_A._--I a