rod, is of malleable iron, and is formed with a T head,
through which bolts pass up through the brass, to keep the cover of the
brass in its place.

637. _Q._--Is the piston of an oscillating engine made deeper than in
common engines?

_A._--It is expedient, in oscillating engines, to form the piston with a
projecting rim round the edge above and below, and a corresponding recess
in the cylinder cover and cylinder bottom, whereby the breadth of bearing
of the solid part of the metal will be increased, and in many engines this
is now done.

638. _Q._--Would any difficulty be experienced in keeping the trunnions
tight in a high pressure oscillating engine?

_A._--It is very doubtful whether the steam trunnions of a high pressure
oscillating engine will continue long tight if the packing consists of
hemp; and it appears preferable to introduce a brass ring, to embrace the
pipe, cut spirally, with an overlap piece to cover the cut, and packed
behind with hemp.

639. _Q._--How is the packing of the trunnions usually effected?

_A._--The packing of the trunnions, after being plaited as hard as
possible, and cut to the length to form one turn round the pipe, is dipped
into boiling tallow, and is then compressed in a mould, consisting of two
concentric cylinders, with a gland forced down into the annular space by
three to six screws in the case of large diameters, and one central screw
in the case of small diameters. Unless the trunnion packings be well
compressed, they will be likely to leak air, and it is, therefore,
necessary to pay particular attention to this condition. It is also very
important that the trunnions be accurately fitted into their brasses by
scraping, so that there may not be the smallest amount of play left upon
them; for if any upward motion is permitted, it will be impossible to
prevent the trunnion packings from leaking.

DIRECT ACTING SCREW ENGINE.

640. _Q._--Will you describe the configuration and construction of a direct
acting screw engine?

_A._--I will take as an example of this species of engine, the engine
constructed by Messrs. John Bourne & Co., for the screw steamer Alma, a
vessel of 500 tons burden. This engine is a single steeple engine laid on
its side, and in its general features it resembles the engines of the
Amphion already described, only that there is one cylinder instead of two.
The cylinder is of 42 inches diameter and 42 inches stroke, and the vessel
has been pro