m inclined to give the preference to a variety of the horizontal
steeple engine, such as was first used in H.M.S. Amphion. In this engine
the cylinders lie on their sides, and they are placed near the side of the
vessel with their mouths pointing to the keel. From each cylinder two long
piston rods proceed across the vessel to a cross head working in guides;
and from this cross head a connecting rod returns back to the centre of the
vessel and gives motion to the crank. The piston rods are so placed in the
piston that one of them passes above the crank shaft, and the other below
the crank shaft. The cross head lies in the same horizontal plane as the
centre of the cylinder, and a lug projects upwards from the cross head to
engage one piston rod, and downwards from the cross head to engage the
other piston rod. The air pump is double acting, and its piston or bucket
has the same stroke as the piston of the engine. The air pump bucket
derives its motion from an arm on the cross head, and a similar arm is
usually employed in engines of this class to work the feed and bilge pumps.

442. _Q._--Is not inconvenience experienced in direct acting screw engines
from the great velocity of their motion?

_A._--Not if they are properly constructed; but they require to be much
stronger, to be fitted with more care, and to have the bearing surfaces
much larger than is necessary in engines moving slowly. The momentum of the
reciprocating parts should also be balanced by a weight applied to the
crank or crank shaft, as is done in locomotives. A very convenient
arrangement for obtaining surface is to form the crank of each engine of
two cast iron discs cast with heavy sides, the excess of weight upon the
heavy sides being nearly equal to that of the piston and its connections.
When the piston is travelling in one direction the weights are travelling
in the opposite; and the momentum of the piston and its attachments, which
is arrested at each reciprocation, is just balanced by the equal and
opposite momentum of the weights. One advantage of the horizontal engine
is, that a single engine may be employed, whereby greater simplicity of the
machinery and greater economy of fuel will be obtained, since there will be
less radiating surface in one cylinder than in two.

CYLINDERS, PISTONS, AND VALVES,

443. _Q._--Is it a beneficial practice to make cylinders with steam
jackets?

_A._--In Cornwall, where great attention is paid to eco