blow through valve is shut, and a vacuum very soon forms
within the engine, by the condensation of the steam. If now the slide valve
be moved by hand, the steam from the boiler will be admitted on one side of
the piston, while there is a vacuum on the other side, and the piston will,
therefore, be moved in the desired direction. When the piston reaches the
end of the stroke, the valve has to be moved in the reverse direction, when
the piston will return, and after being moved thus by hand, once or twice,
the connection of the valve with the eccentric is to be restored by
allowing the notch on the end of the eccentric rod to engage the pin on the
valve lever, when the valve will be thereafter moved by the engine in the
proper manner. It will, of course, be necessary, when the engine begins to
move, to open the injection cock a little, to enable water to enter for the
condensation of the steam. In the most recent marine engines, a somewhat
different mechanism from this is used for giving motion to the valves, but
that mechanism will be afterwards described.

117. _Q._--Are all marine engines condensing engines?

_A._--Nearly all of them are so; but recently a number of gunboats have
been constructed, with high pressure engines. In general, however, marine
engines are low pressure or condensing engines.

118. _Q._--Will you now describe the chief features of the oscillating
paddle marine engine?

_A._--In the oscillating paddle marine engine, the arrangement of the
paddle shaft and paddle wheels is the same as in the case already
described, but the whole of the side levers, side rods, cross head, cross
tail, and connecting rod are discarded. The cylinder is set immediately
under the crank; the top of the piston rod is connected immediately to the
crank pin; and, to enable the piston rod to accommodate itself to the
movement of the crank, the cylinder is so constructed as to be susceptible
of vibrating or oscillating upon two external axes or trunnions. These
trunnions are generally placed about half way up on the sides of the
cylinder; and through one of them steam is received from the boiler, while
through the other the steam escapes to the condenser. The air pump is
usually worked by means of a crank in the shaft, which crank moves the air
pump bucket up and down as the shaft revolves.

119. _Q._--Will you give an example of a paddle oscillating engine?

_A._--I will take as an example the oscillating engines constru