128. _Q._--What is the kind of direct acting screw engine employed by
Messrs. Penn.

_A._--It is a horizontal trunk engine. In this engine a round pipe called a
trunk penetrates the piston, to which it is fixed, being in fact cast in
one piece with it; and the trunk also penetrates the top and bottom of the
cylinder, through which it moves, and is made tight therein by means of
stuffing boxes. The connecting rod is attached at one end to a pin fixed in
the middle of the trunk, while the other end engages the crank in the usual
manner. The air pump is set within the condenser, and is wrought by a rod
which is fixed to the piston and derives its motion therefrom. The air pump
is of that species which is called double-acting. The piston or bucket is
formed without valves in it, but an inlet and outlet valve is fixed to each
end of the pump, through the one of which the water is drawn into the pump
barrel, and through the other of which it is expelled into the hot well.



THE LOCOMOTIVE ENGINE.

129. _Q._--Will you describe the more important features of the locomotive
engine?

_A._--The locomotive employed to draw carriages upon railways, consists of
a cylindrical boiler filled with brass tubes, through which the hot air
passes on its progress from the furnace to the chimney, and attached to the
boiler are two horizontal cylinders fitted with pistons, valves, connecting
rods, and other necessary apparatus to enable the power exerted by the
pistons to turn round the cranked axle to which the driving wheels are
attached. There are, therefore, two independent engines entering into the