is passing through the regenerator into the cylinder. The mixture thus
ignited expands, and acting on the full area of the piston propels it
downward, the under side of the piston being at that time subject to
pressure only on its annular area. When the piston has completed its
down-stroke the passage is opened to the discharge-pipe, and the
expanded products of combustion then pass from the cylinder through the
regenerator, and are discharged. In their passage they give out to the
regenerator a large portion of their heat, which the charge entering
the cylinder for the next stroke receives in passing through the
regenerator.

[Illustration: SIEMENS' GAS PRODUCER AND GAS MOTOR. Fig 2.]

Fig. 2 is a vertical section of a gas producer and scrubber, which, as
stated above, may be employed in combination with engines such as have
been described for supplying them with combustible gas. The producer
is a vessel lined with refractory material. At the top it has a supply
opening covered by a cap, U, having a flange dipping into a sand joint.
At the bottom it has an opening surrounded by inclined bars, V, which
rest upon a water-pipe perforated with small holes, by which water
issues to cool the bars and generate vapor. This vapor rises along
with a limited supply of air through the incandescent fuel above, and
combustible gas is produced, which collects in the annular space, and
is led thence by a pipe to the scrubber. The scrubber is a vessel
containing in its lower part water, W, supplied by a pipe, and having an
overflow. By means of a perforated deflecting plate the gas is caused
to bubble through the water, whereby it is cleansed and cooled, and it
passes by a pipe, X, to supply the engine. The upper end of the vertical
pipe of the scrubber is made open and covered by a cap sealed in water
while the producer is at work. In starting the producer this cap is