ng of plumbago. When the piston is at the bottom of its
travel, the eduction valve closes. The slide valve, _b'_, establishes
a communication between the pump chamber and the cylinder. The air
contained in the pump is already compressed in the latter to a
pressure of nearly a kilogramme at the moment of the communication.
This air enters the cylinder, and the communication between the latter
and the pump continues until all the air is forced into the driving
cylinder, the piston of the pump being at the bottom of its travel,
and that of the cylinder about midway.

[Illustration: BENIER'S HOT AIR ENGINE.]

The air forced by the pump piston enters the cylinder through two
conduits, one of which leads a portion of it toward the top of the
cylinder, and the other toward the bottom. The lower conduit debouches
under the grate, and the air that passes through it traverses the fire
box, and the hot gas fills the cylinder. The conduit that runs to the
top debouches in the cylinder, C, at the lower limit of the surface
rubbed by the piston. The air that traverses this conduit is
distributed through the annular space between the piston and cylinder.
The hot gas derived from combustion can therefore never introduce
itself into this annular space, and consequently cannot come into
contact with the rubbing surfaces of the cylinder and piston.

As the quantity of air introduced at every stroke is constant, the
work developed at every stroke is varied by regulating the temperature
of the gas that fills the cylinder. When the temperature falls, the
pressure, and consequently the work developed, diminishes. This result
is obtained by varying the respective quantities of air that pass
through the fire box and around the piston. In measure as less air
passes through the fire box, the quantity that passes around the
piston augments by just so much, and the pressure diminishes. A valve,
_n'_, in the conduit that runs to the fire box is controlled by the
regulator, L', in the interior of the column. When the work to be
transmitted diminishes, the regulator closes the valve more or less,
and the work developed diminishes.

The coke is put by shovelfuls into a hopper, I. Four buckets mounted
upon the periphery of a wheel, I', traverse the coke, and, taking up a
piece of it, let it fall upon the cover, J, of the slide valve, _j_,
whence it falls into the cavity of the latter when it is uncovered,
and from thence into the conduit, _c'_, of t