The cylinder is in two parts, C and C'. The piston, which is very
long, rubs at its upper end against the sides of the cylinder, C. The
lower end is of smaller diameter, and leaves an annular space between
it and the cylinder. The grate is at the bottom of the cylinder, C'.
The sides of the cylinder at the level of the fire box are protected
with a lining 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