type, almost as well known as the Gnome, is the
Clerget, in which both cylinders and crank case are made of steel, the
former having the usual radial fins for cooling. In this type the
inlet and exhaust valves are both located in the cylinder head, and
mechanically operated by push-rods and rockers. Pipes are carried from
the crank case to the inlet valve casings to convey the mixture to the
cylinders, a carburettor of the central needle type being used. The
carburetted mixture is taken into the crank case chamber in a manner
similar to that of the Gnome engine. Pistons of aluminium alloy, with
three cast-iron rings, are fitted, the top ring being of the obturator
type. The large end of one of the nine connecting rods embraces the
crank pin and the pressure is taken on two ball-bearings housed in the
end of the rod. This carries eight pins, to which the other rods are
attached, and the main rod being rigid between the crank pin and piston
pin determines the position of the pistons. Hollow connecting-rods
are used, and the lubricating oil for the piston pins passes from the
crankshaft through the centres of the rods. Inlet and exhaust valves
can be set quite independently of one another--a useful point, since
the correct timing of the opening of these valves is of importance. The
inlet valve opens 4 degrees from top centre and closes after the bottom
dead centre of the piston; the exhaust valve opens 68 degrees before
the bottom centre and closes 4 degrees after the top dead centre of the
piston. The magnetos are set to give the spark in the cylinder at 25
degrees before the end of the compression stroke--two high-tension
magnetos are used: if desired, the second one can be adjusted to give
a later spark for assisting the starting of the engine. The lubricating
oil pump is of the valveless two-plunger type, so geared that it runs
at seven revolutions to 100 revolutions of the engine; by counting
the pulsations the speed of the engine can be quickly calculated by
multiplying the pulsations by 100 and dividing by seven. In the 115
horse-power nine-cylinder Clerget the cylinders are 4.7 bore with a 6.3
inches stroke, and the rated power of the engine is obtained at
1,200 revolutions per minute. The petrol consumption is 0.75 pint per
horse-power per hour.

A third rotary aero engine, equally well known with the foregoing two,
is the Le Rhone, made in four different sizes with power outputs of from
50 to 160 horse-power; th