gine was made of nickel chrome-steel, and was in two parts connected
together at the crank pin; these two parts, after the master-rod had
been placed in position and the other connecting rods had been attached
to it, were firmly secured. The steel crank case was made in five parts,
the two central ones holding the cylinders in place, and on one side
another of the five castings formed a cam-box, to the outside of which
was secured the extension to which the air-screw was attached. On the
other side of the crank case another casting carried the thrust-box, and
the whole crank case, with its cylinders and gear, was carried on the
fixed crank shaft by means of four ball-bearings, one of which also took
the axial thrust of the air-screw.

For these engines, castor oil is the lubricant usually adopted, and it
is pumped to the crankshaft by means of a gear-driven oil pump; from
this shaft the other parts of the engine are lubricated by means of
centrifugal force, and in actual practice sufficient unburnt oil passes
through the cylinders to lubricate the exhaust valve, which partly
accounts for the high rate of consumption of lubricating oil. A very
simple carburettor of the float less, single-spray type was used, and
the mixture was passed along the hollow crankshaft to the interior of
the crank case, thence through the automatic inlet valves in the tops of
the pistons to the combustion chambers of the cylinders. Ignition was
by means of a high-tension magneto specially geared to give the correct
timing, and the working impulses occurred at equal angular intervals of
102.85 degrees. The ignition was timed so that the firing spark occurred
when the cylinder was 26 degrees before the position in which the piston
was at the outer end of its stroke, and this timing gave a maximum
pressure in the cylinder just after the piston had passed this position.

By 1913, eight different sizes of the Gnome engine were being
constructed, ranging from 45 to 180 brake horse-power; four of these
were single-crank engines one having nine and the other three having
seven cylinders. The remaining four were constructed with two cranks;
three of them had fourteen cylinders apiece, ranged in groups of seven,
acting on the cranks, and the one other had eighteen cylinders ranged in
two groups of nine, acting on its two cranks. Cylinders of the two-crank
engines are so arranged (in the fourteen-cylinder type) that fourteen
equal angular impulses occur d