r hour. Both the oil for lubricating
the bearings and the water for cooling the cylinders were circulated by
pumps, and all parts of the valve gear, etc., were completely enclosed
for protection from dust.

A later development of the Green engine was a six-cylindered vertical,
cylinder dimensions being 5.5 inch diameter by 6 inch stroke, developing
120 brake horsepower when running at 1,250 revolutions per minute. The
total weight of the engine with ignition system 398 was 440 lbs., or
3.66 lbs. per horse-power. One of these engines was used on the machine
which, in 1909, won the prize of L1,000 for the first circular mile
flight, and it may be noted, too, that S. F. Cody, making the circuit
of England in 1911, used a four-cylinder Green engine. Again, it was a
Green engine that in 1914 won the L5,000 prize offered for the best aero
engine in the Naval and Military aeroplane engine competition.

Manufacture of the Green engines, in the period of the War, had
standardised to the production of three types. Two of these were
six-cylinder models, giving respectively 100 and 150 brake horse-power,
and the third was a twelve-cylindered model rated at 275 brake
horse-power.

In 1910 J. S. Critchley compiled a list showing the types of engine then
being manufactured; twenty-two out of a total of seventy-six were of the
four-cylindered vertical type, and in addition to these there were two
six-cylindered verticals. The sizes of the four-cylinder types ranged
from 26 up to 118 brake horse-power; fourteen of them developed less
than 50 horse-power, and only two developed over 100 horse-power.

It became apparent, even in the early stages of heavier-than-air flying,
that four-cylinder engines did not produce the even torque that was
required for the rotation of the power shaft, even though a flywheel
was fitted to the engine. With this type of engine the breakage of
air-screws was of frequent occurrence, and an engine having a more
regular rotation was sought, both for this and to avoid the excessive
vibration often experienced with the four-cylinder type. Another, point
that forced itself on engine builders was that the increased power which
was becoming necessary for the propulsion of aircraft made an increase
in the number of cylinders essential, in order to obtain a light engine.
An instance of the weight reduction obtainable in using six cylinders
instead of four is shown in Critchley's list, for one of the
four-cylinder e