connecting the pistons to the crank pins can be seen. The mixture is
drawn through the crank chamber, and to enter the cylinder it passes
through the two automatic valves in the crown of the piston; the exhaust
valves are situated in the tops of the cylinders, and are actuated by
cams and push-rods. Cooling of the cylinder is assisted by the radial
rings, and the diameter of these rings is increased round the hottest
part of the cylinder. When long flights are undertaken the advantage of
the light weight of this engine is more than counterbalanced by its high
fuel and lubricating oil consumption, but there are other makes which
are much better than this seven-cylinder German in respect of this.

Rotation of the cylinders in engines of this type is produced by the
side pressure of the pistons on the cylinder walls, and in order to
prevent this pressure from becoming abnormally large it is necessary
to keep the weight of the piston as low as possible, as the pressure is
produced by the tangential acceleration and retardation of the piston.
On the upward stroke the circumferential velocity of the piston is
rapidly increased, which causes it to exert a considerable tangential
pressure on the side of the cylinder, and on the return stroke there
is a corresponding retarding effect due to the reduction of the
circumferential velocity of the piston. These side pressures cause an
appreciable increase in the temperatures of the cylinders and pistons,
which makes it necessary to keep the power rating of the engines fairly
low.

Seguin designed his first Gnome rotary as a 34 horse-power engine when
run at a speed of 1,300 revolutions per minute. It had five cylinders,
and the weight was 3.9 lbs. per horse-power. A seven-cylinder model soon
displaced this first engine, and this latter, with a total weight of 165
lbs., gave 61.5 horse-power. The cylinders were machined out of solid
nickel chrome-steel ingots, and the machining was carried out so that
the cylinder walls were under 1/6 of an inch in thickness. The pistons
were cast-iron, fitted each with two rings, and the automatic inlet
valve to the cylinder was placed in the crown of the piston. The
connecting rods, of 'H' section, were of nickel chrome-steel, and the
large end of one rod, known as the 'master-rod' embraced the crank pin;
on the end of this rod six hollow steel pins were carried, and to these
the remaining six connecting-rods were attached. The crankshaft of the
en