Mixture is drawn into the cylinder by the suction of the piston through an inlet valve in the piston head, in practically the same way as in the “Gyro” motor, but unlike the latter motor, the valve is lifted by the suction (automatic valve) and not by the mechanical actuation of the connecting rod. The inlet valve is balanced against the effects of centrifugal force by a small counter-weight in the piston head, and the valve is held normally on its seat by a flat spring acting on the valve stem. The gases are brought into the crank case from the carburetor through the hollow crank-shaft as described elsewhere. See Fig. 53. [Illustration: Fig. 53. Longitudinal Section Through Gnome Rotary Motor. ] All exhaust valves are located in the cylinder head and are actuated by long push rods that are moved by individual cams in an extension of the crank case. The exhaust valves are counter-balanced against centrifugal force and are retained on their seats by a flat spring. The counter weights do not entirely overcome the effects of the centrifugal force but allow a slight excess to exist which will permit the engine to run with a broken spring. All of the exhaust gases escape directly to the atmosphere without piping or mufflers. Owing to the fact that the advancing or leading face of the cylinder is cooler than the trailing face, the cylinder bore is thrown out of line by the difference in expansion between the two sides. Because of this distortion of the bore, a special form of piston ring is used, which, by its flexibility, adapts itself to variations in the bore. These rings are of brass and are shaped like the pump leathers of a water pump so that the pressure of the explosion acting on the inside of the ring tends to force the thin shell against the cylinder. In spite of this precaution, the compression pressure is very low at the best, in the most of cases not over 45 pounds per square inch. The exhaust valve screws into the end of the cylinder and may be removed, complete with its seat, for the frequent regrinding necessary to efficient operation. After the cylinders are ground with the greatest care and accuracy, the finishing is carried still further by wearing-in the cylinder with an actual piston carrying an “obturateur” or piston ring. [Illustration: Fig. 54. Gnome Motor on Testing Stand. From Scientific American. ]