In Fig. 4, the piston has now reached the bottom of the stroke, and the ports F have opened as the slots carry below the upper end of the cylinder where the bore is increased. At the same time, as the piston plate E passes the bottom of the cylinder H into the enlarged diameter K, the compressed mixture in A and M rushes through the annular space opened around E into the combustion chamber and drives out the residual burned gases which still remain after the explosion. On starting the second revolution the piston rises and the cycle repeats as shown by Fig. 1. [Illustration: Gnome Rotary, Diagrams 3 and 4. ] This engine may be built with any number of the cylinder units described, preferably with an uneven number, as in the case of the Gnome radial four stroke cycle, and with twice the number of impulses of the four stroke type a very uniform turning movement should be had. [Illustration: Fig. 64-b. Roberts Two Stroke Aero Motor Using a Rotating Tubular Valve that Controls the Mixture from the Carburetor so that it Enters Only One Crank Case at a Time. This Gives Each Cylinder an Equal Charge of Gas. ] [Illustration: Fig. 64-c. Roberts Distributor Valve. The Ports Are Cut in the Valve so that Only One Crank Case is in Communication with the Carburetor at Any One Time. The Central Hole Connects with the Carburetor. ] Since the valves are the parts that give the most trouble in the four-stroke cycle Gnome, this motor should be better adapted for aviation than the original type of Gnome. (62) Variable Speed Two Stroke Motor. A variable speed two stroke cycle motor is described by C. Francis Jenkins in the _Scientific American_ that seems to solve many of the problems encountered in designing a two stroke cycle motor for automobile purposes. As is well known, the present design of the crank-case compression type is wasteful of fuel, and ignites irregularly at low speeds and light running, and as nearly all automobiles are well throttled for a greater portion of the time it means that this type of motor is working under the greatest disadvantage. [Illustration: Fig. 66. Jenkins Two Stroke Cycle Motor. ] Since the greater part of the trouble is due to the dilution of charge by the residual gases, and as the spark plug of the motor is situated in the most diluted portion of the gas, it would seem that a change of spark plug location, or a change in the circulation of the fresh mixture in the cylinder would be a great aid in remedying the difficulty. With the spark continually in contact with fresh undiluted mixture it would be possible to run it as low speeds as with the four stroke motor, with a corresponding increase in the efficiency, and opportunity to run with a constant advance of the point of ignition. This is accomplished by any or all of the following conditions: (1.) By keeping good gas separate from bad. (2.) By placing the spark near the intake port.