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 engines developed 118.5 brake horse-power and weighed 1,100 lbs., whereas a six-cylinder engine by the same manufacturer developed 117.5 brake horse-power with a weight of 880 lbs., the respective cylinder dimensions being 7.48 diameter by 9.06 stroke for the four-cylinder engine, and 6.1 diameter by 7.28 stroke for the six-cylinder type. A list of aeroplane engines, prepared in 1912 by Graham Clark, showed that, out of the total number of 112 engines then being manufactured, forty-two were of the vertical type, and of this number twenty-four had four-cylinders while sixteen were six-cylindered. The German aeroplane engine trials were held a year later, and sixty-six engines entered the competition, fourteen of these being made with air-cooled cylinders. All of the ten engines that were chosen for the final trials were of the water-cooled type, and the first place was won by a Benz four-cylinder vertical engine which developed 102 brake horse-power at 1,288 revolutions per minute. The cylinder dimensions of this engine were 5.1 inch diameter by 7.1 inch stroke, and the weight of the engine worked out at 3.4 lbs. per brake horse-power. During the trials the full-load petrol consumption was 0.53 pint per horse-power per hour, and the amount of lubricating oil used was 0.0385 pint per brake horse-power per hour. In general construction this Benz engine was somewhat similar to the Green engine already described; the overhead valves, fitted in the tops of the cylinders, were similarly arranged, as was the cam-shaft; two springs were fitted to each of the valves to guard against the possibility of the engine being put out of action by breakage of one of the springs, and ignition was obtained by two high-tension magnetos giving simultaneous sparks in each cylinder by means of two sparking plugs--this dual ignition reduced the possibility of ignition troubles. The cylinder jackets were made of welded sheet steel so fitted around the cylinder that the head was also water-cooled, and the jackets were corrugated in the middle to admit of independent expansion. Even the