the one casting, which rotates at seven-eighths engine speed for the seven-cylinder type, and nine-tenths engine speed for the nine-cylinder engines. Each of these cams has four or five points respectively, to suit the number of cylinders. The steel cylinders are machined from solid forgings and provided with webs for air-cooling as shown. Cast-iron pistons are used, and are connected to the crankshaft in the same manner as with the Gnome and Le Rhone engines. Petrol is sprayed into the crank case by a small geared pump and the mixture is taken from there to the piston valves by radial pipes. Two separate pumps are used for lubrication, one forcing oil to the crank-pin bearing and the other spraying the cylinders. Among other designs of rotary aero engines the E.J.C. is noteworthy, in that the cylinders and crank case of this engine rotate in opposite directions, and two air-screws are used, one being attached to the end of the crankshaft, and the other to the crank case. Another interesting type is the Burlat rotary, in which both the cylinders and crankshaft rotate in the same direction, the rotation of the crankshaft being twice that of the cylinders as regards speed. This engine is arranged to work on the four-stroke cycle with the crankshaft making four, and the cylinders two, revolutions per cycle. It would appear that the rotary type of engine is capable of but little more improvement--save for such devices as these of the last two engines mentioned, there is little that Laurent Seguin has not already done in the Gnome type. The limitation of the rotary lies in its high fuel and lubricating oil consumption, which renders it unsuited for long-distance aero work; it was, in the war period, an admirable engine for such short runs as might be involved in patrol work ‘over the lines,’ and for similar purposes, but the water-cooled Vee or even vertical, with its much lower fuel consumption, was and is to be preferred for distance work. The rotary air-cooled type has its uses, and for them it will probably remain among the range of current types for some time to come. Experience of matters aeronautical is sufficient to show, however, that prophecy in any direction is most unsafe. V THE HORIZONTALLY-OPPOSED ENGINE