To prevent overheating, there were two cooling systems available. In one, known as water-cooling, a stream of water flowed through metal chambers round the cylinders, and tended to reduce their heat. But this implied extra weight, and the carrying also of a radiator, or framework of thin pipes exposed to the air, through which the water was pumped, so as to cool it after it had passed round the heated cylinders. The second system, which obviated the use of water or radiators, was known as air-cooling. In this the tops of the cylinders were ringed with metal flanges, or fins, upon which the air impinged as the machine flew. This method is adopted in most motor-cycle engines, and answers admirably. But with aeroplane motors, in the early days, the high speed at which they worked, and the fact that they obtained no rest in flight, or slackening in the number of their revolutions, made them overheat very often, in spite of their cooling fins. One might liken their work to taking a motor-cycle and running it at top speed up a hill without end. [Illustration: FIG. 47.--The seven-cylinder 50-h.p. Gnome motor.] The difficulty with air-cooling--although it had obvious advantages over water-cooling--was to bring enough air to play upon the surfaces of the cylinders; and it was here that the Gnome won so complete a success. In other engines the cylinders were stationary, and their pistons, moving up and down in the cylinders, turned a crank-shaft to the end of which the propeller was fixed. Therefore the only air the cylinders obtained was what rushed upon them through the speed of the machine in flight. But in the Gnome, instead of the cylinders remaining stationary and the crank-shaft revolving, the cylinders themselves spun round, and the crank-shaft did not move. An illustration of this motor with one end of the crank-chamber removed, so that the piston-rods can be seen, is given in Fig. 47. It will be noted that there are seven cylinders, set in the form of a star, and that the seven piston-rods projecting from them come together upon a single crank-pin, which is attached to the stationary crank-shaft and turns round it. The propeller, instead of being fitted to the crank-shaft, as was the case with other motors, was bolted to a plate upon the engine itself, so that when this turned around its crank-shaft, it carried the propeller with it.