In the early days when scientists were groping for the secret of air navigation the best that the clumsy steam engines they had at their disposal would do was to generate one horsepower of energy for every ten pounds of weight. These days the light powerful aeroplane engines we hear roaring over our heads are generating one horsepower of energy for every three or three and a half pounds of dead weight, and engines have been constructed weighing only one pound to every horsepower, though they are impractical for general use. The first engines that were used in aeroplanes were simply automobile engines adapted to air navigation. The main question in those days was lightness and power. This was achieved by skimming down the best available automobile engines so that they were as light as safety would allow. Although lightness is still an important factor in aeroplane engine construction, many authorities declare that it is growing less so as the science advances and aeroplanes are able to carry heavier loads. There were many intricate and difficult problems, however, that attended taking a motor aloft to drive an aeroplane. The motor had to run at top speed every second, for it could not rest on a low gear as an automobile engine could. First one part and then another would give out and the motors were constantly overheating. Experience taught the makers how to make their machines light enough and yet strong enough to do the required work. It was in cooling that the greatest difficulties were met, and it was this that brought about the great innovations in motor building. The system of cooling the engine with water required much heavy material, such as pipes, pumps, water, water jackets, and radiator. On account of the general efficiency of a water-cooled engine many builders of aeroplanes stuck to it and developed it to a very high standard. At present many of the prize-winning engines are water cooled, as, for instance, the Wright and Curtiss. All of these water-cooled engines and several standard air-cooled makes are of the reciprocating type that have stationary cylinders and crankcase while the crankshaft rotates like that of the motor boat. The famous Curtiss, Anzani, Renault, and others are all engines of this type. They all differ, but all have a high capacity, as we know from the records they have broken. The Anzani and R. E. P. makers, whose motors are air cooled, have used to great advantage the plan of making their motors star-shaped--that is, with the cylinders arranged in a circle around the crankshaft.