The abnormal needs of the war period brought many British firms into the ranks of Vee-type engine-builders, and, apart from those mentioned, the most notable types produced are the Rolls-Royce and the Napier. The first mentioned of these firms, previous to 1914, had concentrated entirely on car engines, and their very high standard of production in this department of internal combustion engine work led, once they took up the making of aero engines, to extreme efficiency both of design and workmanship. The first experimental aero engine, of what became known as the ‘Eagle’ type, was of Vee design--it was completed in March of 1915--and was so successful that it was standardised for quantity production. How far the original was from the perfection subsequently ascertained is shown by the steady increase in developed horse-power of the type; originally designed to develop 200 horse-power, it was developed and improved before its first practical trial in October of 1915, when it developed 255 horse-power on a brake test. Research and experiment produced still further improvements, for, without any enlargement of the dimensions, or radical alteration in design, the power of the engine was brought up to 266 horse-power by March of 1916, the rate of revolutions of 1,800 per minute being maintained throughout. July, 1916, gave 284 horse-power; by the end of the year this had been increased to 322 horse-power; by September of 1917 the increase was to 350 horse-power, and by February of 1918 the ‘Eagle’ type of engine was rated at 360 horse-power, at which standard it stayed. But there is no more remarkable development in engine design than this, a 75 per cent increase of power in the same engine in a period of less than three years. To meet the demand for a smaller type of engine for use on training machines, the Rolls-Royce firm produced the ‘Hawk’ Vee-type engine of 100 horse-power, and, intermediately between this and the ‘Eagle,’ the ‘Falcon’ engine came to being with an original rated horse-power of 205 at 1,800 revolutions per minute, in April of 1916. Here was another case of growth of power in the same engine through research, almost similar to that of the ‘Eagle’ type, for by July of 1918 the ‘Falcon’ was developing 285 horse-power with no radical alteration of design. Finally, in response to the constant demand for increase of power in a single plant, the Rolls-Royce company designed and produced the ‘Condor’ type of engine, which yielded 600 horse-power on its first test in August of 1918. The cessation of hostilities and consequent falling off in the demand for extremely high-powered plants prevented the ‘Condor’ being developed to its limit, as had been the ‘Falcon’ and ‘Eagle’ types.