The high performance engine, whether military or commercial, is essentially the light one.

The motor which made flying possible was an internal combustion engine, and the basic reason of its success was the direct use of an easily volatile fuel of high heat value, namely, petrol.

The first engine developed to use this fuel was that of Daimler, and all motor-car, boat and aviation engines have sprung from this engine.

In order to more readily have in mind our problem, we can with advantage briefly run through the main aero engines so far constructed with the progressive results of tests.

The engine of Gottlieb Daimler is illustrated by Slide No. I. It had automatic inlet valves and exhaust governing, the latter being a point to be noted.

The next slide shows the engine of the Wright Brothers. This engine weighed 8.75 lbs. per B.H.P. inclusive; the most successful engine of to-day weighs under 3 lbs. inclusive.