The present time is one of considerable importance in the history of aerial flight: the motive power engine has, during the last few years, reached a stage in its development at which its weight has become sufficiently reduced to render mechanical flight possible, and already several partially successful machines have been built, and flights of several miles have been made. Some of these machines are deficient in many important respects; the propellers for example are of relatively small diameter and are commonly of too quick a pitch and too high a revolution speed for best efficiency. The longitudinal stability also, instead of being automatic, is only maintained by the watchful attention of the aeronaut and the dexterous manipulation of a horizontal rudder. Further than this the cooling of the motor cylinders is not altogether effective, and the duration and range of flight is largely a matter of how long elapses before the water is boiled away. The question of weight makes it difficult to employ a thoroughly efficient "radiator" as used on road vehicles, and without doubt direct air cooling will sooner or later come into vogue. For the above reasons the flights so far made have been of comparatively brief duration.
In the future it is unlikely that the flying-machine will be
