[Illustration: WRIGHT BIPLANE ON STARTING RAIL, SHOWING PYLON AND WEIGHT] To accelerate a machine from rest to a given velocity in a given time or distance involves the use of propulsive force additional to that necessary to maintain the velocity attained. Apparently, therefore, any self-starting machine must have not only the extra weight of framework and wheels but also extra motor power. [Illustration: LAUNCHING SYSTEM FOR WRIGHT AEROPLANE (From Brewer's _Art of Aviation_)] Upon closer examination of the matter, we may find a particularly fortunate condition of things in the aeroplane. Both sustaining power and resistance vary with the inclination of the planes, as indicated by the chart on page 24. It is entirely possible to start with no such inclination, so that the direct wind resistance is eliminated. The motor must then overcome only air friction, in addition to providing an accelerating force. The machine runs along the ground, its velocity rapidly increasing. As soon as the necessary speed (or one somewhat greater) is attained, the planes are tilted and the aeroplane rises from the ground. [Illustration: THE NIEUPORT MONOPLANE Self-Starting with an 18 hp. motor (From _The Air Scout_)] The velocity necessary to just sustain the load at a given angle of inclination is called the _critical_ or _soaring_ velocity. For a given machine, there is an angle of inclination (about half a right angle) at which the minimum speed is necessary. This speed is called the "least soaring velocity." If the velocity is now increased, the angle of inclination may be reduced and the planes will soar through the air almost edgewise, apparently with diminished resistance and power consumption. This decrease in power as the speed increases is called _Langley's Paradox_, from its discoverer, who, however, pointed out that the rule does not hold in practice when frictional resistances are included. We cannot expect to actually save power by moving more rapidly than at present; but we should have to provide much more power if we tried to move much more slowly. [Illustration: A BIPLANE (From _Aircraft_)] [Illustration: ELY AT LOS ANGELES (Photo by American Press Association)]