HOW MOMENTUM IS A FACTOR IN INVERTED FLYING.-- When flying "upside down," the convex side of the plane takes the pressure of the air, and maintains, so it is asserted, the weight of the machine. This is true during that period when the loop is being made. The evolution is made by first darting down, as shown in Fig. 31, from the horizontal position, 1, to the position 2, where the turn begins. _Fig. 31. Flying upside down._ TURNING MOVEMENT.--Now note the characteristic angles of the tail, which is the controlling factor. In position 1 the tail is practically horizontal. In fact, in all machines, at high flight, the tail is elevated so as to give little positive angle of incidence to the supporting planes. In position No. 2, the tail is turned to an angle of incidence to make the downward plunge, and when the machine has assumed the vertical, as in position 3, the tail is again reversed to assume the angle, as in 1, when flying horizontally. At the lower turn, position 4, the tail is turned similar to the angle of position 2, which throws the rear end of the machine down, and as the horizontal line of flight is resumed, in an inverted position, as in position 4, the tail has the same angle, with relation to the frame, as the supporting planes. During this evolution the engine is running, and the downward plunge develops a tremendous speed, and the great momentum thus acquired, together with the pulling power of the propeller while thus in flight, is sufficient to propel it along horizontally, whatever the plane surface curve, or formation may be. It is the momentum which sustains it in space, not the air pressure beneath the wings, for reasons which we have heretofore explained. Flights of sufficient duration have thus been made to prove that convex, as well as concave surfaces are efficient; nevertheless, in its proper place we have given an exposition of the reasoning which led to the adoption of the concaved supporting surfaces. WHEN CONCAVED PLANES ARE DESIRABLE.-- Unquestionably, for slow speeds the concaved wing is desirable, as will be explained, but for high speeds, surface formation has no value. That is shown by Pequod's feat. THE SPEED MANIA.--This is a type of mania which pervades every field of activity in the building of aeroplanes. Speed contests are of more importance to the spectators on exhibition grounds than stability or durability. Builders pander to this, hence machines are built on lines which disregard every consideration of safety while at normal flight. USES OF FLYING MACHINES.--The machine as now constructed is of little use commercially. Within certain limitations it is valuable for scouting purposes, and attempts have been made to use it commercially. But the unreliable character of its performances, due to the many elements which are necessary to its proper working, have operated against it.