When the speed increases to 50 miles an hour, the angle of incidence in the plane D must be decreased, so that the rear end of the frame must be raised, which is done by giving the tail an angle of incidence, otherwise, as the upper side of the tail should meet the air it would drive the rear end of the frame down, and thus defeat the attempt to elevate that part. _Fig. 12. Angles of Flight._ As the speed increases ten miles more, the tail is swung down still further and the rear end of the frame is now actually above the plane of flight. In order, now, to change the angle of flight, without altering the speed of the machine, the tail is used to effect the control. Examine the first diagram in Fig. 12. This shows the tail E still further depressed, and the air striking its lower side, causes an upward movement of the frame at that end, which so much decreases the angle of incidence that the aeroplane darts downwardly. In order to ascend, the tail, as shown in the second diagram, is elevated so as to depress the rear end, and now the sustaining surface shoots upwardly. Suppose that in either of the positions 1 or 2, thus described, the aviator should lose control of the mechanism, or it should become deranged or "stick," conditions which have existed in the history of the art, what is there to prevent an accident? In the first case, if there is room, the machine will loop the loop, and in the second case the machine will move upwardly until it is vertical, and then, in all probability, as its propelling power is not sufficient to hold it in that position, like a helicopter, and having absolutely no wing supporting surface when in that position, it will dart down tail foremost. A NON-CHANGING BODY.--We may contrast the foregoing instances of flight with a machine having the sustaining planes hinged to the body in such a manner as to make the disposition of its angles synchronous with the tail. In other words, see how a machine acts that has the angle of flight controllable by both planes,--that is, the sustaining planes, as well as the tail. _Fig. 13. Planes on Non-changing Body._ In Fig. 13 let the body of the aeroplane be horizontal, and the sustaining planes B disposed at the same angle, which we will assume to be 15 degrees, this being the imaginary angle for illustrative purposes, with the power of the machine to drive it along horizontally, as shown in position 1. In position 2 the angles of both planes are now at 10 degrees, and the speed 60 miles an hour, which still drives the machine forward horizontally. In position 3 the angle is still less, being now only 5 degrees but the speed is increased to 80 miles per hour, but in each instance the body of the machine is horizontal. Now it is obvious that in order to ascend, in either case, the changing of the planes to a greater angle would raise the machine, but at the same time keep the body on an even keel. _Fig. 14. Descent with Non-changing Body._