This principle, however, affords a safer means of navigating than the orthopter type, because the blades of such an instrument can be forced through the air with infinitely greater speed than beating wings, and it devolves on the inventor to devise some form of apparatus which will permit the change of pull from a vertical to a horizontal direction while in flight. CHAPTER VI THE LIFTING SURFACES OF AEROPLANES THIS subject includes the form, shape and angle of planes, used in flight. It is the direction in which most of the energy has been expended in developing machines, and the true form is still involved in doubt and uncertainty. RELATIVE SPEED AND ANGLE.--The relative speed and angle, and the camber, or the curved formation of the plane, have been considered in all their aspects, so that the art in this respect has advanced with rapid strides. NARROW PLATES MOST EFFECTIVE.--It was learned, in the early stages of the development by practical experiments, that a narrow plane, fore and aft, produces a greater lift than a wide one, so that, assuming the plane has 100 square feet of sustaining surface, it is far better to make the shape five feet by twenty than ten by ten. However, it must be observed, that to use the narrow blade effectively, it must be projected through the air with the long margin forwardly. Its sustaining power per square foot of surface is much less if forced through the air lengthwise. Experiments have shown why a narrow blade has proportionally a greater lift, and this may be more clearly understood by examining the illustrations which show the movement of planes through the air at appropriate angles. _Fig. 22. Stream lines along a plane._ STREAM LINES ALONG A PLANE.--In Fig. 22, A is a flat plane, which we will assume is 10 feet from the front to the rear margin. For convenience seven stream lines of air are shown, which contact with this inclined surface. The first line 1, after the contact at the forward end, is driven downwardly along the surface, so that it forms what we might term a moving film. The second air stream 2, strikes the first stream, followed successively by the other streams, 3, 4, and so on, each succeeding stream being compelled to ride over, or along on the preceding mass of cushioned air, the last lines, near the lower end, being, therefore, at such angles, and contacting with such a rapidly-moving column, that it produces but little lift in comparison with the 1st, 2d and 3d stream lines. These stream lines are taken by imagining that the air approaches and contacts with the plane only along the lines indicated in the sketch, although they also in practice are active against every part of the plane. THE CENTER OF PRESSURE.--In such a plane the center of pressure is near its upper end, probably near the line 3, so that the greater portion of the lift is exerted by that part of the plane above line 3.