The shape and curve of a plane, is of vital importance. A machine may be built, and an engine and propellers fitted, but the question is: Will the planes support through the air the load they have been given to carry? Phillips made many experiments, and in the end he produced a wing-shape which he patented. He pointed out that an advantage might be gained in lifting effect if the main curve or camber was situated near the front edge of the plane, and not in the centre (Fig. 10). The theory Phillips worked upon was this--and it is interesting if it can be expressed clearly. Taking a plane curved as he recommended, with this “hump” towards the front, and forcing it through the air as would be the case were an aeroplane in flight, the rush of wind which meets the edge of the plane is split into two currents--one sweeping above and one below. The air current below the plane, following its curve, is thrust downward, and in being so thrust down imparts a lift to the plane; while the current thrown above the plane--rushing up and over the “hump” which, as has been shown, is situated close to the front edge--will sweep rearwards in such a way that there is a partial vacuum or air space between the fast-moving wind current and the curved-down section of the plane behind the “hump.” The value of such a vacuum is this: it has a raising effect upon the surface of the plane, which is thus not only pushed up from below, but drawn from above. [Illustration: FIG. 11.--Suction above a Cambered Surface.] How a vacuum is caused, by air passing over such an arched surface as Phillips recommended, may be shown in a simple experiment. Take a sheet of paper and curve it in the way shown in Fig. 11, allowing the rear portion to hinge in such a way that it will move freely up and down. Then, if the sheet of paper is held between the finger and thumb and one blows across the top edge, the hinged flap at the rear will be found to raise itself--drawn up by the influence of the vacuum, such as Phillips describes. Apart from his theory as to the dipping front edge of a plane, Phillips agreed with a suggestion made by Wenham; and this was that a plane, in order to be most effective in its “lift,” should be narrow from front to back. This theory meant that, as a plane moved forward, it was the curving front section which gripped and acted up the air; and that, if the plane was carried too far towards the rear, its lifting influence fell away, while the surface that was superfluous acted as so much resistance to the machine’s progress through the air. [Illustration: FIG. 12.--Phillips’s Experimental Craft.]