There is also a secondary lifting impulse derived from this simple curve. We have seen that the air which has been cut by the front edge of the plane pushes up from below, and is arrested by the top of the arch, but the downward dip of the rear portion of the plane is of service in actually DRAWING THE AIR FROM ABOVE. The rapid air stream which has been cut by the entering edge passes above the top of the curve, and "sucks up", as it were, so that the whole wing is pulled upwards. Thus there are two lifting impulses: one pushing up from below, the other sucking up from above. It naturally follows that when the camber is very pronounced the machine will fly much slower, but will bear a greater weight than a machine equipped with planes having little or no camber. On high-speed machines, which are used chiefly for racing purposes, the planes have very little camber. This was particularly noticeable in the monoplane piloted by Mr. Hamel in the Aerial Derby of 1913: the wings of this machine seemed to be quite flat, and it was chiefly because of this that the pilot was able to maintain such marvellous speed. The scientific study of the wing lift of planes has proceeded so far that the actual "lift" can now be measured, providing the speed of the machine is known, together with the superficial area of the planes. The designer can calculate what weight each square foot of the planes will support in the air. Thus some machines have a "lift" of 9 or 10 pounds to each square foot of wing surface, while others are reduced to 3 or 4 pounds per square foot. CHAPTER XXV. The Wright Biplane (Cont.) The under part of the frame of the Wright biplane, technically known as the CHASSIS, resembled a pair of long "runner" skates, similar to those used in the Fens for skating races. Upon those runners the machine moved along the ground when starting to fly. In more modern machines the chassis is equipped with two or more small rubber-tyred wheels on which the machine runs along the ground before rising into the air, and on which it alights when a descent is made. You will notice that the pilot's seat is fixed on the lower plane, and almost in the centre of it, while close by the engine is mounted. Alongside the engine is a radiator which cools the water that has passed round the cylinder of the engine in order to prevent them from becoming overheated. Above the lower plane is a similar plane arranged parallel to it, and the two are connected by light upright posts of hickory wood known as STRUTS. Such an aeroplane as this, which is equipped with two main planes, known as a BIPLANE. Other types of air-craft are the MONOPLANE, possessing one main plane, and the TRIPLANE, consisting of three planes. No practical machine has been built with more than three main planes; indeed, the triplane is now almost obsolete.