If we designate the angle made by the wings (_ab_) with the horizontal (_V_) as _B_, then _P_ increases as _B_ increases, while (as has been stated) the ratio of _L_ to _R_ decreases. When the angle _B_ is a right angle, the wings being in the position _a´b´_, _P_ has its maximum value for direct wind--1/300 of the square of the velocity, in pounds per square foot; but _L_ is zero and _R_ is equal to _P_. The plane would have no lifting power. When the angle _B_ becomes zero, position _a´´b´´_, wings being horizontal, _P_ becomes zero and (so far as we can now judge) the plane has neither lifting power nor retarding force. At some intermediate position, like _ab_, there will be appreciable lifting and retarding forces. The chart shows the approximate lifting force, in pounds per square foot, for various angles. This force becomes a maximum at an angle of 45° (half a right angle). We are not yet prepared to consider why in all actual aeroplanes the angle of inclination is much less than this. The reason will be shown presently. At this stage of the discussion we may note that the lifting power per square foot of sail area varies with the square of the velocity, _and_ the angle of inclination. The total lifting power of the whole plane will also vary with its area. As we do not wish this whole lifting power to be consumed in overcoming the dead weight of the machine itself, we must keep the parts light, and in particular must use for the wings a fabric of light weight per unit of surface. These fabrics are frequently the same as those used for the envelopes of balloons. Since the total supporting power varies both with the sail area and with the velocity, we may attain a given capacity either by employing large sails or by using high speed. The size of sails for a given machine varies inversely as the square of the speed. The original Wright machine had 500 square feet of wings and a speed of forty miles per hour. At eighty miles per hour the necessary sail area for this machine would be only 125 square feet; and at 160 miles per hour it would be only 31-1/4 square feet: while if we attempted to run the machine at ten miles per hour we should need a sail area of 8000 square feet. This explains why the aeroplane cannot go slowly. It would seem as if when two or more superposed sails were used, as in biplanes, the full effect of the air would not be realized, one sail becalming the other. Experiments have shown this to be the case; but there is no great reduction in lifting power unless the distance apart is considerably less than the width of the planes. In all present aeroplanes the sails are concaved on the under side. This serves to keep the air from escaping from underneath as rapidly as it otherwise would, and increases the lifting power from one-fourth to one-half over that given by our 1/300 rule: the divisor becoming roughly about 230 instead of 300. [Illustration: SHAPES OF PLANES]