This is not a technical treatise, but some further facts of signal importance in the theory and practice of flight are better explained at once, in so far as the beautiful exactitude of mathematical demonstration can be expressed in the crudities of popular speech. The lift produced by the reaction of the air acts on the whole plane, but not equally on all parts of it. At a flying angle, that is, when the angle of incidence of the plane is small, the upward force is greatest on those parts of the plane which are immediately behind the leading edge. The wings of any soaring bird are long and narrow, and thus are perfectly designed for their work. A square-winged bird would be a poor soarer; a bird the breadth of whose wings should be greater than their length could hardly fly at all. The wings of a flying machine are called planes, or aerofoils; the length of the wing is called the span of the plane; the breadth of the wing is called the chord of the plane. The proportion of the span to the chord, that is, the proportion of the length of the wing to its breadth, is called the 'aspect ratio' of the plane; and a plane, or wing, that is long and narrow is said to have a high aspect ratio. A higher aspect ratio than is found in any bird or any flying machine would theoretically improve its powers of flight, but the practicable span of the plane, or length of the wing, is limited by the need for rigidity and strength. The albatross, nevertheless, the king of soaring birds, has enormously long and narrow wings; and the planes of some flying machines have an aspect ratio almost as high as the slats of a Venetian blind. The wings of a flying machine, it has been said, are called planes, but they are not true planes. Like the wings of a bird, they are 'cambered', that is to say, they curve upward from the leading edge and downward again to the trailing edge. Some of the most valuable work contributed by the laboratory to the science of flight has had for its object the determination of the best form of camber, or curve of the plane. In the result, that form of camber has been found to be best which attains its maximum depth a little way only behind the leading edge, and gradually becomes shallower towards the trailing edge. Such a form of curve produces a comparatively smooth and untroubled partial vacuum above the plane, just behind its leading edge, and this vacuum is the factor of chief importance in the lift of the plane.