along the top and bottom surfaces. It does not, however, follow the surface closely, but in the case of the lower stratum is deflected downwards at an angle to the surface, which results in an upward reaction. The upper of the two streams of air is correspondingly deflected upwards at an angle to the surface for a short distance. This causes an 'area of discontinuity of flow', or eddy, which results in 'negative pressure', causing an upward suction. This fact was first discovered by Sir Hiram Maxim, though it was G. Eiffel who measured the effects of the positive pressure on the lower surface and the negative pressure on the upper surface, and found, contrary to all expectation, that the latter is responsible for three-quarters of the total lifting effect of the wing. In addition to the lift, the wings offer resistance to progress through the air, which effect is known as 'drag'. The ratio of lift to drag is a measure of the efficiency of a wing-section. A well-designed wing will have a L/D ratio at an angle of incidence of 4° of about 16, i.e. the lift effect in pounds will be 16 times that of the drag. The fundamental equation of an aeroplane is R = KSV2, where R = the resistance, K = a constant (usually 0.003), S = area of surface, and V = the velocity in feet per second. From this it will be seen that the resistance for the same area increases as the square of the speed, which shows the importance of reducing the resistance to the lowest possible degree if high speeds are to be obtained. For this purpose it is necessary that the flow of air round the component parts of the aeroplane caused by its passage should be as little disturbed and broken up into eddies as possible. It is found that the best theoretical shape for this purpose is a body of circular cross-section tapering from front to rear, with the maximum cross-section toward the front. The 'fineness ratio' (ratio of length to maximum diameter) should be about 6 to 1, and the maximum cross-section situated about one-third of the distance from the nose. Such a form will offer only about 1/20 the resistance of a flat plate of similar cross-section, and is known as a 'stream-line form'. The width of a wing from side to side at right angles to the wind is known as the 'span', and the breadth from front to back as the 'chord'. The ratio of span to chord is the 'aspect ratio'. Owing to the increase in drag resulting from low aspect ratio (large chord relative to span) the higher the aspect ratio the more efficient the wing. This is in practice about 6, owing to structural difficulties in constructing a wing of larger relative span. The essential parts of an aeroplane are the wings, fuselage (body), tail (comprising fixed vertical and horizontal surfaces behind which are hinged movable rudders and elevators), and chassis, or landing-carriage. The majority of modern machines are biplanes, i.e. with one set of wings superposed on the other and connected by upright wooden members called 'struts'. Aeroplanes with one set of wings only are called 'monoplanes'; those with three, 'triplanes'; with four, 'quadruplanes'; and with more than four, 'multiplanes'. Aeroplanes are also divided into 'tractor' and 'pusher', according to whether the propeller is situated in front or rear of the wings.
When the engine is started, the revolution of the propeller causes the aeroplane to move along the ground until such a speed is reached (usually about 35-50 miles per hour) that it is able to support its own weight in the air when it leaves the ground. When in the air it is made to ascend or descend by moving the elevators, which are operated by a vertical stick in front of the pilot through control cables or levers. Steering to right or left is effected by the rudder, which is operated by a foot-bar through cables or levers. Lateral balance is obtained by means of 'ailerons' or flaps on the outer extremities of the wings. If one wing tends to dip, the aileron on that side is depressed. This increases the resistance of that wing and so causes it to rise. By a combination of movements of the elevators, rudder, and ailerons almost any evolution can be performed with a modern aeroplane. A well-designed machine will, on cutting off the engine-power, turn its nose slightly down and automatically assume its own 'gliding-angle' to the ground. The gliding-angle is the ratio of descent to forward travel and is usually 1 in 12 to 1 in 14.
Speeds of 190 miles per hour have been attained and a height of 34,600 feet reached. The greatest distance covered in one flight is the crossing of the Atlantic—slightly more than 1900 miles—while an aeroplane has remained in the air for 24 hours. Aeroplanes range in size from small single-seater 'scouts' with a duration of only some three hours, to large multiple-engined machines with a weight, fully loaded, of from 15 to 20 tons. The essential feature of the aeroplane is, as already stated, that it is heavier than air and therefore subject to the laws of gravity in the event of engine failure. Its choice of a landing-ground is then dependent upon its height at the moment and gliding-angle.
Aeroplanes are normally constructed throughout of wood, though steel is occasionally used. The wings are built of wooden 'spars', of which there are usually two along the length of each wing, connected together by wooden 'ribs'. The wings of a biplane are braced by the struts (see above) and by wires. 'Landing-wires' support the weight of the wing on the ground, while 'flying-wires' prevent them folding upwards under the influence of the lift in flight. 'Drift-wires' are to prevent the wings folding
backwards under the pressure of the air in flight. See also Aeronautics, Sea-planes.—Bibliography: H. Barber, The Aeroplane Speaks; H. Barber, Aerobatics; Hamel and Turner, Flying; Borlase Mathews, Aviation Pocket Book; Pippard and Pritchard, Aeroplane Structures; Judge, Design of Aeroplanes; Judge, Properties of Aerofoils; Loening, Military Aeroplanes.
Aerostatic Press, a contrivance for extracting the colouring matter from dye-woods and for similar purposes. A liquid intended to carry with it the extract is brought into contact with the substance containing it, and a vacuum being made by an air-pump suitably applied, the pressure of the atmosphere forces the liquid through the intervening mass, carrying the colour or other soluble matter with it.
Aerostat′ics, that branch of physics which treats of the weight, pressure, and equilibrium of air and gases. See Air; Air-pump; Barometer; Gases, Properties of; Hydrostatics; Meteorology; &c.
Aerotherapeutics is the treatment of disease by atmospheres artificially prepared and differing from the normal in compression or pressure or temperature. It is divided into:
1. Medical atmospheres artificially produced by changing the proportions of the normal gases of the atmosphere, or by adding gases to the atmosphere. These are applied by inhalation in various ways:
(a) By the inhalation of gases—ether; chloroform; nitrous oxide (see Anæsthetics). Oxygen under pressure in a cylinder, with outlet applied close to the patient's mouth and nose, is used in severe cases of pneumonia, cardiac disease, or wherever breathing is difficult. Amyl nitrate is inhaled on the breaking of the glass capsules in which it is contained close to the patient's mouth; this treatment is used in cardiac disease and other conditions to recover blood pressure. Chlorine and iodine are used in cases of throat and bronchial affections by inhaling the vapour itself for a short time, or by inhaling air strongly impregnated with the substance.