Provision is also made for balancing the machine. This is sometimes in the form of two small planes hinged to the main plane, one at either end, connected together and to a controlling lever by wires, so that by their use the pilot can steer the right-hand side of his machine upwards and the left-hand downward, or vice versa, if through any cause he finds a tendency to capsize. In some machines the same effect is produced not by separate planes but by pulling the main plane itself somewhat out of shape, but precisely the same principle is involved. The planes are usually made with a slight curve in them, so that they may the better catch the air and "scoop" it downwards, so to speak. They usually consist of fabric specially made for the purpose, stretched upon a light wooden framework. The whole framework is usually of wood with metal fittings frequently made of aluminium for the sake of lightness. The engines have been mentioned in another chapter. The propeller which is almost invariably fixed directly upon the shaft of the engine has two blades only and not three as is usual with those of ships. Precisely why this should be so is not clear, but experience shows that two-bladed propellers are preferable for this work. They are made of wood, several layers being glued together under pressure, the resulting log being then carved out to the required shape. This makes a stronger thing than it would be if cut out of a single piece of wood. All parts, engine, elevator, rudder and balancing arrangement, are controlled by very simple means from the pilot's seat. In monoplanes there is but one main plane, resembling a pair of bird's wings. Or if we care to look upon it as two planes, one each side of the "body," then we must call it a pair. Since the name "mono" indicates one it is best to think of it as one plane although it may be in two parts. The biplane has, as its name implies, two planes, but in that case there can be no doubt, since they are placed one above the other. Machines have been made with three planes and even with as many as five, but monoplanes and biplanes appear to hold the field. It is not possible for an aeroplane to be in any sense armoured for protection against bullets: for defence the pilot has to depend upon his own cunning man[oe]uvres combined with the fast speed at which he can move. For offensive purposes he usually has a machine gun mounted right in front of him with which he can pour a stream of bullets into an opponent or even, by flying low, he can attack a body of infantry. It is recorded that one German prisoner during the war, speaking of the daring of the British pilots in thus attacking men on foot, exclaimed, "They will pull the caps off our heads next."