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It is well to bear the following distinctions in mind: aerostation is the art of flying in a balloon; when the balloon is equipped with motor and propellers so as to be navigable, it is dirigible; an aerocar is any kind of a flying machine; an aeronaut is any one who navigates the air in a balloon; an aeroplane is a flying machine which is heavier than air; a monoplane is a one-planed and a biplane a two-planed flying machine; a triplane consists of three superposed planes; a quadruplane of four planes; airmen are either aeronauts or aviators; aviation is the art of flying in an aeroplane and an aviator is one who so flies; aeronef is an aeroplane as defined by International Congress; a hangar corresponds to a garage for an automobile; ornithopter is a heavier-than-air machine, with wings upon which it depends for support and propulsion; petrol is the European name for gasoline.