The Aeroplane Speaks. Fifth Edition

=Gravity, Specific=-- Density of substance -------------------- Density of water. Thus, if the density of water is 10 lb. per unit volume, the same unit volume of petrol, if weighing 7 lb., would be said to have a specific gravity of 7/10, i.e., 0.7. =Gap= (of an aeroplane)--The distance between the upper and lower surfaces of a biplane. In a triplane or multiplane, the distance between any two of its surfaces. [16] =Gap, Propeller=--The distance, measured in the direction of the thrust, between the spiral courses of the blades. =Girder=--A structure designed to resist bending, and to combine lightness and strength. =Gyroscope=--A heavy circular wheel revolving at high speed, the effect of which is a tendency to maintain its plane of rotation against disturbing forces. =Hangar=--An aeroplane shed. =Head-resistance=--Drift. The resistance of the air to the passage of a body. =Helicopter=--An air-screw revolving about a vertical axis, the direction of its thrust being opposed to gravity. =Horizontal Equivalent=--The plan view of a body whatever its attitude may be. =Impulse=--A force causing a body to gain or lose momentum. =Inclinometer=--A curved form of spirit-level used for indicating the attitude of a body relative to the horizontal. =Instability=--An inherent tendency of a body, which, if the body is disturbed, causes it to move into a position as far as possible away from its first position. =Instability, Neutral=--An inherent tendency of a body to remain in the position given it by the force of a disturbance, with no tendency to move farther or to return to its first position.