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 a surface and the one first above it.
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.
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