While every craft that navigates the air is an airship, all airships
are not flying machines. The balloon, for instance, is an airship, but
it is not what is known among aviators as a flying machine. This latter
term is properly used only in referring to heavier-than-air machines
which have no gas-bag lifting devices, and are
made to really fly by the application of engine propulsion.Imitation of Bird In Aeroplane Design.
[Description: Black and white illustration: One the left, a view of a bird's
wingspan as from above; on right, a view of an aeroplane as from above.]