the four anterior
wings are to be opened, the four posterior ones remaining closed. The
forepart of the ship being now relieved from the downward pressure of
the air, caused by the upward movement of the balloons, this pressure
still acting on the posterior wings, its equilibrium is destroyed; the
forepart rises, the hindpart dips, thus changing the direction of the
ship's course, by converting its vertical into an oblique movement,
which is to carry it onward upon a plane inclined slightly upward.

'This operation is to be followed by its converse. The four posterior
wings are to be opened, and the four anterior ones closed; the vessel
now dips in the opposite direction, and moves forward on a plane
inclined slightly downward; and so on. Thus, by alternately opening
and shutting the two sets of lateral wings, M. Petin proposes to make
his ship sail forward on a series of inclined planes, upwards and
downwards. He takes care to assure us, however, that the requisite
degree of inclination will be so slight as to be imperceptible to his
passengers; and instances, in corroboration of this opinion, the beds
of rivers, where a very slight degree of inclination suffices to
produce a rapid current.

'In order to determine perpendicular movement, the central
wings--which, according to M. Petin, when placed in an oblique
position, will constitute the fulcrum--are to be brought into an
upright position, thus offering no resistance to the air; the two
pairs of screws are then made to turn in opposite directions with
great velocity, forcing powerful convergent currents of air upon the
two sets of lateral wings, maintained in oblique and opposite
positions. The force of these currents, being decomposed by the
resistance of the wings, is thus changed into a perpendicular
pressure, acting upwards or downwards according to the position of the
wings; by means of which the aeronaut hopes to be enabled to ascend or
descend without losing either gas or ballast.

'This decomposition of the force of the currents produced by the
screws, is analogous to that effected by the sails of a ship sailing
across the wind; where, the sails being inclined at an angle of 45
degrees to the course of the wind, the ship is impelled onwards in a
direction at right angles to that of the wind: the only difference in
the two cases being this--namely, that in the sails of the ship, the
axis of inclination, represented by the mast, is _vertical_, creating
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