the Screw. Being so contrived as to be capable of being turned
_flat_, and also directed upwards or downwards as well as to the
right or left, it enables the aeronaut to transfer the resistance of
the air, which, in any inclined position, it must generate in its
passage, to any side upon which he may desire to act, and thus give a
determination to the course of the Balloon in the opposite direction.
This will appear more clear as well as more certain when we consider,
that the aerial vessel being in a state of perfect equipoise, as
it ever must be when proceeding on the same level, the slightest
alteration in its buoyancy is sufficient to send it to a considerable
distance either up or down as the case may be: the rejection of a
pound of ballast, or of an equivalent amount of gas, being enough to
conduct the aeronaut to the extremest limits of his desires in either
direction, whatever may be the size of his Balloon. Now a resistance
equal to many pounds is attainable by an inclined plane of even
moderate dimensions when propelled even with moderate velocity; and
being readily governed by the mere inclination of the impinging plane
at the will and by the hand of the aerial voyager, it will be in his
power to vary the level of his machine with very considerable nicety;
enabling him to approach the surface of the earth, or in a gentle
curve to sweep away from its occasional irregularities, and proceed to
a very considerable elevation without interrupting the progress of his
course, and, what is of more importance, without sacrificing any part
of his resources in gas or ballast, upon the preservation of which the
duration of his career so entirely depends. These properties of the
rudder it is not possible to display in the present exhibition, owing
to the confined nature of the course which it is necessary to pursue;
but they were sufficiently tested in the preliminary experiments at
Willis's Rooms, where the space being larger, a circular motion was
conferred upon the machine by connecting it with a fixed centre round