ted at their
outer extremities by two bands of flattened wire, form the frame work
of the Screw, which is completed by a covering of oiled silk cut into
gores, and tightly stretched, so as to present as nearly uniform a
surface as the nature of the case will permit. This Screw is supported
at either end of the axis by pillars of hollow brass tube descending
from the hoop, in the lower extremities of which are the holes in
which the pivots of the axis revolve. From the end of the axis which
is next the car, proceeds a shaft of steel, which connects the
Archimedean Screw with the pinion of a piece of spring machinery
seated in the car; by the operation of which it is made to revolve,
and a progressive motion communicated to the whole apparatus. This
spring is of considerable power compared with its dimensions, being
capable of raising about 45 pounds upon a barrel of four inches
diameter after the first turn, and gradually increasing as it is wound
up. It weighs altogether, eight pounds six ounces.

The rudder is a light frame of cane covered with silk, somewhat of the
form of an elongated battledoor, about three feet long, and one foot
wide, where it is largest. It might be made considerably larger if
required, being exceedingly light and yet sufficiently strong for any
force to which it could be subjected. It weighs altogether only two
ounces and a half. This instrument possesses a double character.
Besides its proper purpose of guiding the horizontal course of the
Balloon, it is capable of being applied in a novel manner to its
elevation or depression, when driven by the propulsive power of
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 extreme