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 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 which it was thus made to revolve, without the necessity of confining it to the one level.