Independent of the advantage of reduced resistance in this form, there is another of nearly, if not quite, equal importance, in the facility it affords of directing its course; an object scarcely, if at all, attainable with a Balloon of the usual description however powerfully invested with the means of motion; as any one will readily perceive who has ever noticed or experienced the difficulty, or rather the impossibility, of guiding a tub afloat in the water, compared with the condition of a boat or other similarly constructed body, in the same element. The efficacy of this provision and its necessity will appear more forcibly when we observe that whenever the Balloon in the machine here described is thrown out of its direct bearing by the shifting of the net-work which connects it with the hoop, or by any other accident whereby its position is altered with respect to the propelling power, its course is immediately affected, and it ceases to progress in a straight line, following the direction of its major axis, unless corrected by the intervention of a sufficient rudder. The second object, after establishing a proper form for the floating body, was to contrive a disposition of striking surface that should be able to realise the greatest amount of propulsive re-action, in proportion to its magnitude and the force of its operation, which it is possible to accomplish. To shew by what steps and in consequence of what reasoning this point was determined as in the plan adopted, would occupy considerably more space than the few pages we have to spare would admit of our devoting to it. Suffice it to say that of all the means of creating a resistance in the atmosphere capable of being applied to the propulsion of the Balloon, the Archimedean Screw was ascertained to be undoubtedly the best. It is true that by a _direct_ impact or stroke upon the air, as for instance by the action of a fan, or the wafting of any _flat_ surface at _right angles_ to its own plane, the maximum effect is accomplished which such a surface is capable of producing with a given power. The mechanical difficulties, however, which attend the employment of such a mode of operation are more than sufficient to counterbalance any advantage in point of actual resistance which it may happen to possess; at least in any application of it which has hitherto been tried or proposed: so that here, as in the case of ships propelled by steam, the _oblique_ impact obtained by the rotation of the striking surface is found to be the most conducive to the desired result; and of these, that arrangement which is termed the Archimedean Screw is the most effective.