Indeed, in all other respects, except the nature of its material, the screw here represented is exactly analogous to that used by Mr. Smith in its most perfect form, having been, in fact, designed, and in part constructed under his own supervision.[[A]]
The model upon which these principles have been now, for the first time, successfully, at least, tried in the air, is constructed upon the following scale. The Balloon is, as before stated, an ellipsoid or solid oval; in length, 13 feet 6 inches, and in height, 6 feet 8 inches. It contains, accordingly, a volume of gas equal to about 320 cubic feet, which, in pure hydrogen, would enable it to support a weight of twenty-one pounds, which is about its real power when recently inflated, and before the gas has had time to become deteriorated by the process of endosmose.[[B]] The whole weight of the machine and apparatus is seventeen pounds; consequently there is about four pounds to spare, in order to meet this contingency.
[Footnote A: The frame was made at Mr. Smith's request, by Mr. Pilgrim, of the Archimedes; the original experimental vessel in which this mode of propulsion was first tried upon the large scale. Mr. Pilgrim has been long versed in all that relates to the mechanism of this instrument, and is indeed a most expert and ingenious artist.]
[Footnote B: Endosmose is that operation by which gases of different specific gravities are enabled, or rather forced to come together through the pores of any membranous or other flexible covering by which it is sought to restrain them. As above referred to, it is the introduction of atmospheric air into the body of the Balloon through the pores of the silk, however accurately varnished, by which the purity of the hydrogen gas is contaminated, and its buoyant power ultimately exhausted This it is impossible to prevent by any process, except the interposition of a metallic covering; as for instance, by gilding the Balloon, which would be effectual could it be contrived to endure the constant friction and bending of the material itself.]
Beneath the centre of the Balloon, and about two-thirds of its length, is a frame of light wood, answering to the hoop of an ordinary Balloon; to which are attached the cords of the net which encloses the suspending vessel, and which serves to distribute the pressure of the appended weight equally over its whole surface, as well as to form an intermediate means of attachment for the rest of the apparatus. This consists of a car or basket in the centre; at one end the rudder, and at the other the Archimedean Screw. The car is about two feet long and eighteen inches broad, and is laced to the hoop by cords, which running through loops instead of being fastened individually, allow of unlimited play, and equalize the application of the weight of the car to the hoop, as of the whole to the Balloon above. The Archimedean Screw consists of an axis of hollow brass tube eighteen inches in length, through which, upon a semi-spiral of 15° of inclination, are passed a series of radii or spokes of steel wire, two feet long, (thus projecting a foot on either side) and which being connected 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 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.
The rate of motion which the Balloon thus equipped is capable of accomplishing varies according to the circumstances of its propulsion. When the Archimedean Screw precedes, the velocity is less than when it is made to follow, owing to the reaction of the air in the former instance against the car, the under surface of the balloon, and other obstacles, by which its progress is retarded. Again, when the cord upon which it travels is most tense and free from vibration, the rate is found to be considerably accelerated, compared with what it is when the contrary conditions prevail. But chiefly is its speed affected by the proper ballasting of the machine itself, upon which, depends the friction it encounters from the cord on which it travels. Under ordinary circumstances it proceeds at a rate of about four miles an hour, but when the conditions alluded to have been most favourable, it has accomplished a velocity of not less than five; and there is no doubt that were it altogether free from restraint, as it would be in the open air, with a hand to guide it, its progress would be upwards of six miles an hour.
Having now, I trust, sufficiently explained the principles exemplified in the model here described, it may be expected that I should add a few words regarding their reduction into practice upon a larger scale and in the open air, with such difficulties to contend with as may be expected to be encountered in the prosecution of such a design. In the first place, however, it will be necessary to disabuse the public mind of some very prevailing misconceptions with respect to the conditions of a Balloon exposed to the action of the winds, pursuing its course under the exercise of an inherent propulsive power. These misconceptions, which, be it observed, are more or less equally participated in by the scientific as by the ignorant, when devoid of that practical experience which is the basis of all aeronautical proficiency, are of a very vague and general character, and consequently not very easy accurately to define. In order, therefore, to make sure of meeting all the objections and removing all the doubts to which they are calculated to give rise, it will be advisable, even at the risk of a little tediousness, to separate them into distinct questions and treat them accordingly.
One of the most specious of these misconceptions regards the effects of the resistance of the atmosphere upon the figure of the Balloon when rapidly propelled through the air, whereby it is presumed its opposing front will be driven in, and more or less incapacitated from performing the part assigned to it; namely, to cleave its way with the reduced resistance due to its proper form. To obviate, this imagined result, various remedies have been proposed--such as, to construct that part of the machine of more solid materials than the rest, or else (as suggested by one of the most scientific and ingenious of those who have devoted their attention to the theory of aerial navigation), to subject the gaseous contents of the Balloon to such a degree of artificial condensation by compression, as shall supply from within a force equal to that from without; adopting, of course, materials of a stronger texture than those at present in use, for the construction of the balloon. Now the contingency against which it is here sought to provide, and which I grant is a very reasonable one to anticipate, has nevertheless no real existence in practice; at least in such a degree as to render it necessary to have recourse to any particular expedient for its prevention. Taking it for granted that the hypothesis in which it is involved is founded upon a presumed analogy with a Balloon exposed to the action of the wind while in a state of attachment to the earth, I would first observe that the cases in question, however apparently analogous, are in reality essentially dissimilar. In the one case (that where the Balloon is supposed to be attached to the earth) all the motion, and consequently all the momentum, is in the air; in the other case (where the Balloon is supposed to be progressive), it is in the constituent particles of the machine itself and of its gaseous contents. And this momentum, which is ever proportioned to the rate of its motion, and, consequently, to the amount of resistance it experiences, is amply sufficient to secure the preservation of the form of its opposing front, however partially distended, and whatever the velocity with which it might happen to be endowed. Independently, however, of this corrective principle, another, equally efficacious is afforded in the buoyant power of the included gas, which, occupying all the upper part of the Balloon so long as it is in a condition to sustain itself in the air, and generally extending to its whole capacity, presses from within with a force far greater than any it could experience from the external impact of the atmosphere, and sufficiently resists any impression from that quarter which might tend to impair its form. To what extent this is effective, will appear more clearly when we observe that in any balloon inflated, it is the sides of the distended globe that bear out the weight of the appended cargo, through the intervention of the network; a weight only limited by the sustaining power of the machine itself, and in the case of the great Vauxhall or Nassau Balloon, amounting to more than two tons, and consequently pressing with a force far exceeding any that could arise from the impact of the air at any rate of motion it could ever be expected to accomplish. And this statement, which represents the theoretical view of the question, is fully borne out by the real circumstances of the case as they appear in practice. So far from justifying the apprehensions of those who conceive that the front of the Balloon would be disfigured by its compulsory progression through the air, the result is exactly the reverse; the only tendency to derangement of form displaying itself in the part behind, where the rushing in of the atmospheric medium to fill the place of the advancing body (in the nature of an eddy, as it is termed in water), might and no doubt would, to some extent (though perhaps but slightly) affect the figure of that part, in a manner, however, calculated rather to aid than to impair the general design in view,
Another error of more universal prevalency, because of a more superficial character, regards the condition of the Balloon as affected by the currents of air, in and through which it might have to be propelled. The arguments founded upon such a view of the case, generally assume some such form as the following--"It is true you can accomplish such or such a rate of motion; but that is only in a room, with a calm atmosphere, or with a favourable current of wind. In the open air, with the wind at the rate of twenty or thirty miles an hour, your feeble power would be of no avail. You could never expect to direct your course against the wind, and if you were to attempt it and the wind were strong, you would inevitably be blown to pieces by the force of the current." Now this argument is equally nought with the preceding. The condition of the Balloon, as far as regards the exercise of its propulsive powers, is precisely the same whether the wind be strong or gentle, with it or against it. In neither case would the Balloon experience any opposition or resistance to its progress but what itself, by its own independent motion, created; and that opposition or resistance would be exactly the same in whatever direction it might be sought to be established. The Balloon, passively suspended in the air, without the exercise of a propulsive power, experiences no effects whatever from the motion of the atmosphere in which it is carried, however violent; and the establishment of such a propulsive power could never subject it to more than the force itself, with which it was invested. The way which the Balloon so provided would make through the air would always be the same, in whatever direction, or with whatever violence the wind might happen to blow; and the condition of the Balloon would always be the same that was due to its own independent rate of motion, without regard to any other circumstances whatever. If it was furnished with the means of accomplishing a rate of motion equal to ten miles an hour, it would experience a certain amount of atmospheric resistance due to that rate; and this amount of resistance with all its concomitant consequences, neither more nor less, would it experience, whether it endeavoured to make this way against a wind blowing at the rate of 100 miles an hour, or with the same in its favour. The result, so far as regards its distance from the place of starting, would, I grant, be very different; but at present we are only considering the conditions of its motion through the air, and these, I repeat, would be the same whatever the rate or course of the wind; so that all speculations on this score must resolve themselves into questions of quantity, not of quality, in the effect sought to be accomplished: in other words, all consideration of the rate of the wind must be left out of the argument, except, in so far as it shall be taken to regulate the limit which shall be assigned to the rate of the aerial machine, as sufficient to justify its claims to the title of a successful mode of navigating the skies.[A]