ballast, would bring them banging together immediately, it it could be attached to them while they were at the same level, but at a distance.' What useful result would be obtained in his case from the sliding weight, if it were a possibility, is not apparent.1 Finally, as I have before mentioned,2 M. Mongé suggests the shifting ballast, 'pour incliner l'appareil.'

Now this sliding ballast is not only a good notion, but it is an appurtenance absolutely indispensable to the air-craft for maintaining its trim. Indeed the fact that those projectors who mention it, do so, as it were, by the way, as a thing perhaps to be adopted, not seeing that without it aerial navigation is an impossibility, is almost an evidence that they did not entertain a conception of its only legitimate purpose. However, a mere shifting weight is not sufficient. Although without it it would not be possible to keep the vessel long together in a horizontal set, it would not compensate and rectify any temporary bias which the system might receive, and which might throw it into a slant, and interrupt its steady progress. There must be a selfregulating apparatus, tending to keep the vessel horizontal, and to render the true level position one of stable equilibrium for the whole system. I have not yet seen any hint of such an adjustment for an aerial vessel. I hope to show in a future page how the air-craft may be made to keep its level by a self-acting equipoise.

I have met with but one other attempt, or rather supposition, for it is no more, towards the solution of this problem of maintaining the level trim of the air-craft; this is M. Sanson's scheme of suspension. He thus describes it: 'Moyen spécial et nouveau de suspension, et d'équilibre horizontal indéstructible (trigonométrique) permettant aux voyageurs de se porter, isolement ou tous ensemble, sur n'importe quel point du plancher, sans danger aucun de faire chavirer la machine. Ce sont des cordes lesquelles portant des lambourdes sur lesquelles est posée la galerie, vont en se croisant, et après s'être bifurquées deux foix, s'attacher, aux

'M'Sweeny, 'Aer. Nav.' 2nd ed. p. 29. There are in this production three useful pages (pp. 83, 4, 5), which contain a list of books on the subject of the author's fervid fancies.

2 See p. 77; Mongè, 'Études,' p. 110.

équatoriales.'1 Fortunately M. Sanson, to aid the reader in the conception of his design, represents it also by a drawing. His

method of slinging the 'gallery' (which would be represented by the boat of a real air-craft), differs from the ordinary system of conveying cords only in this, that the suspending lines are united in pairs, not each to the one next to it, but to the seventh in order of succession; and that the single cord which continues each pair, instead of being carried down to a corresponding part of the gallery' towards the same end, is attached to a point towards its opposite extremity. Thus by the crossing of these lines the ends of the 'gallery' are hung from the middle parts of the length of the gas-vessel, and the middle of the former from the ends of the latter; and each half of the gallery, as divided across the middle of its length, is suspended from the opposite half of the gas-float. How the contriver expects to derive from this arrangement the advantages he proclaims, I am at a loss to imagine. The most favourable result that can ensue is that the apparatus should behave as if it was all united into one rigid system, which under ordinary circumstances would be the case.

1 Sanson, 'Nav. Atmosph.' p. 12, and cover, p. ii.

144

STATEMENT OF THE PROBLEM.

PART I.

The worst effect would be, if the whole weight was thrown on the extreme cords at either end, for instance at the head, in which case the head of the gallery' would go down, and the head of the gas-vessel would be tilted up-like a shark gaping. The centre of gravity of the system must, of course, case place itself exactly under the centre of buoyancy, and this in every cannot be effected if the position of the chief weight, and therefore of the centre of gravity, be altered, without a change in the position of the whole system. With a short man-vessel like that in M. Sanson's sketch of his queer craft, the change in position arising from shifting of the weight would not be much, but it would be no greater with the common mode of suspension, which, as he proposes to propel from an equatorial framework round the gas-vessel, and not from the 'gallery,' would answer all the purposes of equilibrium for his system.

However, in aerial navigation neither system of suspension, the common or the Sansonian, will answer the serving the horizontal equilibrium. Neither will any mode of purpose of presuspension be of itself sufficient for this end, which, as I have shown, is absolutely necessary to be fulfilled.

It is therefore requisite, as another of the essential conditions of the art

That the air-craft should be provided with means of maintaining both gas-vessel and man-vessel, as well as the propelling agent, in a horizontal position, and of readily restoring the system to a true level if the balance should be disturbed.

145

CHAPTER XII.

THE QUESTION OF POWER.

IF, then, all that we have yet learned to be necessary can be accomplished, a likely-looking air-craft may be built, and may be made to float. But this is not all that we desire; it must be moved through the air, driven at a certain velocity. What speed must be attained I do not think it the least necessary to enquire at present. It is usual, with those who have considered this question with hopes of success, to acknowledge at starting on the one hand, that unless a certain amount of speed can be ensured, the experiment of locomotion in the air is not worth the making; and, on the other hand, that there is but little hope of attaining any considerable swiftness. Now I can make neither of these admissions. In the first place, I consider that a very slow motion secured in still air would be a most valuable step. And whatever wise people may say, I have no doubt that if an air-craft were constructed, which should be guided with certainty, in a perfectly calm atmosphere, to any given point, at any time, everybody would go to see it, and that everybody who was capable of being pleased with a human victory would be delighted, although its rate of motion might not exceed that of a brisk walking pace. On the other hand, I believe that when this is once done--and the less magnificent the beginnings, the more hopeful I should be for the end-improvements will gradually follow, and that no limit can be set to the velocity which we may expect to achieve in our passage through the air. Certainly no mode of locomotion of animals, vital or mechanical, can be compared for speed with that which I anticipate for human flight.

L

But force is requisite to produce motion, and motion through the air is opposed by the resistance of that fluid. And from experiments that have been made on bodies, of forms not suited for air-craft, as well as from theory, there is reason for believing that whatever may be the form of the vessels, the resistance of the air to their progress will increase as the speed increases, in ratios not differing very much from those of the squares of the velocities. The greater therefore the speed required, the greater must be the force supplied-greater according to this law—so that if a uniform force equal to one pound weight will move a given vessel at a speed of five miles an hour, it will require a force of about four pounds to drive it at ten miles, and of about four hundred pounds at one hundred miles per hour.

So, without any farther disquisition, our next condition may be thus announced. The air-craft must be provided with a source of power sufficient to counterbalance the resistance of the atmosphere to the motion of the vessels, at any speed that may be required.

Now there is really very little to report about the means that have been proposed hitherto for providing this power. The aerial projector usually, either-firstly, has shirked the question of power altogether, and has contented himself with planning mechanism; or secondly, has discovered some wonderful secret, which he will disclose on no account, but which is in fit season to do all the duty required of it; or thirdly, has suggested, as sources of force, devices that were utterly inadequate to the purpose, or were even in fact not sources of force at all; or fourthly, has relied upon means which were not practically available without so great a weight of appliances, as to render the scheme useless even on his own theory, except upon a scale of such huge bulk as to be an effectual barrier to its adoption as a first attempt; or fifthly, has fairly looked about him for the motive agent best suited by its lightness and energy for the required end. Besides these, a few good notions have been thrown out as bald hints, generally without any serious hope that anyone would take them up and work them out, which, of course, has never been done.

I shall give a few instances of the modes in which suggestions have been made, or not made, towards the end we are now considering.