out to mean in his view of a length about three, or at most four times as great as the breadth of the figure;' and with these dimensions he proposes, for reasons which are simply absurd, a form which has nothing to recommend it. Indeed, his adherence to his notion of a cylindro-conic' gas-vessel utterly vitiates the whole superstructure of his scheme. This fairly deducts one from M. Mongè's points.

Of the remaining two conditions, the fourth is one which, as I have just had occasion to state, is true in general design but false in special principle; it corresponds in object with that which will be the fourth of my conditions. The ninth is simply a general truth common to all constructions;-all things that are to be made, must be easily made. It is only inserted by the author to give weight to his unhappy crotchet about 'développable' surfaces. The statement of it seemed perhaps to be entailed upon him by the belief which he holds in common with Sir George Cayley, and, it must be admitted, with the other ablest writers on this subject-that enormous magnitude is necessary for air-craft to give them any chance of success. Twenty yards of breadth and forty of length is to be the measure of the first experimental machine which he proposes. And he talks quite coolly of an aerostat colossal cylindro-conique' 140 yards in diameter and 560 yards in length. This author states propositions that correspond with eight of the twelve conditions we have arrived at, but of the chief part of one of the eight which he touches truly (the condition of anchorage), he is as silent as he is of the remaining four. The necessity, indeed, for three of these latter, those that respect the stiffness and the balance of level of the air-craft, would not be very urgent with such short vessels as he has in contemplation. I do not think M. Mongè can be allowed to count more than six for his ascertained requisites.

1 Mongè, ‘Études,' p. 174.

AERIAL NAVIGATION.

PART THE SECOND.

HINTS FOR THE SOLUTION OF THE PROBLEM.

'Possunt etiam fieri instrumenta volandi, ut homo sedens in medio instrumenti, revolvat aliquod ingenium, per quod alæ artificialiter compositæ aerem verberent ad modum avis volantis....Hoc instrumentum volandi non vidi, nec hominem qui vidisset cognovi, sed sapientem, qui hoc artificium excogitavit, explicite cognosco.'

Friar ROGER BACON, 'De Secretis Operibus artis et naturæ,'

CHAPTER I.

INTRODUCTORY.

THE object of this part of the book is to point out how the navigation of the air may be accomplished. The previous pages will have prepared the reader, if he has had patience for their perusal, for the matter which is now to follow. I may, however, briefly state what now remains for me to put before him. I have in the first part endeavoured to show, that though the propulsion through the air, at a rapid rate, of vehicles buoyed up by light gas, has not hitherto been accomplished, it has never been proved impossible. I have now, if I can, to establish the grounds of a belief that this achievement is possible with the simple aid of resources with which the present state of science provides us, and that without any difficulty, and without the necessity of making the first experiments on any scale of colossal magnitude. In the former part of my book I have stated the conditions which it was necessary to satisfy in any attempt to solve this problem. In this concluding part my task is to endeavour to show how these requisites may be fulfilled. I said in my preface that I am unable to give to my suggestions, as was my wish, the additional claim to attention which the results of experimental enquiry might put forth. I will not occupy more space by repeating the apology which I there gave in explanation of this. I trust that the number of experiments which will propose themselves under the several heads of this part of my book, will confirm what I said before of the impossibility of a single individual presenting the subject in a complete form. Indeed, I think it will be allowed that any contribution of a few experiments where such a number of details are required, would be

far less likely to impress a thinking mind with the possibility of the ultimate end to which they might remotely point, and to which they might be intended to lead,—far less likely, I say, than a body of suggestions founded on established facts, not only all converging to the same centre, but as a whole claiming to be sufficient for the perfect solution of the problem. Such a set of materials I have here to offer; not, perhaps, all shaped and smoothed and ready to be put together, but requiring, I believe, nothing but the finishing hand of the artificer to prepare them for arrangement into an organic whole, for whose completion they are sufficient without default of any element essential to the structure. I shall arrange my suggestions under separate heads corresponding to the conditions ascertained in the second part. Taking the conditions in the order in which I arranged them at the end of my former disquisition, I hope to show, for each of them severally, the means by which it may be satisfied. Finally, I shall endeavour to group together the results of this enquiry so as to show that none of the means suggested are incompatible with the use of the others, but that they will all work together for the fulfilment of their appointed end. I have no discoveries to announce, scarcely anything that may be called an invention to describe, none certainly to claim.' I have merely to point out the bearing of certain well-known principles in science, chemical, mechanical, and geometrical, upon the problem which it occurred to me to illustrate-principles whose application to this purpose seem to me to have been neglected by those who have gone before me, if indeed the general utility of one or two of them has not been entirely neglected by those who, in this Dædalic age, have been binding science to the service of man. The wide application, indeed, which some of the means to which I shall have occasion to advert, seem to me to have to numerous important purposes of everyday life on solid earth, are such as almost to stagger my faith in the correctness of my facts. Indeed, were there no other instances of our oversight and of our slowness even in this generation, to apply to our benefit all the natural powers at our command, I might be inclined to doubt whether the forces which I fancy I see close at hand are not mere fictions of my brain, and whether I have not been misled by some strange