1843.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



191 



impossible precisely to define tlie actual quantities or proportions of 

 each wliich are calculated to produce the greatest amount of resistance 

 available to the purpose in view. But though we are thus precluded 

 from appointing any specific terms to the size and number, in like 

 manner as we have been able to do with respect to the obliquity of 

 the impinging planes, yet are there certain considerations by which 

 these conditions are affected, and under which a rough estimate may 

 be formed of their efficiency or aptitude to perform the work assigned 

 to them. These considerations are drawn from the properties of the 

 medium, and the obligations of speed imposed upon the aerial vehicle 

 by the already stated exigencies of its support. It is well known and 

 easily conceivable, that when surfaces of any specific dimensions are 

 set in motion, a disturbance of the equilibrium of the density of the 

 air must ensue proportioned to the extent and speed of the impingiu g 

 planes. Now if the system of planes by which this disturbance is 

 effected, should be so constituted as that they come iu succession into 

 the same or adjacent portions of the atmosphere before this equilibrium 

 shall have been restored, they no longer operate in or act upon a me- 

 dium of equal resistance to that by which the other conditions of the 

 case are governed, and upon the hypothesis of which their own effec- 

 tive power has been calculated ; and in fact, if this state of things be 

 extreme, so that either by reason of the proximity of the planes or the 

 rapidity of their succession, they pass over the space allotted to them 

 in the same or less time than the air takes to rush into a vacuum (which 

 is at the rate of about 1339 ft in a second) they would no longer have 

 any medium at all to act upon, and consequently be productive of no 

 amount of reaction, however great their number or their size. To 

 mitigate these consequences there are two modes of proceeding; 

 either to increase the distance between the planes, or to diminish the 

 velocity with which they are impelled. Now to the latter of these 

 we are precluded from having recourse by two considerations ; first t 

 by the general consideration that, as rale is the condition of impact 

 upon which the actual amount of the resistance of a given plane en- 

 tirely depends, to subdue the rate is to impair the efficiency of the 

 machine essentially and without regard to any particular disposition 

 of its parts ; and, secondly, because a rate of motion the highest, pro- 

 bably (if not indeed higher than any) that will be found attainable, is 

 required by the condition of rate assigned to the aerial vehicle itself 

 which rate must at all events be equalled by that of the instruments of 

 its propulsion; for it requires no great process of reasoning to perceive 

 that by no appointment of propulsive machinery can a body be in- 

 dued with a greater rate of motion than that at which the propelling 

 agents are themselves proceeding; so that it will be readily admitted 

 the contemplated relief cannot with propriety be expected from the 

 diminished speed of the impinging vanes. To increase the distance 

 between them is therefore the only available means of sustaining the 

 propulsive energy of the revolving apparatus, which in a construction 

 of prescribed dimensions can only be accomplished by restricting the 

 number; and the only question that remains for our discussion is, 

 whether this condition has been sufficiently regarded in the plan be- 

 fore us. Now this question can be answered satisfactorily by refer- 

 ence to experiment alone ; to experiment, with the particular machine 

 in the process of its construction, or to general experience in ope- 

 rations of the same description. To the former of these, of course, 

 we have no means of referring, but to the latter we have happily 

 some pretensions; and in accordance with that experience, we have 

 no hesitation in alleging a very great redundancy in the number of the 

 impinging vanes in Mr. Henson's propelling apparatus, viewed with 

 reference to the special object for which they are designed. Indeed, 

 so convinced are we of the prejudicial influence which even one su- 

 perflous member in a system of revolving vanes is calculated to exer- 

 cise over the effective product of their impact, that we feel quite 

 assured that had the number of arms been but three or even two, in- 

 stead of six, in each compartment, they would have realised under 

 the prescribed conditions of the case, a very superior amount of at- 

 mospheric reaction ; perhaps the greatest that with an equal amount 

 of surface it would have been possible to have accomplished. We 

 know that to this it may be answered, that in truth the modifications 



alluded have been tried, and that it was not until the superior efficacy 

 of the larger number had been attested and approved, that the pre- 

 sent arrangement was ultimately adopted. But before we admit any 

 force to this conclusion, we must be satisfied that the experiments 

 upon which it is based, embrace all the modifications to which the 

 conditions of the case are liable ; that not only has the proper angle 

 of inclination been equally retained throughout, but that the efficacy 

 of the machine has been tested in all cases at its utmost speed, and 

 subject to an equable distribution of surface according to the varying 

 circumstances of the case ; for there is no doubt that with a subdued 

 rate of motion a larger number of the like planes will produce a 

 greater amount of resistance, or that a larger number of planes will 

 produce a greater amount of resistance than a smaller number of the 

 same planes, where the rate of their motion is not such as to require 

 an augmented interval between them to preserve the integritv of the 

 resisting medium. 



And here, in concluding this branch of our investigations, we 

 would just briefly warn our readers against a very common and falla- 

 cious mode of regarding the operation of this particular sort of in- 

 strument, (which may not inaptly be termed the "aerial screw,") by 

 which many are led to overlook or reason away the obstacle here set 

 forth to its success; namely, that this obstacle, arising from a rapid 

 action of the parts in a circumscribed space, would be avoided by the 

 anticipated result of the operation — the progress conferred upon the 

 machine — whereby they would be constantly introduced into fresh 

 portions of the resisting medium ; in other words, that the ultimate 

 success of the undertaking will be an efficient means of removing the 

 difficulty by which that success is mainly threatened. The argument 

 is absurd to a degree. It is a petttio principii of the most flagrant 

 character ; because, not only is the insufficiency of the grounds of the 

 progressive motion of the machine (the impoverishment of the me- 

 dium) here set forth as the thing to be cured by the result of its own 

 operation, but, the truth is, the very progress of the machine, apart 

 from the operation of the vanes, is itself a main contributer to that 

 deficiency, the effects of which it is expected to repair, in the with- 

 drawal of the medium by the advance of the body within it, imposing 

 fresh obligations of speed upon the agents of its propulsion to enable 

 them to realise the prescribed amount of resistance. It is an argu- 

 ment not merely in a circle, but (if we may be allowed to coin a figure) 

 in a spiral, in which the premises are rendered even less consequen- 

 tial by the conclusion which they are intended to support. 



We have now left, of the questions we originally proposed to con- 

 sider with reference to Mr. Henson's scheme of flight, the power by 

 which the parts are to be indued with motion, and the principle upon 

 wliich a first impetus is expected to be acquired. The insufficiency 

 of the one and the fallacy of the other we purpose to expose in our 

 next number. 



NEW FLAX MILL AT CASSANO D'ADDA, IN LOMBARDY. 



(With an Engraving, Plate VIII.) 



We give in this number a plate showing a perspective view of the 

 Flax Mill atCassano in Lombardy, erected under the immediate direc- 

 tion of Mr. Albano, C.E., of London, and which at present is exciting 

 much attention on the continent. To us it has additional interest, as 

 attesting the wide diffusion of English industry, the whole of the mill- 

 wright work having been supplied from the works of Mr. W.Fairbairn 

 of Manchester, after the designs of Mr. Albano. It serves to show 

 at the same time the field of employment open to the English manu- 

 facturer, and the resources available to foreign enterprise, who are 

 thus able to carry out their own plans, and to profit by the proficiency 

 and talent of the best factories in the world. 



The following description of the manufactory we extract from a 

 report of Giulio Sarti, the government engineer, who with his col- 

 leagues surveyed the building at its completion, on the 31st of De- 

 cember last. 



