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THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[July, 



Were it not that this principle and the mode by which it is in- 

 tended to be applied, had got hold of the public mind by some of 

 those fatalities, by which the most preposterous opinions occasionally 

 get disseminated and established, we should not have held ourselves 

 justified in administering a formal refutation to a position so exceed- 

 ingly absurd, that a reference to the first rudiments of the physical 

 sciences is more than sufficient to overset. For what, after all, is the 

 principle which this position is calculated to uphold? Simply, that 

 a body can be sustained in motion against a given amount of opposi- 

 tion by a less degree of force than what is actually required to es- 

 tablish it. That this is a true statement of the principle which is in- 

 volved in this hypothesis, and that it is utterly fallacious and inane, 

 we trust we shall not be at much pains to demonstrate ; and the mode 

 we purpose to adopt with this view, is to examine the forces which 

 are inherently concerned in the establishment of a given motion, and 

 those concerned in its continuance, to show that a perfect equality 

 exists between them, and consequently that nothing less than what is 

 required for the former would satisfy the exigencies of the latter. 

 These forces, by which the induction or contiruance of motion in any 

 degree or direction, are solely affected, are reducible to four heads; 

 the attraction of gravitation— the pit inertia of matter— the friction 

 of the parts in contact — and the resistance of the medium in which 

 it is moved. 



1. Of these forces, the first, assuredly, opposes no impediment to 

 the induction of motion which it does not equally oppose to its con- 

 tinuation. Acting in one direction only, and (appreciably at least) 

 with equal force in that direction at any elevation attainable by man, 

 it is clear that whenever it interferes at all, it can never operate with 

 less force in one than in another stage of the same proceeding. To 

 initiate, therefore, or establish a given motion even in direct opposi- 

 tion to gravity, not a particle more force is exerted than is required 

 for its continuance. In an horizontal direction, however, with which 

 we are here more immediately concerned, the attraction of gravita- 

 tion literally interposes no obstacle whatever either to the induction 

 or perpetuation of motion to any extent in any body, however ponde- 

 rous and inert. This might, indeed, have been concluded, without 

 reference to principles, from a simple consideration of its effects; as 

 in .. vertical or upright direction gravity oppose* the induction of 

 motion with its mholt power, and in a contrary or descending direction 

 it favours the same in exactly the same degree, so in a direction 

 which partakes of neither, but is intermediate between both, it fol- 

 lows as a matter of course, it can really exercise no influence at all. 

 The weight of a body has in fact nothing to do with the difficulty of 

 imparting to it an horizontal motion, except in so far as it increases 

 the friction, in regard to (he medium of its support; a property 

 which, as the weight is not altered by its translation, it must exert 

 with equal efficacy in the ultimate as in the preliminary stages of its 

 progression. So far, therefore, as the attraction of gravitation is 

 concerned, it is certain that the exigencies of the aerial machine in 

 respect of power, are not a whit less for the maintenance than the es- 

 tablishment of the motion which is sought to be conferred upon it. 



2. Wit!) regard to the second of the forces enumerated, the pit fa. 

 cr/nr literally opposes, of itself, no definite obstacle to the induction or 

 continuation of motion in any direction whatsoever, and literally re- 

 quires n.. definiti amount of power to overcome. Every body which 

 is pressed against by another must reciprocate the pressure, and, 

 unless restrained by some extrinsic impediment, must participate in 

 the motion which is the measure of the force applied ; the only con- 

 dition affecting the establishment of a given rate of motion, so far as 

 this property is concerned, being that a certain time must elapse be- 

 fore the operation be completed. In all transitions from quiescence 

 to motion, or from motion to repose, a certain time is due to the trans- 

 action in respect of and proportionate to the quantity of matter which 

 is the subject of the change ; and the only obligation which is imposed 

 upon the force employed, so far as regards the Pit inertia, is that it be 

 continuously applied. With this reservation, there is no limit to the 

 effects which can be produced, however great the mass to be moved, 

 or small the force employed to move it. A flv attached bv a human 



hair to a ton of marble, would not only invest it with motion in an hori- 

 zontal direction, if it were possible to suspend it free from all friction, 

 but, presuming the absence of atmospheric resistance, bring it up, by 

 continuous effort, to any rate of motion at which it could itself proceed . 

 nay, the mighty globe of the earth could be propelled round the 

 universe by an infant as fast as its legs could carry it, if it could find 

 a path to walk on, provided the pressure was continued a sufficient 

 time to allow all the particles to be indued with motion at the rate 

 required. These results in relation to the hypothesis involved in Mr. 

 Henson's scheme, may be conveniently and familiarly illustrated by 

 the case of a man towing a heavily laden barge in still water; where 

 the counteracting forces, friction and the resistance of the medium, 

 being indefinitely assignable, and the impulse in an horizontal direc- 

 tion, the pit inertia is free to display its peculiarities without ambi- 

 guity or interference. Here is no question as to whether the strength 

 of the individual be sufficient to enable him to set his load in motion, 

 or bring it up to that rate in which his own powers shall be equal to 

 the extrinsic forces developed in the operation. As soon as he begins 

 to pull, the boat begins to move, slowly at first, and increasing gradu- 

 ally until it reaches that point at which the friction and resistance it 

 develops is equal to the power he is able to employ : nor would he 

 accomplish anything by obtaining additional assistance in the com- 

 mencement, which he would not immediately forfeit by its discon- 

 tinuance, except a more speedy attainment of the same result. No 

 one, not even the most unintellectual of bargemen, ever dreamt of be- 

 ing able at any time to drag his boat at a greater rate than he could, 

 by his own unaided efforts, have brought it up to; or with let 

 culty at the same rate, because he happened to have had the assist- 

 ance of a friend to enable him to begiu. And yet these are only the 

 conclusions which the adoption of Mr. Henson's hypothesis requires 

 us tu admit. All, in fact, that the mediation of any extrinsic tempo- 

 rary force can effect, is to dispense with a part of the time that would 

 otherwise be required to produce the result due to the inferior power 

 alone ; and it is to the forced limitation of this element of invested 

 motion that are to be ascribed those appearances in certain familiar 

 cases which have been so triumphantly referred to as affording illus- 

 trations and attesting the correctness of the hypothesis upon which 

 Mr. Henson's scheme and the hopes of his friends are founded. The 

 circumstances of the horse straining to drag into motion a heavily 

 laden cart, which he afterwards appears to draw with the utmost 

 facility, of the railway train and steam vessel, with so much difficulty 

 seemingly set agoing, and ultimately proceeding with so much appa- 

 rent ease, together with others of the like description, are all owing 

 to the neglect of this consideration in the attempt to invest large 

 bodies with motion at a given rate, in less time than is due to the 

 proportion between their masses and the forces employed to move 

 them. The conclusions, therefore, which have been drawn from these 

 premises are utterly fallacious. Not a particle, absolutely, more 

 power is required to initiate or establish a given rate of motion in 

 any bodv than is requisite to compete with the extrinsic forces, the 

 friction and resistance, developed at the rate in question ; neither in 

 the beginning, nor subsequently at any stage of the proceeding, there 

 being anything over to be accounted for on the score of, or by which 

 to measure the effects of, the vis inertia in respect of the motion so 

 superinduced and established — a certain proof that, so far as this also 

 of the forces enumerated is concerned, no more power would be re- 

 quired to produce, and, therefore, no less would suffice to maintain, 

 the rate of motion assigned to the aerial machine as necessary to its 

 support, than what is due to the exlrimic forces, the resistance and 

 friction, developed at the rate required. 



:i. Now the friction, it is well known, is a force in no way affected 

 by the rate under which it is developed ; but is just as great in •' State 

 of quiescence, however strange this may appear, as under the extrem- 

 est condition of velocity. So far, therefore, as regards the question 

 before us, this may be considered as a neutral element; interposing 

 no more obstruction to the establishment of n given motion than to 

 its continuance, and requiring, therefore, exactly as much power to 

 keep in subjection, if we may so speak, as originally to subdue. 



