1819.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



107 



where they are arranged so as to be disi-onnected, being included 

 in the determined resistance ; and in these cases, this would have 

 to be allowed for and be deducted. As however the resistance to 

 flat surfaces has been so fully investigated, this would occasion 

 little difficulty. An example perhaps will serve better to render 

 the foregoing proposition clear. Suppose a vessel to be going 

 tlirough the water with a velocity of 15 feet pei second, and on 

 the stopping of the engines, the speed be observed in one second 

 to sink to 14.i feet per second, the velocity destroyed in one second 

 of time would be equal to 6 inclies ; this is ^th part of the velo- 

 city which would have been destroyed, in the same time, by a force 

 equal to the weight of the vessel, and the force which destroyed it 

 is therefore equal to j'.th part of that weight. Now suppose this, 

 as ascertained from the displacement, to be 1,000 tons, then the 

 mean force of resistance between the velocities of 1.5 feet and H^ 

 feet per second is 'gf tons, and the povser expended in overcoming 

 this resistance, at tlie mean between the two velocities, or l-tf feet 

 per second, is — 



1000 X 2240 



— —= 63,30 H.r. 



61 X 550 ' 



Havingthus described the mode in which the observation should he 

 conducted, it may not be uninteresting briefly to notice the advan- 

 tages which might result from a weU-arranged set of experiments 

 on this plan. 



In the first place, it would offer an infallible means of testing 

 the qualities of any particular vessel, apart from those of her ma- 

 chinery ; since the action of the engines being stopped during the 

 observation, has clearly no influence whatever on the rate of dimi- 

 nution of tlie velocity. The observer v.ould thus, in case of any 

 deficiency of speed, be enabled to fix the fault with certainty upon 

 the vessel, or the engines, as the case might be. Indeed, if all 

 vessels were submitted to the observation here described, there 

 seems to be no reason why their resistance should not be as pre- 

 cise and definite a quantity, and as capable of accurate expression 

 for any particular draught, as their displacement ; and it might be 

 confidently asserted, that the vessel which in this way showed the 

 least resistance, ought to beat all others cceteris paribus, and if she 

 were not found to do so, that the fault was either in the engines, 

 or in the propelling apparatus, and certainly not in the ship. 



This however, though one advantage, would not be the only one, 

 as a series of observations made upon the same vessel, at various 

 velocities, could hardly fail to demonstrate some expression or law 

 of relation, between the observed velocities and the resistances, 

 which if f(uind to be uniform within the limits of the experiment, 

 might fairly be presumed to extend to some distance beyond them, 

 so that there would be a strong ground for predicting, with confi- 

 dence, the increase of speed wliich might be expected to result 

 from any proposed increase of power. 



Again, if these observations were repeated for various draughts 

 of water, in the same vessel, tlie means would be furnished of 

 knowing, a priori, tiie precise amount of power which should be 

 necessary for maintaining a given speed, with any required load. 

 Or if the power remained constant, what would be the speeds 

 which should correspond to various loads, and as before, if the 

 performance of the vessel should fall short of what had been so 

 determined, the fault would be in the machinery. 



It may be mentioned, that although hitherto no opportunity has 

 been afforded for testing the correctness of the views here pro- 

 pounded, by a practical application of them to the point in 

 question, yet the author has on several occasions adopted a 

 method, similar in principle, in determining the resistance of 

 shafting and machinery, by observing the rate of diminution of the 

 velocity, on shutting off the steam from the engine, and having 

 had good reason for believing the results, in these cases, to have 

 been tolerably accurate, in spite of the difficulty of correctly es- 

 timating the aggregate momentum of so many bodies revolving at 

 various velocities, he is encouraged to suppose, that in the case of 

 a floating body, whose momentum is so easily and precisely ascer- 

 tainable, the result would be more exact and unquestionable. 



The method adopted by M. de Pambour, for ascertaining the 

 amount of resistance to the motion of railway trains, by the cir- 

 cumstances attending their descent and stoppage upon two consecu- 

 tive inclined planes, is based upon precisely the same principle as 

 that here advanced. 



It remains only to copsider, by what means the variable velocity 

 of the vessel can be measured, so as to ascertain it, at any instant, 

 with the necessary precision. In the absence of a better, the fol- 

 lowing arrangement might perhaps he adopted with advantage : — 



To the bowsprit of the vessel (Fig. 1), sufficiently a-head to be 

 beyond the disturbed water, should be screwed a small iron 



bracket, carrying a pin, which should pass through a hole in a 

 slender rod, hanging down below the surface of tlie water, and 

 prolonged a few inches upwards above the bracket. Upon this rod. 



Fig. I. 

 a metal sphere should be fitted, so as to be fixed at any required 

 distance from the point of suspension ; from the upper extremitv 

 of the rod, a small cord or wire should be brought, passing to some 

 convenient spot on the fore-part of the deck, where it should be 

 attached to one arm of a rectangular lever, whose other arm or 

 index should move along an arc of metal. 



It will be clear, that when the vessel is at rest in the water, the 

 rod, with the sphere attached, will hang vertically, and the cord 

 being properly adjusted, the index of the bent lever will be hori- 

 zontal, at which position the zero of the graduated arc should be 

 marked. If the vessel moves through the water, the resistance ou 

 the sphere will throw the rod out of the perpendiculai-, as indi- 

 cated by the dotted line, and the angle which it makes with its 

 former position, will be shown by the new position of the index on 

 the metal arc. It is obvious, therefore, that the same degree of 

 deflection will always be produced by the same resistance, and the 

 same resistance by the same velocity. So that if the arc has once 

 been carefully graduated, by moving with known velocities, which 

 would have to be done once for all, it would always afterwards 

 furnish a correct indication of the velocity with which the sphere, 

 and consequently the vessel, was moving through the water, at the 

 instant of observation. A slight consideration of the nature of 

 this apparatus in action, will make it ajiparent that the position 

 of the ball, or sphere, on the rod is indiffei-ent, and that the same 

 angular deflection will always correspond to the same resistance. 

 This would contribute to render the arrangement convenient as 

 being applicable to vessels of various heights. 



The mode of using this apparatus is obvious ; an observer, with 

 a seconds watch, would note the positions of the index on the arc, 

 at such intervals of time as should be determined upon, when the 

 diminution of velocity and consequently the retarding forces would 

 become known, as before described. 



Mathematical expressions have in the course of this paper been 

 expressly abstained from, as being unnecessary to a general view 

 of the method proposed. It may however not be out of place to 

 remark, that perhaps the best way of treating the observations 

 when obtained, would be to endeavour to discover an expression, 

 representing accurately the velocity in terms of the time, in which 

 case, its differential co-efficient would be accurately proportional 

 to the resistance. 



Remarks made at the Meeting after the reading of the above Paper. 



Mr. Scott Russell said he could not venture, without more care- 

 ful consideration of the suliject thaa was permitted by merely hearing 

 the paper read, to draw any comparison between tlie results stated by the 

 author and those which he had arrived at from the extensive series of ex- 

 periments he had made; but his first impression was, that the method pro- 

 posed was not the most direct, and that it was lial)le to several objectioni. 

 He doubted whether there was a sufficient knowledge of the resistance of 

 flat bodies moving through fluids, to enable the portion of resistance due to 

 the floats of the paddle-wheels, in a state of rest, to be separated, in the 

 general result, from the resistance due to the body of the vessel. Assuming 

 such to be the case, he must contend, that unless some means existed of 



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