234 REPORT— 1875. 



lower tlian this, the wave-resistaoice is considerably less, and at low speeds it is 

 insignificant. Lengthening- the entrance and run of a ship tends to decrease the 

 wave-resistance ; and it is better to have no parallel middle body, but to devote the 

 entu-e length of the ship to the entrance and run, though in tlais case it be neces- 

 sary to increase the midship section in order to get the same displacement in a 

 given length. 



With a ship thus formed, with fair water-lines from end to end, the speed at 

 which wave-resistance is accumulating most rapidly, is the speed of an ocean wave 

 the length of which, from crest to crest, is about that of the ship from end to end. 



I have said we may practically dismiss the item of eddy-resistance. The problem, 

 then, to be solved in designing a ship of any given size, to go at a given speed with 

 the least resistance, is to so form and proportion the ship that at the given speed 

 the two main causes of resistance, namely surface-friction and wave-resistance, 

 when added together, may be a minimum. 



In order to reduce wave-resistance we should make the ship very long. On the 

 other hand, to reduce the surface-friction we should make her comparatively short, 

 so as to diminish the surface of wetted skin. Thus, as comnionh' happens in such 

 problems, we are endeavouring to reconcile couilicting methods of improvement ; 

 and to work out the problem in any given case, we require to know actual quan- 

 tities. We have sufficient general data from which the skin-resistance can be de- 

 termined by simple calculation ; but the data for determining, wave-resistance must 

 be obtained by direct experiments upon difierent forms to ascertain its value for 

 each form. Such experiments should be directed to determine the wave-resistance 

 of all varieties of water-line, cross section, aud proportion of length, breadth, smd 

 depth, so as to give the comparative results of difierent forms as well as the absolute 

 result for each. 



An exhaustive series of such experiments could not be tried with full-sized 

 ships; but I trust that the experiments I am now carrying out with models, for the 

 Admiralty, are gradually .accumulating the data required on this branch of the subject. 



I wish in conclusion to insist again, with the greatest lU'gency, on the hopeless 

 futility of any attempt to theorize on goodness of form in ships, except under the 

 strong and entirely new light which the doctrine of stream-lines throws on it. 



It is, I repeat, a simple fact that tlie whole framework of tliought by which the 

 search for improved forms is commonly directed, consists of ideas which, if the 

 doctrine of stream-lines is true, are absolutely delusive and misleading. And real 

 improvements are not seldom attributed to the guidance of those very ideas which 

 I am characterizing as delusive, while in reality they are the fruit of painstaking, 

 but incorrectly rationalized, experience. 



I am but insisting on views which the highest mathematicians of the day have 

 established irrefutably ; and my work has been to appreciate and adapt these views 

 when presented to me*. 



No one is more alive than myself to the plausibility of the unsound views against 

 which I am contending ; but it is for the very reason that they are so plausible that 

 it is necessary to protest against them so earnestly ; and I hope that in protesting 

 thus, I shall not be regarded as dogmatic. 



In truth, it is a protest of scepticism, not of dogmatism ; for I do not profess to 

 direct any one how to find his way straight to the form of least resistance. For 

 the present we can but feel our way cautiously towards it by careful trials, using 

 only the improved ideas which the stream-line theory supplies, as safeguards against 

 attributing tbis or that result to irrelevant or, rather, non-existing causes. 



* I cannot pretend to frame a list of the many eminent mathematicians who originated 

 or perfected the stream-hue theory ; but I must name, fi'cm amongst them, Professoi- 

 Rankine, Sir William Thomson, and Professor Stokes, in order to express my personal 

 indebtedness to them for information and explanations, to which chiefly (however imper- 

 fectly \it)lized) I owe such elementary knowledge of the subject as alone I possess. 



