TRANSACTIONS OF THE SECTIONS. 233 



form is better than some otlier, it will be with the simple object of illustrating the 

 application of principles, by following which it wonld be possible to design a ship of 

 given displacement to go at given speed, with miuinium resistance, in smooth water — 

 in fact, to make the best performance in a " measured mile " trial. 



I have pointed out that the causes of resistance to the motion of a ship through 

 the water are : — first, surface-friction ; secondly, mutual friction of the particles 

 of water (and this is only practically felt when there are features sufficiently abrupt 

 to cause eddies); and thirdly, wave-genesis. I have also shown that these are the 

 only causes of resistance. I have shown that a submerged body, such as a iish, or 

 torpedo, travelling in a perfect fluid, would experience no resistance at all ; that in 

 water it experiences practically no resistance but that due to surface-friction and 

 the action of eddies ; and that a ship at the surface experiences no resistance in 

 addition to that due to these two causes, except that due to the waves she makes. 

 I have done my best to make this clear ; but there is an idea that there exists a 

 form of resistance, a something expressed by the term " direct head-resistance," 

 which is independent of the above-mentioned causes. Tliis idea is so largely 

 prevalent, of such long standing, and at first sight so plausible, that I am anxious 

 not to leave any misunderstanding on the point. 



Lest, then, I should not have made my meaning sufficiently clear, I s.ay distinctly, 

 that the notion of head-resistance, in any ordinary sense of the word, or the 

 notion of axiy opposing force due to the inertia of the water on the area of the 

 ship's way, a force acting iipou and measured by the ai-ea of midship section, is, 

 from beginning to end, an entire delusion. No such force acts at all, or can act, 

 as throughout the greater part of this address I have been endeavouring to explain. 

 No doubt, if two ships are of precisely similar design, the area of midship section 

 may be used as a measure of the resistance, because it is a measure of the size of 

 the ship ; and if the ships were similar in every respect, so also would the length of 

 the bowsprit, or the height of the mast, be a measure of resistance, and for just the 

 same reason. But it is an utter mistake to suppose that any part of a ship's resist- 

 ance is a direct effect of the inertia of the water which has to be displaced from 

 the area of the ship's way. Indirectly the inertia causes resistance to a ship at the 

 surface, because the pressm-es due to it make waves. But to a submerged body, 

 or to the submerged portion of a ship travelling beneatli rigid ice, no resis- 

 tance whatever will be caused by the inertia of the water which is pushed aside. 

 And this means that, if we compare two such submerged bodies, or two such sub- 

 merged portions of ships travelling beneath the ice, as long as they are both of suffi- 

 ciently easy shape not to cause eddies, the one which will make the least resistance 

 is the one which has the least skin smface, though it have twice or thrice the area 

 of midship section of the other. 



The resistance of a ship, then, practically consists of three items — namely, surface- 

 friction, eddy-resistance, and wave-resistance. 



Of these the first-named is, at least in the case of large ships, much the largest 

 item. In the 'Greyhound,' a bluff ship of 1100 tons, only 170 feet long, and 

 having a thick stem and sternposts, thus making considerable eddy-resistance, and 

 at 10 knots visibly making large waves, the surface-friction was 68 per cent, of the 

 whole resistance at that speed ; and there can be no doubt that with the long iron 

 ships now built, it must be a far greater proportion than that. Moreover the ' Grey- 

 hound ' was a coppered ship ; and most of the work of om- iron ships has to be done 

 when they are rather foul, which necessarily increases the surface-friction item. 



The second item of resistance, namely the formation of eddies, is, I believe, imper- 

 ceptible in ships as finely formed as most modern iron steamships. Thick square- 

 shaped stems and sternposts are the most fruitful source of this land of resistance. 



The third item is wave-resistanco. On this point, as we have seen, the stream- 

 line theory rather suggests tendencies, than supplies quantitative resiilts, because, 

 though it indicates the nature of the forces in which the waves originiite, tlie laws 

 of such wave-combinations are so very intricate, that they do not enable us to pre- 

 dict what waves will actually be formed under any given conditions. 



There are, however, some rules, I will not call them principles, which have to 

 some extent been confirmed by experiment. At a speed dependent on her length 

 and form, a ship makes a very large wave-resistance. At a speed not much 



