O.\ 7 frAVAL ARCHITECTURE. 311 



existence, in virtue of the frictionai force exerted on the contiguous 

 particles by the friction cf the plane, a pair of forward streams, along- 

 side, and if the plane is fifty feet long, they are seen to have, at the 

 tail end of the plane, a breadth of, say, seven or eight inches on each of 

 its sides, and to have a mean velocity, something like half that of the 

 plane itself. It is extraordinary how it happens that particles posscs^fi 

 of such large concurrent speed as those which form these streams, can 

 still exert a not very greatly diminished backward drag on the plane ; 

 it is true that the front edge of the plane the intensity is greatly 

 increased, and is quite incommensurable with what it is at the after 

 end. If the anterior edge is of polished metal quite keen, and a 

 very thin coating of tallow is put on it, the friction will strip this back, 

 and arrange it in a pair of thread-like ridges immediately behind the 

 edge ; but after this very narrow strip of mixinuni intensity is past 

 there is for the nVst few feet a rmderat^ly graduated d'.minution of 

 intensity ; and after ten feet cf length the diminuti< n of intensity is 

 relatively insensible. However, without affecting to have solved the 

 question theoretically, we have gone through it practically v/ith sufficient 

 completeness to know what mean resistance per square foot is exerted 

 by any given lengths of the different qualities of surface, and we know 

 that surface friction is a very sensitive fi. nation, and that until we learn 

 the exact quality of the surface of the ship or body, it is impossible 

 to predicate what its resistance will be. I may tell you that a plane, 

 coated with tin foil six or eight inches long, makes only half the 

 resistance of a plane coated with clean paint or varnish, but when you 

 get to fifty feet long the tin foil makes almost exactly the same resis- 

 tance as the clean paint ; and again a slimy surface, like that of a fish, 

 makes more resistance than a perfectly hard smooth surface. By the 

 help of these experiments I am able to say approximately what the 

 sv.rface friction of a ship of given quality of surface should be, and in 

 virtue of that determination I am able to tell you that with a model 

 of say twelve feet in length, at a speed of fifty or sixty feet, per minute, 

 which is a moderate speed for the model, and is equivalent to a speed 

 of six knots for a ship 300 feet long similar to the model, the resistance 

 is nothing more than that due to surface friction : we find, in fact, by the 

 dynamometer that there is not any more than 1 that. I have, therefore, 

 ro hesitation in saying that the fundamental proposition I have 

 asserted is borne out by facts. 



