232 REPORT— 1875. 



except tbis, tliat it constitutes almost the wliole of the resistance experienced hy 

 bodies of tolerably easy shape traTelling under water at any reasonable speed. 



Secondly. The mutual friclional resistance experienced by the particles of water 

 in moving past one another, combined willi the almost imperceptible degree of vis- 

 cosity which water possesses, somewhat hinders the necessary stream-line motions, 

 alters their nice adjustment of pressm-es and velocities, and thus defeats the balance 

 of stream-line forces and induces resistance. This action, however, is imperceptible 

 in forms of fairly easy shape. On the other hand, angular or very blunt features 

 entail considerable resistance fi'om this cause, because the stream-line distortions 

 are in such cases abrupt, and degenerate into eddies, thus causing gi-eat differences 

 of velocity between adjacent particles of water, and great consequent friction be- 

 tween them. " Dead water," in the wake of a ship with a full run, is an instance 

 of this detrimental action. 



So far we have dealt with submerged bodies only ; we will now take the case of 

 a ship travelling at the surface of a perfect fluid. But first, let us suppose the 

 surface to be covered with a sheet of rigid ice, and the ship cut off level with 

 her water-line, so as to travel beneath the ice, floating, howe-\-er, exactly in the same 

 position as before (see Plate XI. fig. 28). As the ship travels along, the stream-line 

 motions will be the same as for a submerged body, of which the ship may be regarded 

 as the lower half; and the ship will move without resistance, except that due to the 

 two causes I have just spoken of, namely surface-friction and mutual friction of the 

 particles. The stream-line motions being the same in character as those we have 

 been consideiing, v.e shall still have at each end an excess of pressure which will tend 

 to force up the sheet of ice, and along the side we shall have defect of pressure 

 tending to suck down the sheet of ice. If, now, we remove the ice, the fluid will 

 obviously rise in level at each end, so that excess of hydrostatic head may afford 

 the necessary reaction against the excess of pressxu-e ; and the fliud will sink by 

 the sides, so that defect of hydrostatic head may afford reaction against the defect 

 of pressure ; and the same actions and reactions will happen in the imperfect fluid, 

 water, making only the same allowance for the modification of velocities and pres- 

 sures by friction as were sho'wn to be necessary in treating of wholly submerged 

 bodies. 



The hills and valleys thus formed in the water arc, in a sense, waves ; and, though 

 originating in the stream-line forces of the body, yet when originated, they come 

 rmder the dominion of the ordinary laws of wave-motion, and, to a large extent, 

 behave as independent waves. 



The consequences which result from this necessity are most intricate ; but the 

 final upshot of all the different actions which take place is plainly this — that 

 the ship in its passage along the surface of the water has to be continually sup- 

 plying tlie waste of an attendant system of waves, which, from the nature of tlieir con- 

 stitution as independent waves, are continually difliisiug and transmitting them- 

 selves into the surroundii^g water, or, where they form what is called broken water, 

 crumbling away into froth. Now waves represent energy, or work done ; and 

 therefore all the energy represented by the waves wasted from the system attend- 

 ing the ship, is so much work done by the propellers or tow-ropes which are urging 

 the ship. So much wave-energy wasted per mile of travel, is so much work done 

 per mile ; and so much work done per mile is so much resistance, and this cause of 

 resistance at least would operate with full effect even in a perfect fluid. 



The actions involved in this cause of resistance, which is sometimes termed " Wave- 

 Genesis," are so complicated that no extensive theoretical treatment of the subject can 

 be usefullj' attempted. All that can be known about the subject must, for the present 

 I believe, be sought by direct experiment. 



Having thus briefly described the several elements of a ship's resistance, I will 

 proceed to draw yoiu' attention more particularly to certain resulting conside- 

 lations of practical importance. Do not, however, suppose that I shall ■\enture on 

 dictating to shipbuilders what sort of ships they ought to build : I have so little 

 experience of the practical requirements of ship-owners, that it would be presump- 

 tuous in me to do so ; and I could not venture to condemn any feature in a ship as 

 a mistake, when, for all I know, it may be justified by some practical object of 

 which I am ignorant. For these reasons, if I imply that some particular element of 



