THE LAWS OF FLUID RESISTANCE. Ill 



and, as I have said, it will be imperceptible in forms of 

 fairly easy shape, such, for example, as Fig. 2. Such a 

 form of submerged body will experience practically no 

 resistance except that due to surface-friction, and will 

 therefore experience practically only the same total re- 

 sistance as a thin plane, like Fig. 1, moving edgeways, 

 which possesses the same area of wetted skin. In fact, we 

 may say generally, that all submerged bodies of fairly fine 

 lines experience no resistance except surface-friction. 



I have hitherto, throughout the whole of this reasoning, 

 been dealing with submerged bodies only, by which I mean 

 bodies travelling at a great depth below the surface of the 

 fluid ; and I have shown the sole causes of their resistance 

 to be the two I have termed respectively surface-friction 

 and eddy-making resistance. But when we come to the 

 case of a ship, or any other body travelling at or indeed 



near the surface, we find a new cause of resistance intro- 

 duced ; a cause, the consideration of which is often of most 

 vital importance in the design of the forms of ships, and 

 which renders the question of the form of least resistance 

 for a ship, entirely different from that of the form of least 

 resistance for a submerged body. This new cause of re- 

 sistance, like the eddy-making resistance, operates by 

 altering the stream-line motions and defeating their balance 

 of forward and backward forces. It arises as follows : 



Imagine a ship travelling at the surface of the water, 

 and first let us suppose the surface of the water 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, 

 however, exactly in the same position as before (see Fig. 

 17). 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 



