92 LECTURES TO SCIENCE TEACHERS. 



mating the resistance of bodies moving through water were 

 not only wrong in detail, but that the supposed cause of 

 resistance, with which alone they professed to be dealing, 

 was in reality no cause at all ; and that the real cause of 

 resistance, whatever it might be, was entirely left out. 



It is easy to imagine how fruitful, in false aims and false 

 principles of nautical construction, would be the assignment 

 of the resistance of ships to a supposed cause which has no 

 existence at all. And the old theory, though now discarded 

 by scientific men, has obtained such a hold on the minds of 

 the general public, that I hope you will excuse my devoting 

 considerable space to its refutation. 



I will now briefly sketch an elementary view of the 

 stream-line theory so far as it is relevant to our present 

 purpose. Let it be understood that I am still dealing only 

 with the supposed frictionless fluid ; that for reasons which 

 will hereafter appear, I am dealing not with a ship at the 

 surface, but with a submerged body ; and that I am sup- 

 posing it to be travelling at a steady speed in a straight 

 line. I am going to prove to you that under these circum- 

 stances the inertia of the fluid which has to be set in motion 

 to make way for the body, will cause no resistance to it. 

 Not that such inertia will cause no pressures and suctions 

 acting upon the surface of the body ; far from it ; but that 

 the pressures and suctions so caused must necessarily so 

 arrange themselves, that the backward forces caused to the 

 body on some parts of its surface will be neutralised by the 

 forward forces caused on other parts. In effect, although 

 the inertia of the fluid resists certain portions of the body, 

 it propels the other portions of the body with a precisely 

 equal force. 



In showing how this comes about, I prefer to substitute 

 for the submerged body moving through a stationary ocean 

 of fluid, the plainly equivalent conception of a stationary 

 submerged body surrounded by a moving ocean of fluid. 

 The proposition that such a body will experience no total 

 endways push from the fluid flowing past it arises from a 

 general principle of fluid motion, which I shall presently 

 put before you in detail, namely, that to cause a frictionless 

 fluid to change its condition of flow in any manner what- 

 ever, and ultimately to return to its original condition of 



