THE LAWS OF FLUID RESISTANCE. 113 



the particles of the fluid which have to be set in motion by 

 the body. But so far from thereby rendering less obstruc- 

 tion to. the passage of the body, these pressures are 

 enabled by that very escape to result in a resistance, which, 

 if they were confined by the fluid overhead, as with a sub- 

 merged body, they would have been unable to produce ; in 

 fact at the surface the particles are able to escape the duty 

 of restoring to the body the power which the body em- 

 ployed to set them in motion. There can be no doubt that 

 in this way a fish, when swimming so close to the surface 

 as to make waves, experiences more resistance than when 

 deeply immersed. 



It is worth remark that this cause of resistance, " wave- 

 genesis " or "wave- making resistance," as it has been 

 termed, would be equally a cause of resistance in a fric- 

 tionless fluid, and it is for this reason that in proving to 

 you just now that a body would experience no resistance 

 in moving through a frictionless fluid, I limited the case to 

 that of a submerged body. It is true that in a frictionless 

 fluid the wave system generated by a ship would not waste 

 away, as in water, by its internal friction; but it would 

 none the less be diffused into the surrounding fluid, and 

 thus, as the ship proceeded, she would cover a larger and 

 larger area of ocean surface with the waves she was making. 



Having arrived at this point, I think it will be useful 

 briefly to review the several cases of motion through fluid, 

 in order to trace where the several causes of resistance we 

 have dealt with come into operation. 



Case I. A plane moving edgeways through frictionless 

 fluid. Here there will be no resistance. 



Case II. A plane moving edgeways through frictional 

 fluid. Here there will be resistance due to surface friction. 



Case III. A submerged body moving through friction- 

 less fluid. The inertia of the fluid undergoing stream-line 

 motion, causes excess of pressure at the two ends, and defect 

 of pressure along the middle. The forward and backward 

 pressures balance one another, and therefore cause no re- 

 sistance. 



Case IY. A submerged body moving through frictional 

 fluid. Here there is resistance due to surface friction. 

 Also, if the body is abrupt enough to cause eddies, part of 



VOL. II. I 



