110 LECTURES TO SCIENCE TEACHERS. 



Since then a frictionless fluid would offer no resistance 

 to a submerged body moving through it, we have next to 

 consider what are the real causes of the resistance which 

 such a body experiences when moving through water. The 

 difference between, the behaviour of water and that of the 

 frictionless fluid is twofold, as follows : 



First, the particles of water, unlike those of a friction- 

 less fluid, exert a drag or frictional resistance upon the 

 surface of the body as they glide along it. This action is 

 commonly called surface-friction or skin friction, and its 

 amount in any given case can be calculated from general 

 experimental data. The resistance due to the surface- 

 friction of a body such as that which we have been con- 

 sidering is practically the same as that of a plane surface 

 of the same length and area, moving at the same speed 

 edgeways through the water. 



The second difference between the behaviour of water 

 and that of the imaginary frictionless fluid surrounding 

 the moving submerged body, is that the mutual frictional 

 resistance experienced by the particles of water in moving 

 past one another somewhat hinders the necessary stream- 

 line motions, alters their nice adjustment of pressures and 

 velocities, defeats the balance of forward and backward 

 forces acting against the surface of the body, and thus in- 

 duces resistance. This action, however, seems imperceptible 

 in forms of fairly easy shape such as that shown in Fig. 2, 

 and only operates tangibly where there are angular fea- 

 tures, or very blunt sterns, like the blunt round tail, for 

 instance, of the bodies shown in Figs. 15 and 16. In such 

 a case, the stream-lines, instead of closing in round the 

 stern, as shown in the figures, form a swirl or eddy, from 

 which it results that the excess of pressure which would 

 exist at the tail-end in a frictionless fluid, and which would 

 there counterbalance the similar excess of pressure at the 

 nose of the body, becomes in water greatly reduced, and in 

 part converted into negative pressure, and thus a very 

 great resistance may result. It is worth mentioning, how- 

 ever, that it is blunt tails rather than blunt noses that 

 cause these eddies, and thus a body with one end round 

 and the other sharp, no doubt experiences least resistance 

 when going with the round end first. 



I call this source of resistance " eddy-making resistance, ' 



