Let us observe a little more closely what happens when any body is 

 moved through the water. It is not certain that eddies should form at 

 any given instant or specified place. But the smallest of them, still in 

 an embryonic state, has the ability to develop and to grow. The water 

 alongside the ship is thus in an unstable condition. In order to under- 

 stand what we mean by that, let us study a little more closely an experi- 

 ment that everyone is familiar with. 



Let us place a long cane vertically on its point. We know that 

 necessarily it will not be long before it falls: and it will fall in that 

 direction in which it was originally leaning. If we had been able to 

 place its centre of gravity exactly above the point of rest it would re- 

 main upright indefinitely. We know that it is impossible to reach this 

 perfect position of equilibrium. The slightest initial eccentricity in- 

 creases rapidly, like an avalanche rushing down a mountain. 



Nevertheless, we can see in any circus a juggler maintaining in 

 perfect immobility a long cane at the end of his finger. Perfect immobi- 

 lity ? If we look closely we shall observe that his hand is continually 

 making little movements. It is evident that these motions compensate 

 all the while the slightest defects of equilibrium. Each of us could, 

 with some training, repeat the experiment with more or less success. 



What the juggler does with his cane the dolphin does with the water. 

 An eddy is forming and tends to develop and grow, but the dolphin 

 intervenes. A very slight movement on his part is sufficient to arrange 

 things. A fraction of a second later this would be impossible : the eddy 

 would have grown and would have become unmanageable. The 

 analogy is perfect : if the juggler had waited till the rod leaned too far 

 in one direction, he could not have brought it back to equilibrium. 



We begin to see the dolphin's secret. He must have under the skin 

 nerves of an extreme sensitivity, which act like pressure gauges, and 

 by means of which he perceives the slightest sign of a developing eddy. 

 Then, with a well-timed movement of his skin, he neutralizes the 

 scarcely- formed eddy. Thin currents of water glide along his body and 

 collect again behind him. The pressure which, necessarily, is exerted 

 upon his head is compensated by that produced by the thin currents of 

 water produced at his tail. Porpoises, dolphins and trout do not know 

 the theory that we have just explained. Reflexes and instinct with 

 them take the place of higher mathematics. 



Can the naval designer draw any practical conclusions from the 

 teachings of the dolphin ? I am tempted to reply in the affirmative. 



[138] 



