88 OUR HUMBLE HELPERS 



sistance of the water ; and their fan-shape gives them 

 purchase enough to send the duck forward. When 

 the duck draws them in again under its breast, they 

 are closed automatically by the resistance of the li- 

 quid acting in a contrary direction; the membrane 

 refolds in the manner of a closed umbrella, thus do- 

 ing away with all shock or recoil. The twofold es- 

 sential of a perfect oar lies in its presenting to the 

 water the greatest possible surface on the stroke, and 

 the least possible surface on the recovery, so as to 

 furnish adequate purchase against the water in the 

 first movement and to offer only very feeble resist- 

 ance in the second. If the oar moved alternately 

 forward and backward while presenting the same 

 extent of surface to the water and driven with the 

 same vigor, the recoil would equal the advance and 

 there would be no progress. Man, with all his skill, 

 does not yet know how to ply his oar so that it shall 

 offer this alternating maximum and minimum of sur- 

 face. Therefore, in propelling a boat, he is obliged 

 to bring the oars back to their first position through 

 the air instead of through the water, which latter 

 would be much more direct. The duck scorns this 

 clumsy method: with its foot, which opens wide of 

 itself in the backward thrust and closes again of its 

 own accord in the return movement, it moves for- 

 ward or puts about, without ever lifting the oars 

 from the water. 



"Thus the duck is an expert swimmer; the shape 

 of its feet tells us as much, and a glance at any duck- 

 pond demonstrates it. Who has not admired the 



