292 



THE REASON WHY : 



By them there sat the loving pelican, 



Whose young ones, poisoned by the serpent s sting, 



With her own blood to life again cloth bring.&quot; DRAYTON. 



941. Thus in the case of single waves, the middle of the slope is a point of rest, 



on which the sea-bird can 

 sit with little more difficulty 

 than on the calm surface. 

 This will, perhaps, be made 

 plainer by the accompanying 

 diagram, in which two birds 

 are represented as being at 

 rest on the wave : a b is the 

 mean level or calm line of 

 the sea, cutting both the 



black and dotted curve on the points o o. The figure 1 represents the ridge, and 3 

 the hollow, at one end of the vibration ; 4 the ridge, and 2 the hollow, as shown 

 by the dotted line at the other. The bird at b on the turning-point is not moved 

 either up or down ; and as that point is alternately on the windward and the 

 leeward of the wave, the wave keeps the bird from drifting in the first case, and 

 shelters it in the second. 



942. Why has the, pelican a large pouch attached to its 

 lower mandible ? 



The pouch answers nearly 

 the same- purposes that the 

 crop does in birds which 

 possess such an organ. The 

 food is taken into it in much 

 ^ larger quantities than the 

 ^^^ digestive stomach can receive 

 at once ; and is gradually 

 received into the stomach 

 as the process of digestion 

 goes on. 



But the pouch serves 

 another and a remarkable 

 purpose. The pelican, 

 though seeking its food in 

 the sea, builds its nest at 

 gg- a distance from it, gene 

 rally in ruins which have 

 become dry and waste ; and this is the reason why the name 



