422 



NATATORES. 



abundantly to the surface, disperses that of the storm petrels. 

 Hence, they are rare upon the narrow seas, and do not come 

 much within the breakers. When the sea is in a state of 

 agitation, its surface is proportionally increased ; so that, if 

 all the ridges, and hollows, and flexures, and dimples, are 

 taken into account, it is no exaggeration to say that, even in 

 a moderately severe gale, it is more than doubled. That far- . 

 ther increases the friction of the air, so that the trouble of 

 the sea, like most other troubles, contains in itself some of 

 the elements of its own increase, and the waves rise more 

 rapidly in proportion than the wind freshens. The oil is 

 thus dispersed over a large surface, and as, different from 

 those small substances which are in the water, it is thrown 

 to the ridge and the hollow, and away from the line on which 

 the wave turns in its vibrations, it is rendered much less 

 acquirable by the birds. 



We can thus readily see that they must flit before the gale 

 which agitates the water sooner than other surface birds ; 

 and as they fly considerably faster than the wind does in 

 ordinary storms, they keep before it, and court both the 

 shelter of the ship, and the greater abundance of food which 

 is to be found in its wake. 



Storm petrels are very abundant birds : but though in the 

 breeding season they throng in vast multitudes to their 

 favourite nesting places, they inhabit the sea rather dis- 

 persedly during that larger portion of the year when they are 

 Pelagic. In consequence of that, and also of the very few 

 points of the sea which are seen at once, even by all the 

 vessels which are afloat, they not only appear much less 

 numerous than they are in reality, but much less so than 

 many of the shore birds which they thousands of times out- 

 number 



Looking at the petrels as a group, (and the species differ 

 from each other little in habit, and not much in appear- 



