400 NATATORES. 



form a very considerable part of their food, and at certain 

 seasons they follow these to some distance seaward. When 

 on the shores, they are not only inaccessible upon the wing, 

 but screaming and clamouring about without intermission, 

 and that too in more loud and ear-piercing strains than, 

 from their size, one would be apt to anticipate. It is diffi- 

 cult to ascertain whether their most shrilly and ear-piercing 

 cry is a song of love or of sorrow ; for they have it equally 

 in the breeding season, and during those long tracts of calm 

 and stilly weather, when it may be presumed that the water 

 furnishes them with only a diminished supply of food. At 

 those times they alternately skim close to the surface, with 

 level wing, and wheel upward, and pounce down again, 

 leaving the water merely momentarily, and either catching 

 nothing or catching something very small, for their wailing 

 is not interrupted. 



Still, though the terns are thus fleet and flitting over the 

 waters, and seem to have the air all their own, they are not 

 the birds that range farthest to seaward. The substances 

 upon which they feed, all appear to be either inhabitants 

 within the lines of sounding, or, if they are caught more to 

 sea by the waves, to be in motion towards the shore. All 

 that can float, and is capable of being wetted by the sea, 

 which is the case with all animals in a state of exhaustion 

 or dead, the sea casts ashore ; and such products of the 

 farther and greater deep, as become terns' food, appear to 

 be in that state. But there is an animal substance which 

 is produced, or, if the term suit better, shed largely at sea, 

 which cannot be whetted even by the ocean, and which, 

 therefore, upon a well-known principle in hydrodynamics, 

 has no tendency to come on shore ; the more so, that it is 

 not acted on by the wind, which often drifts matters to 

 the land, in spite of their own repulsions. This substance 

 is oil, which is produced abundantly by various animals, 



