202 PHYSICAL GEOGRAPHY OF THE SEA: 



other hand, the more delicate marine productions, whether 

 animal or vegetable, which multiply and prosper by warmth, 

 are redundant in the GKilf-stream even after it has quitted 

 the tropical regions whence its heat is derived. The food is 

 thus matured for the whale-field of the Azores, where this 

 huge denizen of the seas flourishes in colder waters amidst 

 the abundance so provided. 



Our author describes yet other peculiarities of this won- 

 derful current. Its waters are found to be warmest at or 

 near the surface, cooling gradually downwards, so as to 

 render it probable that there is a layer or cushion of cold 

 water between them and the solid earth which lies below. 

 Again, the surface of the stream is shown to be not strictly 

 a plane ; but having its axis or central portion raised some- 

 what higher than the level of the adjoining Atlantic; thus 

 giving it a sort of roof-shaped outline, and causing the 

 surface water to flow off on each side. The existence of such 

 surface current has been proved by boats floated near the 

 centre of the stream, which drift either to the east or west, 

 according to the side of the axis on which they may be. 

 This curious fact has been attributed to the central waters 

 of the current being warmest, and therefore of least specific 

 gravity. It may be so ; but we cannot altogether discard 

 another physical cause ; viz. the enormous lateral compression 

 exercised upon the stream by the ocean waters through which 

 it forces its way; tending to heap it up towards the axial 

 line. Those who have beheld the wonderful spectacle of the 

 Niagara River, four miles below the Falls, - so urged and 

 compressed into a narrow ravine that the middle of the 

 stream rises twelve or thirteen feet above the sides, will be 

 able to conceive this hydrodynamic influence, even on the 

 wide scale of operation which we have now before us. 



There is some evidence that the waters of the Gulf-stream, 

 when emerging from the Caribbean Sea, are salter than those 



