176 PHYSIOGRAPHY. [chap. 



That the Gulf Stream is an extremely shallow body of 

 water is well seen in Fig. 45, which is reduced from one of 

 Sir G. Nares' reports to the Admiralty on the Challenger 

 expedition. It represents a section of the North Atlantic, 

 between New York and Bermuda ; and it shows in a very 

 striking manner, that, when compared with the great depth 

 of the ocean, the Gulf Stream is extremely superficial. It 

 may indeed be regarded as a mere rill of warm water 

 running over the surface of the sea;, for, while the water 

 below is considerably more than 2,000 fathoms in depth, 

 the Gulf Stream itself is not more than 100 fathoms deep. 

 It is seen, too, that, while the Gulf Stream has here a tem- 

 perature of 75°, the bottom water has as low a temperature 

 ^s 35°3 r. Incidentally, the diagram in Fig. 45 serves to 

 show the character of the sea-bottom along the line of 

 section ; it shows, for example, that the island of Bermuda 

 rises as an isolated peak from water of great depth.^ 



After what has been explained in Chapter IV. respect- 

 ing the effect of heat in altering the bulk of bodies, it 

 will be understood that a body of warm water, like that 

 of the Gulf Stream, can easily float upon water which is 

 colder and therefore denser. When a mass of water is 

 unequally heated, by raising its temperature below, or by 

 lowering it above, currents are at once established ; and, 

 if light matter, such as sawdust, be suspended in the liquid, 

 the direction of these currents becomes very evident. 

 Thus in Fig. 46, where heat is applied at the bottom of 

 a vessel, the liquid becomes specifically lighter and therefore 

 rises, whilst the surrounding colder watw being denser, runs 

 down in streams to supply the place of that which has 

 ascended to the surface. This is, in fact, the ordinary way 



^ The caution which has already been given respecting the exaggera- 

 tion of the vertical height in the diagrams, must not be forgotten. 



