368 THE DErniS OF THE SEA. [chap, viii, 



be true, or neither ; but it is logically impossible that 

 both of these can, for the simple reason that the 

 waters of the equator cannot at the same time be 

 both lighter and heavier than the water at the poles." 

 " So long as the two causes continue in action, no 

 current can arise unless the energy of the one cause 

 should happen to exceed that of the other, and even 

 then a current will only exist to the extent by 

 which the strength of the one exceeds that of the 

 other."! 



It seems scarcely- necessary to enter further into 

 detail in reference to Captain Maury's theory of ocean 

 currents, which is really chiefly remarkable for its 

 ambiguity, and for the pleasant popular style in 

 whicli it is advocated; since my friend and col- 

 league Dr. Carpenter has latterly brought into great 

 prominence what appears to be a modification of the 

 same view, put in a more definite form. 



Professor Buff, in his excellent little volume on the 

 Physics of the Earth, speaking of the layer of cold 

 water derived from the Arctic seas which underlies 

 the tropical ocean, and its method of transport, says : 

 " The following well-known experiment clearly illus- 

 trates the manner of the movement. A glass vessel 

 is to be filled with water with which some powder 

 has been mixed, and is then to be heated at bottom. 

 You will soon see, from the motion of the particles 

 of powder, that currents are set up in opposite direc- 

 tions through the water. Warm water rises from the 

 bottom, up through the middle of the vessel, and 

 spreads over the surface ; while the colder, and there- 

 fore heavier liquid, falls down at the sides of the 



' James Croll, op. cit. 



