240 MARINE BIOLOGICAL LABORATORY. 



warm water is simply an exchange brought about within 

 narrow Umits, and involves no other motion than that of 

 conduction in a fluid. Besides, later observations by 

 Forschhammer and Buchanan have shown us that these 

 variations are not as important as they were supposed 



to be. 



About the same time V. Schilling, a Russian, advanced 

 a tidal wave theory. It is interesting only from the 

 fact that it is exactly the reverse of the theory of Vos- 

 sius. As the tidal wave passes around the equator, it 

 produces a flood tide at that point, and consequently an 

 ebb tide must be formed at the higher latitudes. Exist- 

 ing facts and the theory cannot be harmonized without 

 straining both, and so it must be rejected. 



There are two modern schools of theorists with regard 

 to the causes of ocean currents. 



I. Those who advocate a thermic circulation in expla- 

 nation. This idea is defended by Schmid, Mubry, Car- 

 penter, and Krummel. It is believed by this school that 

 the differences in temperature between the tropics and 

 the poles are sufficient to account for the currents. 

 This belief is based largely upon an experiment of 

 Carpenter's, in which the water at one end of a trough 

 was cooled, whereas that at the other end was warmed. 

 Small objects, such as pigment granules which floated 

 and colored the water as they advanced, show a motion 

 of a character tending to support this view. Mubry 

 adds to this the idea that the motion is aided by the 

 rotation of the earth. 



The difficulties in the way of this theory are : first, 

 that centrifugal motion does not do what it is supposed 

 to, i.e. start a rotary current. I have already shown its 



