112 THE OCEAN 



(2) The existence of temperature oscillations 

 in the ocean and in lakes has lately attracted 

 much attention. In fresh- water lakes in 

 autumn there forms a Sprungschicht, or 

 " discontinuity layer," through which the 

 rate of fall of temperature is much greater 

 than at other depths. The lake then behaves 

 as if it consisted of two layers of liquid of 

 different density which do not mix ; the 

 difference in density is produced by the 

 difference in temperature between the surface 

 and the bottom waters. There is also a 

 marked difference in the viscosity of the 

 two layers. The effect of wind blowing 

 along the surface of a lake when the tempera- 

 ture distribution is of this nature is to accumu- 

 late the warm surface water at the lee end of 

 the lake. The discontinuity layer is deeper 

 at the lee than at the windward end, but when 

 the wind ceases the discontinuity layer swings 

 back through the normal horizontal position, 

 and oscillations of the lower layer of cold 

 water commence. No motion is observable 

 at the surface of the lake, but the oscillations 

 of the discontinuity layer are often of great 

 amplitude. These oscillations have a node 

 at the centre of the lake, but binodal or 

 plurinodal oscillations are also possible. The 

 period of these oscillations or temperature 

 seiches is determined by two things : 



