On Stocking. 61 



to active, and currents of varying temperatures 

 result, bearing the finer particles of sediment 

 through the whole pond and producing the milky 

 appearance ; and this continues morning frost after 

 morning frost, until the bottom layer attains the 

 temperature of the greatest density, viz., 39 Fahr. 

 after which no action is apparent, or, if any, the 

 ponds appear clearer after a frosty night than be- 

 fore. 



In this suggestion I have assumed that no per- 

 ceptible effect takes place except after the first 

 sharp frosts that is, only when the surface layer 

 is so rapidly cooled that no great action takes 

 place in passing 39 Fahr. The temperature of 

 the greatest density would occur in the case of a 

 gradual cooling, when the bottom water would be 

 reduced to this temperature without any distri- 

 bution, and the colder but lighter water could not 

 pass through to stir the sediment. 



I also assume that the top layer of water would 

 descend in semi-vertical currents through the in- 

 termediate layers. 



The position of feeding trout in lakes, some- 

 times in mid- water and sometimes at the surface, 

 has possibly much to do with temperature currents, 

 more so than with the average temperature of the 

 layers of water through which, I suppose, these 

 currents to pass. With the variation that sea- 

 water attains its greatest density at a temperature 



