Transparency 27 



the water and constitute its normal population. One 

 who has carefully watched almost any of our small 

 northern lakes through the year will have seen that 

 its waters are clearest in February and March, when 

 there is less organic life suspended in them than at 

 other seasons. But it is the suspended inorganic 

 matter that causes the most marked and sudden 

 changes in turbidity the washings of clay and silt 

 from the hills into a stream; the stirring up of mud 

 from the bottom of a shallow lake with high winds. 

 The difference in clearness of a creek at flood and at 

 low water, or of a pond before and after a storm is often 

 very striking. 



Such sudden changes of turbidity occur only in the 

 lesser bodies of water; there is not enough silt in the 

 world to make the oceans turbid. 



The clearness of the water determines the depth 

 at which green plants can flourish in it. Hence it is 

 of great importance, and a number of methods have 

 been devised for measuring both color and turbidity. 

 A simple method that was first used for comparing the 

 clearness of the water at different times and places 

 and one that is, for many purposes, adequate, and one 

 that is still used more widely than any other,* consists 

 in the lowering of a white disc into the water and record- 

 ing the depth at which it disappears from view. The 

 standard disc is 20 cm. in diameterf; it is lowered 

 in a horizontal position during midday light. The 

 depth at which it entirely disappears from view is 

 noted. It is then slowly raised again and the depth 

 at which it reappears is noted. The mean of these 

 two measurements is taken as the depth of its visibility 



*Method of Secchi: for other methods, see Whipple's Microscopy of Drink- 

 ing Water, Chap. V. Steuer's Planktonkunde, Chapter III. 



tWhipple varied it with black quadrants, like a surveyor's level-rod target 

 and viewed it through a water telescope. 



