LIGHT AND PRESSURE 63 



snows and the beginning of the rainy season. The drainage area of Lake 

 Michigan is very small and has little relief, and the amount of sediment 

 carried in is small at all times. The depth of light penetration is there- 

 fore not so much influenced by these factors as in Lake Geneva. Wave- 

 action is also important in stirring the bottom materials near shore. 

 We would expect the light penetration in Lake Michigan to be least 

 during the rainy and windy seasons, and greatest in calm, dry weather 

 late summer and autumn. 1 All of the surrounding physiographic con- 

 ditions are factors controlling light. Table IV shows the seasonal 

 distribution of rainfall and light penetration in Lake Geneva, and the 

 seasonal distribution of winds and rainfall at Chicago. 



TABLE IV 



SHOWING DEPTH OF LIGHT PENETRATION IN LAKE GENEVA AND CONDITIONS AFFECT- 

 ING THE SAME IN BOTH LAKE GENEVA, AFTER FOREL (76, Vol. II, 



p. 439), AND LAKE MICHIGAN 



In the eighth column the results are given in seconds, in terms of the effect on the 

 photographic plate, of equivalent exposures to the sun. 



4. PRESSURE (76) 



Pressure in water increases with depth. The results given by Forel 

 are shown in Table V. 



'The Lake Michigan Water Commission has reported greatest turbidity in 

 January, February, March, and April. 



