CIRCULATION AND TEMPERATURE 61 
We have been able to find no record of the amount of lowering of 
the waters of Lake Michigan at a given point, by the wind, nor any 
discussion of the relations of the surface currents to the effects of winds 
and the vertical circulation. The waves of large lakes rise to consider- 
able heights, as is familiar to all. They are of much importance in 
keeping a large amount of gas in solution in the lake waters. 
The current in streams differs from that in lakes in that it is for the 
most part in one definite direction, while the lake currents often alternate. 
There are backward flows and eddies at various points in streams, in 
front of and behind every object encountered in the current (57, p. 124). 
On the basis of the current, streams are classified as intermittent, swift, 
Fic. to.—Showing the circulation of the water in a lake of equal temperature. 
W represents the direction of the wind (after Birge). 
Fic. 11.—The circulation of the waters of a lake of unequal temperature (after 
Birge). 
moderately swift, ‘sluggish, and stagnant or ponded. The current 
within the same stream differs at different times, and in different places. 
As we pass across a stream we find the current swiftest near the surface 
in the middle, and least swift at the bottom near the sides. 
2. TEMPERATURE 
Temperature has always been regarded as of great importance in 
the direct control of the distribution of life in water. The tendency of 
modern investigation is to show that its influence is of great indirect 
importance, and the belief in its direct importance is correspondingly 
weakened. 
The temperature in a stream is probably about the same at the 
various points in any cross-section. The extent to which daily, seasonal, 
and weather fluctuations in atmospheric temperature affect a lake is 
