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1904] WELD: BOTANICAL SURVEY OF HURON VALLEY 49 
physical differences, among which the following are to be 
specially considered : 
1. Color. The water of the second lake is clear, or would be 
if the blue-green algae were filtered out, while that of the first is 
highly colored, a yellowish-brown, due to the great amount of 
decomposed vegetable substances about the lake. This latter 
fact is the greatest difference between the two lakes, perhaps the 
cause of all others, but there is as yet no wholly satisfactory 
explanation of the antecedent fact that something like 95 per 
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8 
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& 
Fic. 6.—Section across lake basin from E. to W. at ditch. Scale as in fig. 5. 
cent. of the work of filling one basin has already been done, 
while the process is but fairly started in the other. The greater 
depth of the second may have something to do with this. The 
ice in the first lake is clear as crystal. Freezing in winter and 
evaporation in summer serves to concentrate the amount of 
dissolved salts in the water and thus stil) further deepen the 
color. : 
2. Turbidity. Absorption of light, as is well known, is much 
greater in colored than in clear water, and it is also occasioned 
by turbidity due to the presence of solid matter in suspension. 
Turbidity is most conveniently measured in the field by the wire 
method of Hazen, which consists in observing the depth at 
which a platinum wire 1™™ in diameter disappears when lowered 
horizontally into the water. The scale is furnished by the recip- 
