179 
TURBIDITY. 
Records of the turbidity were made (Tables III.-IX.) in gen- 
eral descriptive terms during the first two years of our work at 
Havana. After April 29, 1896, the turbidity was tested by 
means of a white plate of semi-porcelain, 10 cm. square. The 
depth at which this square disappeared from view was recorded 
in centimeters as a measurement of turbidity. Although the 
method is somewhat primitive and subject to some variations 
with the conditions of sky and daylight, it is still sufficiently 
accurate for the purposes of the present paper. The dise 
method has not as yet been correlated with the platinum-wire 
method, the diaphanometer method, or the silica-standard 
method of Whipple and Jackson (’00), and comparisons with 
these are consequently excluded. 
As might be expected in the river environment, when floods 
occur the turbidity is often extreme, and is exceedingly varia- 
ble according to the locality and the river levels. (Cf. Tables 
I1l.-IX.) The extreme range of our records extends from 1.3 
em., in a Spoon River flood, to 260 em., in Quiver Lake, under 
the ice. 
In the river (Table III.) the great majority, about two 
thirds, of the records lie between 20 and 50 cm., while the ex- 
treme range is from 2 em., in the flood of May, 1897, to 115 em., 
in the declining waters of July, 1896. The clearer waters ap- 
pear, as a rule, with declining floods and stable low stages, 
especially under the ice. With the inception of floods the most 
turbid water is found, which gradually clears even while the 
rise continues. The river varies in clearness according to the 
instability of the river levels, as will be seen on comparison 
of the turbidity in 1896 and 1897, the latter year being more 
stable and having relatively fewer records of a marked turbidity. 
The turbidity of the river is due to both plankton and silt, 
the latter being as varied as the character of its tributaries, 
with the added contamination from the cities along its banks. 
In Spoon River (Table IV.) the extremes are even more 
marked than in the main stream, varying from 1.3 cm., in flood 
