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 disc 

 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 

 III.-IX.) The extreme range of our records extends from 1.3 

 cm., in a Spoon River flood, to 260 cm., 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 cm., in the flood of May, 1897, to 115 cm., 

 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 1.8 cm., in flood 



