549 
in 1895 and 1896 reach limits of + 118 per cent. and — 61 per 
cent. respectively. This greater uniformity in total production 
is brought about in part by the compensating effects of lessened 
plankton content in years of high water, and increased con- 
tent in years of low water. 
The mean total discharge is 67,750 cubic meters, or 149,- 
050,000 pounds if we compute its weight as equal to that of the 
same volume of water. This represents the mean annual loss 
of living organic matter from the watershed of the Illinois. 
The total production, including the plankton impounded in the 
backwaters and that utilized there and in the channel as food 
by other organisms, as well as that perishing within the water- 
shed and contributing by its wastes and decay to the growth of 
~ the coarser aquatic vegetation,—this total must indeed be much 
greater than the amount indicated by these imperfect computa- 
tions. If we add to this the undetermined but probably not in- 
considerable volume which leaks through the meshes of the silk 
net and is therefore not at all represented in our computations, 
we reach a total annual production of still greater magnitude. 
TOTAL PRODUCTION AS AFFECTED BY LEAKAGE THROUGH 
THE SILK NET. 
In an earlier paper (97b) I have called attention to the 
extent of leakage through the silk as determined by the enu- 
meration of catches made by silk, tilter-paper, and the Berkefeld 
filter. An attempt was also made later, in connection with 
the enumeration of the organisms of the filter-paper catches, 
to determine the volumetric catch of plankton by this method. 
It was my endeavor to estimate the proportions of silt and 
plankton in these catches, as had previously been done in the 
case of those of the silk net. There is, however, in these filter- 
paper catches a silt of very different character. To that which 
we find in the catches of the silk net there is added a large 
amount of fine loam, clay, and sand which passes directly 
through the silk, and a very considerable quantity of minute 
flocculent particles, presumably bacterial zodglea. This floc- 
