201 
from the population of Chicago, as before stated, because of the 
fact that the drainage of certain districts did not enter the Illi- 
nois River. It is apparent that Chicago, with a population four 
times and a pumpage twenty-five times as great as that of the 
remaining territory, is the principal source of sewage, over- 
shadowing all others by its magnitude. 
The sewage of Chicago during the period of our operations 
was mainly discharged into Chicago River,a tributary of Lake 
Michigan. An area of 50.63 square miles lying within the city 
limits and having in 1897, according to estimates kindly fur- 
nished us by the engineering department of the Sanitary Dis- 
trict,a population of 250,000 to 300,000, drains directly into Lake 
Michigan. The water supply of Chicago is drawn directly from 
the lake,and to decrease its pollution by sewage, pumping works 
were established at Bridgeport which raised the fouled water of 
Chicago River into the Illinois and Michigan Canal, which emp- 
ties into the Illinois River at La Salle. At low-water stages the 
pumpage of Bridgeport prevented the discharge of a considerable 
amount of the sewage into the lake, reversing at times the direc- 
tion of the current in the river. During floods the pumps were 
powerless to prevent the discharge of large amounts of sewage 
into the lake. Under the conditions prevailing during the years 
of our operations a considerable portion of the sewage of Chicago 
thus found its way into the Illinois River. This sewage included 
a large amount of industrial wastes, especially from the Union 
stock-yards and slaughter-houses connected therewith. The 
average daily pumpage of the city water-works in 1597 in 
Chicago was 265,530,910 gallons—an amount 50 per cent. less 
than the pumpage at Bridgeport. The amount discharged into 
the [linois and Michigan Canal thus represents a somewhat di- 
luted sewage as compared with that from other sources. Chemi- 
cal examinations of the canal water indicate (see Palmer, ’97) 
that the maximum period of decomposition of the sewage passed 
before the water entered the river. The location of the crest 
of this wave varied with the temperature, ranging from Lock- 
port to Morris. Bacteriological determinations (see Jordan ’00) 
