MICHIGAN ACADEMY OP SCIENCE. 
ti 5 
In addition to the danger of contamination of lakes from vessels, 
they receive the surface wash from the land tributary to them and in 
numerous instances are also the receptacles for the sewage of cities, 
the refuse of factories, etc. In these several ways they are liable to 
become contaminated, the danger being most imminent in the vicinity 
of the localities where sewage is discharged into them and where ves¬ 
sels pass in large numbers. To be sure, the great volume of water 
present serves to dilute the contributions of sewage, and freedom of 
exposure to the air promotes oxidation, thus diminishing the danger 
to persons who drink the water, but as is well known, the water of 
even the Great Lakes has in several instances served to disseminate cer¬ 
tain diseases. An important investigation in this connection, which so 
far as I have been able to learn lias not been made, is in reference to the 
life histories arid rate of increase or diminution of diseases producing 
germs after entering an open water body such as a large lake. 
While the waters of lakes at a distance from the mouths of sewers 
and away from the tracks frequented by vessels, are among the very 
best of natural water supplies, Lake Superior, for example, being prob¬ 
ably the purest, both chemically and bacteriologically, of any consider¬ 
able water body on the earth, yet it is self evident that a city cannot 
discharge its sewage into a lake and take its water supply from it at 
a nearby locality or from the vicinity of the routes frequented by ves¬ 
sels, without serious danger. While the water supply of an individual 
city, whether taken from a lake or from some oilier source, presents 
concrete problems for study and should be specially considered, it is 
safe to say as a general proposition, that even if the greatest care is 
taken in locating intakes with reference to the places where sewers dis¬ 
charge and with regard also to the routes followed by vessels, filtration 
is still desirable. A common sense reason for establishing such a safe¬ 
guard, is that it is possible to exercise continuous watchfulness when a 
filtration plant is used, while it is impracticable to predict when con¬ 
tamination will occur in case such a precaution is not taken. 
In connection with the use of the water of streams and lakes for 
municipal purposes, the liability of the supply becoming turbid or even 
muddy during heavy rains and in the case of lakes, on account of the 
action of shore currents generated by strong winds, have also to be duly 
considered. To counteract such undesirable conditions, settling basins 
for the water used should be provided, or filtration resorted to. In cer¬ 
tain instances, as is well known, both of these methods have to be 
employed. 
CONCLUSIONS. 
Even a brief review, such as is here attempted, of the ideals which 
municipalities should strive to attain, renders certain conclusions self 
evident. These, as it appears, may be stated as axioms from which there 
is no logical appeal. They are: 
First. When water derived from a dee]) source in the earth is utilized 
for domestic purposes, every possible precaution should be taken to 
prevent its becoming contaminated by disease-producing germs, owing 
to exposure to the air. 
Second. Every care should be taken to prevent the contamination 
