MICHIGAN ACADEMY OF SCIENCE. 
121 
sidered as unreasonable and subject to regulation and prohibitive legis¬ 
lation. Filtration systems of water supply are provisions for public 
safety. When properly constructed and maintained they are absolute 
safeguards against the infectious pollution in a slightly polluted water. 
With the increase of pollution in the water, the safety provided be¬ 
comes more and more precarious. In other words, a filter provides a 
wide margin of safety in the case of a slightly polluted water, but a 
narrow, precarious, and expensive margin in the case of a grossly pol¬ 
luted water. No matter how faithful or how expert the attendants in 
charge of a filtration system may be, there will arise occasions when 
the effluent of that system will not be purified as completely as the 
system of purification would warrant. If the water supply is only 
slightly polluted the danger is not great, but if, on the other hand, it 
is a dangerously polluted water, then the occasional lapses may and 
will allow infection to enter the distribution system with serious re¬ 
sults. 
The above statements indicate with a fair degree of precision the line 
which may be drawn between permissible or inevitable pollution and 
unreasonable or gross pollution. In the former case, we are helpless, 
but in the latter, we should be all powerful. 
There are a few observations which may profitably be made concern¬ 
ing the efforts exerted by many authorities, and the legislation prepared 
and presented by them to prevent stream pollution. Many of the laws 
which legislatures are requested to enact are altogether unreasonable 
because they seek to secure that which it is not within the power of 
man to attain. If those interested in the pollution of streams would 
take the middle course or the reasonable course, and recognize the 
limitations by which they must be guided, they wmuld secure immediate 
recognition by legislative bodies, and the commendation of society at 
large. A statute that recognizes these limitations, that seeks to secure 
reforms gradually and^ expediently, and that tactfully takes account 
of the fact that long imbued municipal and industrial habits are not to 
be effaced by the sounding of a trumpet, is the only one that promises 
success. 
United States Geological Survey, Washington, D. C. 
16 
