—Chicago is supplied with water from Lake Michigan. It is taken two miles from shore and conducted to the city in tunnels. When unpolluted by sewerage the lake water is remarkably pure and pleasant. The sewerage of the city is, however, emptied into it through the Chicago river. In the spring, when the river is full, there is sufficient current to carry the sewerage far out into the lake, and when the wind blows from the south, the current is blown directly across the source of water supply. Except for a little time in the spring the river is so sluggish that the water would be motionless unless artificially moved. Some years ago it was found necessary to create an artificial current. In order to do this, two pumping works were established, one on each branch of the river. The river is somewhat Y shaped, the base of Y being