[Illustration: _MAP showing the Locations of the Cases of Cholera adjacent to the Boundary between HAMBURG and ALTONA in the Epidemic of 1892_ Boundary line indicated by line of dashes. Cases of cholera by solid circles. Cases of cholera imported from Hamburg by circles. Water mains in Hamburg streets by black lines.] On the river Elbe, some miles from the sea, there are three cities adjoining and forming in appearance one large city of 800,000 inhabitants, the combined sewage of which is discharged into the river Elbe. The water supply to the city of Hamburg, a free German city, with a population of 640,400, is derived from the Elbe above where the sewage is discharged into the river but not sufficiently far away to escape contamination from a recision of polluted water at flood tide. This water after some imperfect sedimentation passes direct to the consumer without filtration. The supply of water to Wandsbeck, a city of 20,000 population, is obtained from a lake which is unexposed to contamination and is filtered before being delivered to the mains. The supply to Altona, on the other hand, a Prussian city of 143,000 inhabitants, is obtained from the river Elbe at a point about 8 miles below where it receives the combined sewage of the three cities, with their population of over 800,000. It will thus be seen that the source of supply to Altona is the worst of the three. This most grossly polluted supply, however, is filtered with exceeding care before delivery to the consumers, and to this fact is attributed the freedom from cholera that visited Hamburg in 1892. The story is well told by Dr. Thorne, medical officer of the London Local Government Board.[9]