The area of the watershed of the Elbe above Altona is about 52,000 square miles, and the average rainfall is estimated to be about 28 inches, varying from 24 or less near its mouth to much higher quantities in the mountains far to the south. On this watershed there are 46 cities, which in 1890 had populations of over 20,000 each, and in addition there is a permanent population upon the river-boats estimated at 20,000, making in all 5,894,000 inhabitants, without including either country districts or the numberless cities with less than 20,000 inhabitants each. The sewage from about 1,700,000 of these people is purified before being discharged; and assuming that as many people living in cities smaller than 20,000 are connected with sewers as live in larger places without being so connected, the sewage of over four million people is discharged untreated into the Elbe and its tributaries. The more important of these sources of pollution are the following: City Population On what Approximate in 1890. River. Distance, Miles. Shipping 20,000 Altona 143,353 Elbe 6 Hamburg 570,534 Elbe 7 Wandsbeck 20,586 Elbe 8 Harburg 35,101 Elbe 11 Magdeburg 202,325 Elbe 185 Dresden 276,085 Elbe 354 Berlin and suburbs 1,787,859 Havel 243 Halle 101,401 Saale 272 Leipzig 355,485 Elster 305 Chemnitz 138,955 Mulde 340 Prague 310,483 Moldau 500 The sewage of Berlin and of most of its suburbs is treated before being discharged, and in addition the Havel flows through a series of lakes below the city, allowing better opportunities for natural purification than in the case of any of the other cities. Halle treats less than a tenth of its sewage. Magdeburg will treat its sewage in the course of a few years. Leipzig, Chemnitz, and other places are thinking more or less seriously of purification. The number of bacteria in the raw water at Altona fluctuates with the tide and is extremely variable; numbers of 50,000 and 100,000 are not infrequent, but 10,000 to 40,000 is perhaps about the usual range. The works were originally built by an English company in 1860, and have since been greatly extended. They were bought by the city some years ago. The water is pumped directly from the river to a settling-basin upon a hill 280 feet above the river. From this it flows by gravity through the filters to the slightly lower pure-water reservoir and to the city without further pumping. The filters are open, with nearly vertical masonry walls, as described in Kirkwood’s report. The cross-section of the main underdrain is 1/2800 of the area of the beds.