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Natural Purification of Polluted Streams.—The growing population of the many cities which discharge their sewage into rivers gives increasing importance to the question how great a degree of pollution is allowable in a stream of given flow, the water of which is to be used lower down for domestic or for manufacturing purposes. Mr. Rudolph Bering says that oxidation and decomposition of sewage matter was for a long time thought to be the main cause for the clarification of polluted rivers. To-day it is known to be but a minor cause compared with dilution and subsidence; and if the sewage is discharged in a fresh condition into a stream of water, its destruction is in part due to fish and other aquatic animals. Some of the refuse from stock-yards is disposed of, no doubt, in this way. Most of the decomposition, or oxidation as it is usually termed, of sewage is effected by the myriads of microscopic plants, microbes, or bacteria contained in both air and water, which at once seize upon the dead organic matter. For purifying the sewage discharged into a river, oxidation can be depended upon only to a limited extent, because of the comparative slowness with which it takes place. Subsidence of the heavier matter tends to clarify it before it flows many miles; dilution with a sufficient quantity of clean water prevents an offensiveness almost at once; but oxidation requires many days under continuous aeration of the river. From a comparison of data in regard to the actual purification of polluted streams we may draw the following inference: Rivers not to be used for water-supplies, but to be inoffensive to communities , residing a few miles below, to remain fit for ordinary manufacturing purposes, and to sustain the life of fish, may receive the sewage from one thousand persons for at least every one hundred and fifty to two hundred cubic feet of minimum flow per minute, supposing that natural subsidence of the heavier matter takes place immediately below the town discharging the sewage. Beyond the above limit it appears to be advisable to resort to land or other filtration, or to chemical precipitation. But the whole subject needs further investigation.
The Nebraska City Pontoon Bridge.—Colonel S. N. Stewart, of Philadelphia, has recently built a pontoon bridge for ordinary traffic across the Missouri River, at Nebraska City. It is 1,074 feet long, 241 feet wide, and consists of a flooring laid upon boats which float upon the river and are securely anchored. The city has held a franchise for such a bridge for twelve years, but the project has been opposed by persons interested in steamboats plying on the river. Many predicted that the attempt would fail, for the Missouri River has a swift current, which here attains about its highest velocity, and large numbers of logs and trees are constantly drifting in the stream. These, however, are carried under the floats without doing any damage. In the channel of the river the bridge makes a V, pointing down stream, which is the draw. To open the draw, the connections at the point of the V are cast