66 were considerably increased by mist, rain and low visibility, and the consequent absence of aerial cooperation." The Italian naval exploit at Pola, which resulted in the destruction of an Austrian dreadnaught, was favored by a very dark night, and an offshore wind, which prevented the sounds of preparation from being carried landward. The war in the air is being carried on with steadily increasing intensity. Aviators are flying in weather conditions-rain and snow storms; gales and mists-which were only very lately regarded as prohibitive. As aerial warfare continues on the western front, the disadvantage under which the Allied flyers labor because of the prevailing westerly winds are receiving more and more emphasis. As a well-known aviator has recently expressed it, "if an airman ever wishes for a favorable wind it is when he is breaking for home. . . . These westerly gales were one of the worst things we had to contend with at the Front. They made it very easy for us to dash into enemy territory, but it was a very different story when we started for home and had to combat the tempest." In connection with general air raids, several points are worth noting. On March 11 nine squadrons of German airplanes attacked Paris during a fog, which "was thick enough to cause the general belief that there was little chance that the Germans would attempt an air raid." It may very likely have been for this reason that these weather conditions were selected. A German raid on Hull and its vicinity on March 13 was also "completely unexpected. The night was dark, and a slight drizzle was falling." This raid, and others, have shown that the German aviators no longer depend on moonlight. Early in March, the Germans made their first night air-raid on London when there was no moon. The stars were out, however, and there was little wind. On May 19 another raid was made on a very clear night, when the moon was shining. On April 12 a German air raid on Paris was made on a "still, dark night, of the sort most favorable for an aerial attack, and a raid was generally expected." And on May 21, during another raid, the night was clear and calm, with a brilliant moon, "ideal for an aerial attack." In connection with the work of the German army meteorological service, it has, since the beginning of the war, been a matter of some interest to know how the enemy obtains the observations, especially from the western coast of Ireland, which are very necessary in constructing weather maps and in making forecasts. Captured documents show that their meteorological reports are fairly complete, despite the fact that no pub lication of weather data or forecasts is permitted in English newspapers. An English meteorological expert declares that the answer to the question is not through any system of spies and land wireless, but that the data are obtained from observations taken by submarines. He thinks that a submarine working off the western Irish coast is detailed to send weather reports to Germany by relays through the wireless apparatus working around the British Isles. In the African war zone, where so many political changes have taken place but from which so little direct information has come, the spring months have witnessed an advance of the allied troops on the remnants of the German forces which escaped from German East Africa to Portuguese East Africa. An official despatch dated London, April 11, says: In Portuguese Nyassaland, despite the difficulties caused by heavy rains and flooded rivers, our columns from the coast and from Lake Nyassa are approaching Medo and Msalu, respectively, and their advanced troops are in contact with those of the main enemy forces concentrated in these localities. A later report (April 27) from the British War Office stated: Since April 17 the convergent advance of General Northey and General Edwards's troops has proceeded under better weather conditions. The main enemy force is in the vicinity of Namungo. British and Portuguese troops are moving in the direction of Msalu River, while further south other British and Portuguese columns have been disposed north and south of the River Lurio. VOL. VII.-3. PLANT AND ANIMAL LIFE IN THE PURIFICATION OF A POLLUTED STREAM By C. ELSMERE TURNER, M.A., C.P.H. INSTRUCTOR IN THE DEPARTMENT OF BIOLOGY AND PUBLIC HEALTH, MASSACHUSETTS INSTITUTE OF TECHNOLOGY RIVE IVERS and other streams have always been resorted to by man for the disposal of his wastes. And yet unless greatly overtaxed they remain fairly clean. The problem here raised reminds one of that wonder of the ancients that all the rivers run into the sea and yet the sea is not full. What becomes of all the dirt and the street-wash and the sewage that find their way into our streams? The "self-purification" of streams is an old and captivating phrase graphically describing a process which is patent even to a superficial observer. On the Merrimac, for example, Concord, Manchester and Nashua, important cities of New Hampshire, poured all their sewage into the noble river flowing past their doors, while Lowell, Mass., only sixteen miles below Nashua, did not hesitate to drink the water now again clear and bright, which reached the intake pipe of its city water works. More wonderful still, Lawrence, only nine miles below Lowell, drank directly from the same stream after the sewage of the 80,000 inhabitants of that city had been added to it. Alternating pollution and purification is the common characteristic of streams. The mechanism of pollution is obvious, but how about the process of purification? It was to throw further light if possible upon the self-cleansing of polluted streams, that the Sanitary Research Laboratory of the Massachusetts Institute of Technology carried on for two years an investigation of a small stream polluted by a relatively large quantity of partially purified sewage effluent which reaches it from slow sand filters. Whence comes the water of a normal brook or river? The answer is, partly from the atmosphere as rain or snow, partly from the earth's surface through tributary rills or brooklets, but largely from the ground upon which it has fallen, through which it is filtered, and from which it arrives comparatively pure from all but the smallest suspended matters. This ground water, however, does contain dissolved gases and salts together |