444 



NATURE 



[February i i, 1909 



an accouiit of the diurnal variation of pressure, which is 

 so important in the tropics. The only reference to this 

 phenomenon is contained, incidentally, in an article on 

 rules for handling the ship in hurricanes, given on the 

 back of the chart for December. 



The remaining space on the backs of the charts is utilised 

 to the full. On several of them detailed information is 

 given of the systems of storm signals used in the area 

 covered by the charts. Others give particulars of the time- 

 signal stations. Numerous fully illustrated articles give 

 particulars of meteorological events of special interest, 

 such as the Hong Kong typhoon of September i8, 1906, 

 and other famous hurricanes. In addition, we have a 

 number of monographs on all manner of subjects of interest 

 to the sailor and the meteorologist. Among them we 

 mention specially one on the prevalence of easterly winds 

 to the south of latitude 50° S. 



We congratulate the .Seewarte on the completion of so 

 important and arduous a piece of work, which is sure to 

 prove of the utmost value both to sailors and students. 



THE FILTRATION AND PURIFICATION OF 



WATER FOR PUBLIC SUPPLY.' 

 ^^REAT progress has been made in recent times in the 

 ^-^ appliances for purifying water. It is no longer 

 necessary to go to distant uplands for a pure and palatable 

 supply. By the methods of treating ordinary river water, 

 carrying possibly hundreds of objectionable germs per c.c, 

 drinking water is now being prepared from the lower 

 reaches of the Thames and of many Continental rivers as 

 wholesome as can be obtained from the mountains of 

 Wales or of Scotland. So great has been the activity of 

 scientific workers in this field that a new and complex 

 branch of technology may be said to have come into 

 existence. 



With reference to sources of supply, water companies 

 should not place too much reliance on the innocuousness 

 of supplies drawn from country districts. Water-courses 

 and reservoirs should be protected from the intrusion of 

 harmful matters, and the adjacent ground should be fenced 

 off and planted. Special precautions are needful for pre- 

 venting the ingress of impurities to wells and bore-holes, 

 and where pollution occurs the origin of the same may be 

 detected by suitable experiments. Storage reservoirs are 

 a useful adjunct to a purifying plant, even when not re- 

 quired for conserving the supply, and it has been proved 

 by the researches of Dr. Houston that the bacteria of 

 enteric practically all disappear from impounded water in 

 two or three days. Still, as it appears that even here the 

 survival of the fittest holds good, and that a few germs 

 live on for w-eeks, water undertakers are not relieved from 

 the duty of further treating the supplv. Sedimentation 

 proceeds more or less rapidly in stagnant reservoirs, but it 

 has been found at the Paris installations that effective 

 precipitation can be secured by running the water in 

 channels, with frequent changes of direction. Thus space 

 is economised. 



Discussing the retention of bacteria in filter beds, the 

 lecturer directed particular attention to the functions of 

 the filtering skin. It appears that in the finishing filter 

 at Bedford, which is fed by a sprinkler, no skin is formed 

 at the surface, because the water does not rest there. It 

 sinks at once into the sand, and at a depth of about an 

 inch and a half a slimy growth is easily perceptible on 

 the grains, and this possibly serves the same purpose as 

 the network of algoid grovvths bedecking the open sand 

 beds. There are five distinct ways in which the sand bed 

 operates in eliminating impurities, but what is most 

 important in the operation of these beds is the circum- 

 stance that, after cleaning, a considerable time must elapse 

 before the purifying agencies come into effective action. 

 Water managers should have the means of finding out 

 when the effluent is pure, and in order to do this thev 

 must rely on bacteriological analyses. This is the method 

 adopted on the Continent. Unfortunately, it is generally 

 neglected here, and it is a matter of chance in too many 



I Abstract of a naper by Mr. John Don selected by Ihc Council of the 

 Institution of Mechanical Engineers for the first award of Ihe "Water 

 .Arbitration Prize," 190S, and read before the Institution on January 15, 



XO. 2050, VOL. 79] 



cases whether there may or may not be dangerous germs 

 passing through. Chemical analyses alone cannot reveal 

 whether the filtrate is wholesome or not. The amount of 

 nitrogen present as nitrate and nitrite is important enough, 

 but analysts should not rely on this as the chief criterion 

 for determining the purity of a sample. 





Flc. I.— Compressed Air and Oxidising Waterworks Filter (Candy). 



Recently many mechanical appliances have been brought 

 into use for the purification of water, and among these 

 the Jewell filter is largely used in America. A precipitate 

 of sulphate of alumina forms an efficient skin within a 

 short time after cleaning, and thus there is a great saving 



c Ozonlser. 



a Raw water inlet. 



^ Purine'l water outlet. 



d Chariiher to catch ut 



k Chamber for waste gasea 



1: BalHc plates 



J. 2.— Sketch of Ozonising .\pparatus (Howard-Bridge) Scale about .,'. 

 Action of the Apparatus : — As the raw water passes down the pipe a 

 draws the unabsorbed ozone by way of the tube/ from the chamber 1] 

 Freshly ozonised air is also d ■"*" *^- ""' *"""" *'"^ "^^ 



iir IS also drawn into the current from the ' 



igh the pipe «. After traversing the vertical pipes, the water is 



-J . ,._:.5 of bafRe plates i, and finally flowing under 



of time. The water also passes through the filtering 

 layers forty times more quickly than it does in the open 

 sand filter, but the effluent, subjected to every test, proves 

 to be of a high degree of purity. In Britain, Mather and 

 Piatt's and Bell's filters are of similar construction, and 



