April 30, 1896] 



NATURE 



621 



pumped up to the surface, it usually contains a small number 

 of microbes. Thus during the year 1892 it contained, on the 

 average, 6 per c.c. ; in 1893, 13 ; in 1894, 15 ; and in 1895, 8. 



Thus, although the deep-well water has, from a bacterial 

 point of view, a decided advantage, the filtered river waters are 

 not very fixr behind ; and there is every reason to believe that, 

 with the improvements which are now being carried out by the 

 various river water companies, the Kent Company's water will, 

 l)efore long, be run very hard by the other supplies. 



By the examination of the water as it issues from the filters, 

 the utmost freedom from microbes, or maximum degree of 

 sterility of each sample, is determined. This utmost freedom 

 from bacterial life, after all sources of contamination have been 

 passed, is obviously the most important moment in the history 

 of the water ; for, the smaller the number of microbes found in 

 a given volume at that moment, the less is the probability of 

 jiathogenic organisms being present ; and, although the non- 

 pathogenic may afterwards multiply indefinitely, this is of no 

 consequence in the primary absence of the pathogenic ; but, it is 

 only fair in describing che character of the present water supply 

 of London to say that not a single pathogenic organism has ever 

 been discovered even in the unfiltered water as it enters the 

 intakes of the various companies, although these organisms have 

 been diligently sought for. It is sometimes said that the non- 

 pathogenic organisms found in water may be beneficial to 

 man ; but this idea is not borne out by their entire absence from 

 the food which nature provides for young animals. Milk, if 

 healthy, is absolutely sterile. 



As it is at present impracticable to obtain water, uniformly at 

 least, free from microbes, it is desirable to adopt some standard 

 of bacterial purity, and 100 microbes per c.c. has been fixed 

 upon, by Dr. Koch and myself, as the maximum number allow- 

 able in potable water. This standard is very rarely infringed by 

 the London water companies, whilst I have every reason to 

 hope that, in the near future, now that special attention is 

 directed to bacterial filtration, it will not be approached within 

 50 per cent. This hope is based not only upon my own observa- 

 tions, but also upon the exhaustive and important investigations 

 carried out at the Lawrence Experiment Station by the State 

 Board of Health of Massachusetts, under the direction of Mr. 

 George W. Fuller, the official biologist to the Board. More 

 than six years have already been spent in the prosecution of 

 these American experiments, and many thousands of samples of 

 water have been submitted to bacterial cultivation. 



These important experiments, and my own observations on 

 the London waters continued for four years, lead to the following 

 conclusions : — 



(i) The rate of filtration between half a million and three 

 million gallons per acre per day exercises, practically, no effect 

 on the bacterial purity of the filtered water. It is worthy of 

 note that the rates of filtration practised by the several water 

 companies drawing their supplies from the Thames and Lea are 

 as follows: — Chelsea Company, 1,830,000; West Middlesex, 

 1,359,072; Southwark Company, 1,568,160; Grand Junction 

 Company, 1,986,336; Lambeth Company, 1,477,688; New 

 River, 1,881,792 ; and East London, 1,393,920. Hence, not one 

 of the London companies filters at the rate of two million 

 gallons per acre per day ; at which rate in the Massachusetts 

 filters 99 '9 per cent, of the microbes present in the raw water 

 were removed. 



(2) The effect of the size of the sand-grains used in the filters 

 is very considerable. Thus, by the use of a finer sand than that 

 employed by the Chelsea Company, the West Middlesex 

 Company is able, with much less storage, to attain an equal 

 degree of bacterial efficiency. 



(3) The depth of sand, between the limits of i and 5 

 feet, exercises no practical effect upon bacterial purity, when the 

 rate of filtration is kept within the limits just specified. Thus 

 the New River Company, with i*8 feet of sand on their filters, 

 compares favourably with the Chelsea Company, the sand on 

 whose filters is more than twice that depth. Placed in the order 

 (if thickness of sand on their filters, the Metropolitan companies 

 range as follows :— Chelsea, Lambeth, West Middlesex, South- 

 wark, East London, Grand Junction, and the New River. 

 Placed in the order of efficient bacterial filtration, they range 

 as follows : — Chelsea and West Middlesex (equal), New River, 

 Lambeth, East London, Southwark, and Grand Junction. 



(41 When there is such an accumulation of deposit on the 

 •■urface of a sand-filter that, fjr practical purposes, sufficient 



NO. 1383, VOL. 53] 



water cannot be made to pass through it, the surface of thejfilter 

 has to be scraped ; that is to say, mud and about half an inch of 

 the sand are removed from the surface. After this operation, 

 there is often an increase in the number of bacteria in the filtered 

 water, and it has been noticed that the increase is greater in 

 shallow than in deep filters, and with high than with low rates 

 of filtration ; and there is no doubt that the effect of scraping 

 is considerably magnified when the coarser descriptions of sand 

 are employed, as is the case in the filters of the London water 

 companies. I should therefore like to impress upon the 

 engineers of these companies the desirability of using finer sands 

 than are at present employed. 



The lecturer here described a long series of experiments 

 proving that the temperature of the water and the presence or 

 absence of sunshine has little or no effect upon the number of 

 microbes in river water, whilst the presence of flood water is 

 almost invariably accompanied by an enormous increase in the 

 number of microbes, showing that the microbial population of a 

 river is directly dependent upon the volume of water flowing in 

 its bed. 



The Water Supply of the Future. 



In view of the rapid increase of the population of London, 

 fears have from time to time been entertained that the water 

 supply from the Thames basin— that is to .say, from the rivers 

 Thames and Lea, supplemented by water from springs and deep 

 wells within the basin itself — would soon be insufficient in 

 quantity, whilst the quality of the water taken from the river has, 

 up to comparatively recent date, been considered unsatisfactory. 

 On these grounds various schemes have from time to time been 

 brought forward for the supply of the metropolis from other 

 river basins— from the Wye, the Severn, the river basins of 

 North Wales, and of the lake districts of Cumberland and 

 Westmoreland. It is worthy of note, however, that all the 

 Royal Commissions have arrived unanimously at the conclusion 

 that the quantity of water obtainable from the Thames basin is 

 so ample as to render the necessity of going elsewhere a very 

 remote contingency. 



I shall now endeavour to put, very shortly, before you the 

 facts which, in my opinion, prove that both as regards quantity 

 and quality the Thames basin will, for a very long time to come, 

 afford an abundant supply for the metropolis. There is, indeed, 

 no river basin in Great Britain which affords such an abundant 

 supply of excellent water as that available in the Thames basin. 

 Besides that which flows directly into the riyers, this water is 

 contained in the Chalk, Oolite, and Lower Greensand, which 

 are the best water-bearing strata in the kingdom. From these 

 strata it issues in copious springs of unsurpassed organic purity. 

 P'or dietetic purposes there is no better water in the kingdom 

 than the underground water of the Thames basin. For senti- 

 mental reasons, I should like to see it conveyed to the works of 

 the various companies in special conduits ; but we have seen 

 that, on hygienic grounds, it may safely be allowed to flow down 

 the bed of the Thames, if it be afterwards efficiently filtered. 



So much for quality, now as to quantity. The basins of the 

 Thames and Lea include an area of upwards of five thousand 

 square miles. Of this, more than one-half, including the Oolitic, 

 Cretaceous, and portions of the Tertiary formations, is covered 

 by a porous soil upon a permeable water-bearing stratum. The 

 remainder is occupied by the Oxford, Kimmeridge, Gault, and 

 London clay ; being thus covered by a clay soil upon a stiff and 

 imperviovis subsoil. The annual rainfall of the district averages 

 twenty-eight inches. The rivulets and streams of the Thames 

 basin are formed and pursue their course on the clay land. 

 There are no streams on the chalk. That which falls upon the 

 porous stratum and d(3es not evaporate sinks, mostly where it 

 alights, and heaps itself up in the water-bearing stratum below, 

 until the latter can hold no more. The water then escapes as 

 springs at the lowest available points. Innumerable examples 

 of these springs occur all round the edge of the Thames basin, 

 and at various points within it. Thus from the Chalk they are 

 ejected at the lip of the Gault, and in the Oolitic area by the 

 Fuller's earth below it, or by the Oxford clay, geologically, 

 above it. 



According to the gaugings of the engineer of the Thames- 

 Conservancy Board, there passed over Teddington Weir, in 

 1892, 387,000 millions of gallons, equal to an average flow of 

 1060 millions daily. In the following year, 1893, there passed 

 over this weir an aggregate of 324,227 millions of gallons, or a 



