798 REPOKT — 1888. 



records of meteorologists showing that the average amount of rain falling on the 

 surface of the Thames watershed varies according to position from 25 inches to 28| 

 inches, he has assumed that 27 inches is the nearest approach to the mean quantity 

 upon which the supply of the Thames system depends. Out of the three and 

 a half million acres constituting approximately the superficial area of the Thames 

 watershed he estimates that one-third represents the aggregate of impervious surfaces 

 consisting of clays, gault, and lias, whilst two-thirds represent in like way the 

 numerous formations and beds of chalk, sandstones and sand-beds, cornbrash and 

 coral rag, and marlstones, the former throwing off the rain, and the latter absorb- 

 ing and infiltrating nearly all that falls upon it to satisfy vegetation and evapora- 

 tion and to find discharge by springs at the outcrops. 



Of the 27 inches forming the mean average annual rainfall, about two-thirds, 

 or eighteen inches, are evaporated from the surface, whilst of the remaining third 

 4 inches serve to maintain the river system, and 5 inches pass away as floods and 

 freshets. Instances are numerous in which the year's rainfall exceeds 30 inches, 

 ■whilst they are very few in which it is less than 20 inches — about three times in 

 twenty years. As the amount of evaporation is nearly a constant figure, and the 

 quantity required to maintain effectually the river system necessarily remains the 

 same under all conditions, the amount of flood or excess water greatly varies. 

 It is sometimes double the average. On the few occasions when the rainfall does 

 not reach 20 inches it is insufficient to satisfy the demands of the river system, 

 and then the river becomes a borrower from the stored supply of the subterranean 

 reservoirs. On such occasions the quantity of water flowing down the river to King- 

 ston has been so reduced as not to reach 300 million gallons in twenty-four hours. 

 The importance and bearing of this fact upon the proposal to replenish the sub- 

 terranean supply will be appreciated when it is pointed out that the quantity 

 of water supplied daily to the metropolis by the water companies has already 

 exceeded 150 million gallons. Of these 150 millions the river Thames contributes 

 50 per cent, or 75 millions, which is a quarter of the quantity flowingpast Kingstou. 

 The Lea furnishes 38 per cent., and deep chalk wells the remainder, or 12 per 

 cent. The quality of deep well waters has become of late years more and more 

 approved. Dr. Edward Frankland, in his classification of potable waters, places 

 deep "well waters only second to springs issuing from the outcrops of the same 

 formations. To make good the loss of this superior water the author proposes that 

 whenever the water in the river rises above a certain datum height recognised as 

 the gauge of its full service, the excess shall be diverted out of the river course on 

 to filter-beds formed near at hand. The outlet from these filter-beds would be 

 steined shafts or sumps sunk down to the water-level beneath, and into them the 

 filtered water woidd pass after it is freed from flocculent matter. The steined 

 shafts would be made water-tight and sealed against all surface contamination. 

 The whole of the 150 million gallons forming the metropolitan supply of the water 

 companies is, with exception of the Kent Company's supply, at present filtered 

 daily through filter-beds varying in depth and character of materials from 3 feet 

 6 inches to 9 feet deep of sand and other ingredients of diflerent degrees of coarse- 

 ness, the whole of the beds covering a superficial space rather less than 100 acres 

 in extent. These arrangements having been successfully worked, the same might 

 be adopted in the utilisation and purification of excess or storm waters. 



As far back as 1867 the author in his evidence before the ' Royal Commission 

 on Water Supply ' pointed out that if towns on the banks of rivers, such as the 

 Thames, the Lea, and their tributaries, were to lift their sewage and foul liquids 

 on to absorbent lands lying 100 to 160 feet above them, they would not only free 

 those rivers from pollution but they would help to maintain their flow with 

 certainty. He specified Luton as a town that could adopt such a mode of disposal 

 with good effect and economy (see Question 1613 of Minutes of Evidence). The 

 author now refers to this case in consequence of the authorities of Luton having 

 adopted the treatment by which they have signally proved the facility with which 

 the sewage of towns may be cleansed by filtration through a deep bed or stratum 

 of porous material. Winchester, Basingstoke, and several other towns situated on 

 the chalk have adopted this mode of sewage disposal without any injurious effect. 



