1886.] NEW YORK ACADEMY OF SCIENCES. 43 



3(1. Ifc purifies the polluted local supply. 



In the study of this subject, there is no source of information 

 more valuable than the blue books containing the minutes of 

 inquiry before the Royal Commissions of 1851 and 1868 upon 

 the supply of London. It is there stated that at first London 

 drew its supply directly from the Thames where it flowed 

 through the town, at London Bridge. This was in 1581, and a 

 century later (1691) the Thames was again drawn upon at Char- 

 ing Cross, and this intake remained in use as late as 1829. 

 Again, in 1723, the Chelsea water works were established, and 

 in 1785 those at Lambeth. Whilst some part of the water sup- 

 ply was derived from springs in the chalk formation at Chadwell 

 (brought in. through a canal called the New River, in 1613), and 

 another part from the river Lee (introduced by the East London 

 Water Woi'ks Co. in 1806), yet as late as the year 1829, the 

 metropolis was principally supplied by water taken from the 

 Thames within the reach of the tidal flow. But in 1829, a 

 Royal Commission, consisting of Telford, Brande and Roget, 

 was appointed to inquire into "the description, the quality, and 

 the salubrity" of the water. They reported "that the Thames 

 water, when free from extraneous substances, was in a state of 

 considerable purity; but as it approached the 7netropoUs if became 

 loaded with a quantity of filth luhich rendered it disgusting. 

 It appeared, however, that a very considerable part, if not the 

 whole, of this extraneous matter might be removed by fil- 

 tration through sand, and the Commission decided that it was 

 perfectly possible to filter the whole supply with the requisite 

 rapidity and within reasonable limits of expense.'" Stimulated 

 by this report, and alarmed probably at the prospect of a 

 sweeping change of the sources of supply, the companies 

 directed their attention to the purification of the water by fil- 

 tration. It was soon found that the only appropriate material 

 for mechanical filtration on a large scale, was fine sand; but the 

 great practical difficulty was to prevent the sand from becoming 

 clogged, and to find an easy, practical, and chpap method for its 

 renewal. After long experimentation, a means was discovered 

 of getting over these difficulties. It was found that by far the 

 greater quantity of the impurities was held in suspension by 

 the agitation and motion of the water, and that, if it was allowed 

 to stand for some time at perfect rest, in a reservoir, the heavier 

 and grosser jyarticles tuere deposited hy simple subsidence, \evkv- 

 ing only a small proportion of lighter and finer matters to be 

 dealt with by filtration. It was also found that when the water 

 was allowed to filter downwards through a porous bed of sand, 

 held up in its place by underlying layers of coarse gravel, the 

 dirt did not penetrate into its mass, but was stopped at its 



