PURIFICATION OF AVATER FOR DRINKING PURPOSES 189 



1892. At these works the water of the river Lea, mixed 

 with a certain proportion of well-water, is brought along 

 an artificial cutting, and is made to pass through two 

 large reservoirs before going on to the filter beds. 

 Samples were taken at the cutting j ust above the 

 reservoirs, at the outlet of the first reservoir, and at the 

 outlet of the second reservoir. 



The following results were obtained : — 



Number of Colonies 

 obtained from 

 1 c.c. of water 



1. Cutting above reservoir ... . 677 



2. Outlet of first reservoir . . . 560 



3. Outlet of second reservoir ..... 183 



The above figures show that by storage alone the 

 waters of the Thames at Hampton and the purer water 

 of the New Eiver Company's channel may, so to speak, 

 be brought to the same bacteriological level. 



More recently still we have met with an example of 

 bacterial subsidence in a semi-natural lake, the Loch of 

 Lintrathen, from which the water supply of Dundee is 

 now obtained, and in which the water of the loch itself 

 was found to contain a strikingly smaller number 

 of bacteria than the streams supplying it ; for the par- 

 ticulars of this case see p. 112 in the chapter on the 

 bacterial contents of various waters. 



On the Continent we rarely hear of storage reser- 

 voirs being employed, but in America they are more 

 common, and such reservoirs are said to be frequently 

 used for the wate3;s of the Mississippi and Missouri rivers.^ 

 The water is allowed to remain at rest for many hours 

 in large shallow settling basins, after which it is drawn 

 off through a wire screen to retain fish and other large 

 objects swimming in the water, and the sediment re- 

 moved at regular intervals from the basins. The sedi- 



1 Potable Water, Floyd Davis, Boston, 1891, p. 83. 



