PURIFICATION OF WATER FOR DRINKING PURPOSES 139 



works again exhibited distinctly more micro-organisms 

 than that of the new. The average number found in 

 the filtered water taken directly from the beds amounted 

 to 30 per cubic centimetre, and thus hardly differed 

 from that on the previous occasion. On the other 

 hand, the unfiltered Thames water yielded about 7,000 

 colonies per cubic centimetre as against about 1400 

 before, and this clearly shows that when such small 

 numbers are found in the filtered water hardly any of 

 them can be derived from the unfiltered water, but that 

 they are probably mostly due to" the fact that the 

 filtering materials themselves contain micro-organisms, 

 as do also the culverts and wells into which the filtered 

 water passes ; in fact the micro-organisms found in 

 the filtered water are probably almost entirely due to 

 post-filtration sources. 



New River Waterworks, Stoke Neivington, August 27, 1892 

 Filtered Water 



Ko.rfpnt.Ba, 



No. 1 . . . . 1 . .154 



No. 2 . . . . 8 . . . 16 



No. 3 . . . . 10 . . . 18 



No. 4 . . . . 5 . . 10 



No.5 .... 23 ... 9 



General Filter Well . ... 10 



Unfiltered Water. 



Water supplying Filters Nos. 1 and 2, 8 and 9 . 260 



Nos. 3, 4, 5, 6, and 7 . 171 



New River Cut above Reservoir . . . 677 



Outlet of First Reservoir ..... 560 



Outlet of Second Reservoir .... 183 



The water issuing from only five out of the nine 

 filter beds was accessible. In all cases excepting one 

 the number of micro-organisms found in the filtered 

 water was very small, and it is particularly noteworthy 

 that the filter bed yielding the water with the larger 



