THE WATER SUPPLY OF LONDON. 181 



river water. This condition is the amount of rainfall higher 

 up the river, or, in other words, the volume of water flowing 

 along the river bed, as is seen from the comparison repre- 

 sented in the next diagram (No. 9). 



This diagram shows very conclusively that the volume 

 of water flowing in the Thames is the paramount influence 

 determining the number of microbes. It compares the 

 volume of water in the river gauged at Teddington Weir 

 with the number of microbes found in the raw Thames 

 water at Hampton on the same day. In this diagram, the 

 numbers representing the flow of the river in millions of 

 gallons per day and the number of microbes per cubic 

 centimetre in the water both run from the bottom of the 

 diagram upwards. 



Comparing the curves in the diagram it is seen that, 

 with very few exceptions, a remarkably close relation is 

 maintained between them. 



The only exception of any importance to the rule that 

 the number of microbes varies directly with the flow of 

 the river, occurring during the thirty-two months through 

 which these observations were continued, happened in 

 November, 1892, when the flow increased from 501 mil- 

 lions of gallons in October to 1845 millions in November, 

 whilst the microbes actually diminished in number from 

 2216 to 1868 per cubic centimetre. Neither the sunshine 

 nor the temperature records of these two months, however, 

 afford any explanation of this anomalous result, for there 

 was a good deal of sunshine in October before the collection 

 of the sample and the temperature was higher, whilst in 

 November no ray of sunshine reached the Thames during 

 the three days preceding the taking of the sample and the 

 temperature was nearly 4 C. lower than in the preceding 

 month. I have ascertained, however, that the Thames 

 basin had been twice very thoroughly washed out by heavy 

 floods before the time when the November sample was 

 taken, and this affords a satisfactory explanation of the 

 anomalous result yielded by this sample. 



These comparisons demonstrate that the number of 

 microbes in Thames water depends directly upon the rate 



