Exar)iination of Melbourne Tap Water. 73 



are undoubtedly good for an unfiltered water supply ; at 83 per 

 c.c. they approach the best average results obtained by sand 

 filtration. It will be seen that two of the three abnormal results 

 occurred in the winter months, and the rainfall in the last half of 

 July was unusually heavy. The consequence is that more 

 sediment is washed down the creeks, and the average number of 

 organisms is I'aised. T am inclined to attribute the three large 

 numbers to a little particle of mud being taken up with the 

 sample, but on the other hand the third case happened in 

 January, when the rainfall and amount of sediment are both at 

 the minimum. So far as we could ascertain there was no inter- 

 ference with the mains or reticulation pipes on these occasions. 

 Leaving out of account the three anomalous results it will be seen 

 that the number of organisms steadily fell from July to January 

 thus: — 



July - - - 132 



August - - - 99 . 



September - - 76 



October- - - 83 



November - - 60 



December - - 71 



January - - 54 



As will be seen from the following tables the tap water 

 contains a smaller number of organisms than the mountain 

 creeks. It is considerably higher than the average of the outlet 

 of the Yan Yean Reservoir, or of the Surrey Hills and Caulfield 

 local reservoirs, which are supplied direct from the Yan Yean. 

 As previously explained nearly half the water supplied to 

 Melbourne does not pass through the Yan Yean Reservoir, and 

 therefore loses the advantage of sedimentation. Moreover, the 

 Maroondah water is supplied chiefly in the winter, so as to allow 

 of the large reservoir to be filled ready for summer consumption. 

 This probably accounts for the average improvement as the 

 summer approaches, the results of the examinations of t"lie creeks 

 showing little variation in summer and winter, unless the water 

 was actually discoloured by recent heavy rain. For the same 

 reason the number of micro-organisms bears no direct relation to 

 the temperature of the water. For purposes of comparison 

 the results of the examination of London water are appended. 



