Drinking Water Supplies. 253 
range from 14.9 at Leonora to 36.2 at Nonpareil. On other estates with 
an unprotected supply in no case is the rate below 47.2 and it rises on 
some to as high as 96.5, 
It is important to realise that water thus impounded, lying still, 
undisturbed and free from added contamination, proceeds naturally to 
purify itself. It is on this fact that a large portion of European cities 
depend for the purification of their supplies. The provision of large 
settling reservoirs by the various metropolitan and borough water com- 
panies shows their appreciation of the importance of this process hardly 
second even to filtration. 
When, however, frequent driblets of pollution are being added to 
water, this purification process is unable to make headway against it. 
Analysis of open trench drinking water supplies usually reveal 10—10,000 
faecal organisms per cubic centimetre, whereas the following table shows 
the result in some protected water supplies. 
Plantation, Faecal Organisms. 
Wales 1 per cubic centimetre 
Nismes absent in 5 cubic centimetres 
Port Mourant absent in 5 cubic centimetres 
La Bonne Intention 1 per cubic centimetre 
Some few estates have supplies conserved in concrete tanks or 
reservoirs. These are filled from the roof of the factory buildings or 
from adjacent ranges. In such cases the water is of a purity far sur- 
passing that achieved in any system of trench supply. At Blairmont 
the supply is equal to that of a European city. 
The following results indicate the results obtained :— 
Plantation. Faecal Organisms. 
Providence, Demerara absent in 6 cubic centimetres 
Uitvlugt absent in 3 cubic centimetres 
Blairmont absent in 10 cubie centimetres 
Springlands absent in 5 cubic centimetres 
The chemical analyses give analagous results. 
During the course of the investigation attempts have been made to 
estimate the period at which pollution took place. From the bacteriologi- 
cal analysis and from the types of organism isolated, it was hoped that it 
might be possible to ascertain whether the pollution was added yesterday, 
last week, last month or last year. This has been succeeded in to a certain 
extent within broad limits. Work on this subject was recently carried 
out by Major Clemesha in India and our results correspond very closely 
where surface waters are concerned. By isolating different types of 
faecal bacteria it is found that some are more resistant than others and 
while purifying powers of nature will destroy certain kinds in a few 
days others will remain still living for a few weeks and others for a few 
months. The presence of the Bacillus Coli Communis, if in large 
