ISSO] 



Water supplies of Cidcuita. 



123 



In considering the results of the analysis of the Calcutta water 

 month by month, we find that its composition varies considerably at 

 different parts of the year. A close inspection of the table will show 

 that apparently there are two distinct causes at work in modifying the 

 composition of the water. The first prominent cause, and the one which has 

 by far the greater influence, is to be found in the commencement, 

 and during the continuance of the rainy season ; the second and smaller 

 cause ajjpears to be the melting of the Himalayan snows by the burning 

 sun of March, April, May and June. These changes are most clearly noticed 

 in the column of Total Solid Impurity, and here we read that starting in 

 January the amounts of total solids gradually increase up to March, when 

 22'37 parts are present in every hundred thousand parts of water ; in April 

 and May the quantities steadily and gradually diminish, the numbers being 

 2 L'68 and 2 1'23 respectively ; this decrease continues until June 1st when 

 there are only 19 43 parts of solid impurity present. These nun^bers of 

 course correspond with the gradual and inci-easing diluting effect due to the 

 admixture of pure snow water with the ordinary river water. In the middle 

 of June, however, the rainy season usually commences, and there is a sudden 

 decrease in the solids owing to the diluting action of the enormous volumes of 

 rain water, and we find only 18'04 parts on July 1st ; from this time there 

 is a slight but steady decrease until October, when the water contains the 

 smallest amount of solids present at any time of the year ; the average for 

 October 1st showing 11-30 parts. After the complete cessation of the 

 rains (after November 1st) there is again a sudden rise in the total solids, 

 and on December 1st, 19'4-1 parts are present. Some of the other columns of 

 figures sliow a somewhat similar change, but in the case of the organic 

 matter the change is not very marked. In the amount of nitrates 

 present in the water, there appear to be two distinct maximum quanti- 

 ties during the year, one in March at the time of greatest concentra- 

 tion of tlie water as before mentioned, and the second at the commence- 

 ment of the rains. This second maximum is readily accounted for when 

 we consider, that the first effect of the rains will be to dissolve out the 

 nitrates which have been accumulating in the soil of the drier parts of 

 the country during the hot season ; the amount of nitrates, however, it 

 will be seen, steadily decreases towards the end of the rains, and this to a 

 certain extent confirms the explanation. 



Indeed during the first weeks or even days of the rainy season, the 

 composition of our water supply is undergoing very rapid change, owing to 

 tlie diluting action of the rain, and to the fact that the first showers of rain 

 will wash out considerable quantities of soluble organic matter, nitrates 

 etc., from the soil ; afterwards, however, the rain water will run off com- 

 paratively pure. We shall therefore expect that the first action of the rain 



