PART I.] Maximum Prevalence of Cholera with a Low-water-level. 



253 



stations from which good water-level returns have been furnished extending over 

 a period of three years. 



Average monthly Rainfall [12 years]. 



Average monthly Water-level [3 years] 



otal Cholera [25 to 51 years] 



iBa^azuaaaBaaiiWi 



LLLii.. ii..<^i^cna33iiii^w»M 



?m'^mmKsmmM% 



-Rainfall curve, inverted. 



-Water-level curve, inverted. 



-Cholera curve, not inverted. 



Diagram U. — Illustrating the avei-age monthly variation in the Water-level at Chinsurah, and the Cholera- 

 prevalence among European and Native troops at Calcutta and adjacent Military Stations. 



The above diagram will suffice to illustrate the remarks made in the previous pages 

 regarding the fact that, be the explanation what it may, the prevalence of cholera is at its 

 maximum in Lower Bengal when the water-level is low : not only is this observed in 

 Calcutta itself, nor when judged by data acquired elsewhere from among an indifferently 

 registered civil population, but also when tested by statistics of fairly assured accuracy 

 when they extend over a sufficient number of years. 



Mean Water-level 6 years 

 [Inverted curve]. 



Total monthly Cholera Deaths 



-14'' 7 from surface. 





— ir-5 



— 8' -2 



11 11 



[38 years]. 



Diagram 11a. — Average Water-level and Cholera- prevalence in Calcutta. 



In order to make this matter more evident, we reproduce the diagram showing the 

 relation which exists between the mean water-level of Calcutta for a period of six years. 

 In both diagrams the water-level is seen to be at its lowest in May and nearest the surface 

 in August and September, and cholera attains its maximum in April and May ; as in the 



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