228 



Cholera in Relation to Certain Physical Phenomena. [part i. 



falls steadily, until it reaches a maximum depression in May, from which it again rises 

 rapidly to maximum elevation. There is a considerable amount of coincidence apparent 

 between the lines in the diagram indicating the course of the phenomena of depression 

 of water-level and cholera prevalence. The period of maximum prevalence coincides with 

 part of the period of maximum depression of the water-level, and one of the months of 

 minimum prevalence with the month of minimum depression. When, however, the data 

 are more minutely examined, the coincidence is found to be a general one only, and 

 numerous divergencies between the courses of the two phenomena present themselves. 

 For example, the average maximum depression of water-level occurs in May ; but the 

 prevalence of May is much less than that of April. There is, also, a continued fall in 

 water-level in December and January, coincident w^ith the diminution of prevalence 

 occurring at that time. In fact, very much the same failures in coincidence are 

 encountered here as in the comparison of the course of atmospheric humidity with cholera 

 prevalence ; and though we may again have recourse to the conditions of temperature as 

 possibly accounting for the phenomena of December and January, we still require a 

 satisfactory explanation for those of May. 



Whilst the prevalence of cholera in Calcutta is associated with a low level of the 

 soil-water, the data very clearly show that the absolute water-level, in itself, is 

 of no importance. This cannot be better demonstrated than by comparing the average 

 water-level and prevalence of July, October, and November. 



TABLE XXXIII. 



Gomfi'parison of Water-level and Cholera Prevalence in July, October, and Novembei 



October and November, whilst showing a prevalence much greater than that of July, 

 have a considerably higher water-level than it has. If, then, the concurrence of low 

 water-level and high prevalence of cholera in Calcutta be more than a mere coincidence, — 

 if any casual relation exist between the two phenomena, — it cannot be a direct simple one, 

 dependent on the mere mass of water in the soil. 



Before leaving the subject, it may be well to look into the facts regarding the 

 fluctuations of water-level, compared with those of actual prevalence, during the period in 

 which the observations have been carried on. 



Diagram 7 shows the monthly averages of water-level and rainfall since April 1870, 

 together with the relative annual prevalence of cholera reckoned from the November of 

 one year to the October of the next. 



