PARTI.] Rainfall and Water-level in Relation to Cholera-prevalence in India. 287 



nature, similarity in their effects could not be looked for. The effects on soil- 

 ventilation produced by a fall of 3'^'89, which is the average amount for August in 

 Lahore, must patently be very different from those of the contemporaneous fall of 

 13"-71 in Calcutta. 



Direct observations are still wanting on the effect of the rainfall in the 

 stations of Upper India on conditions of soil-ventilation; but the data furnished by 

 the present example are sufficient to show that it is idle to found conclusions on 

 comparisons of local relative conditions, and that it is the absolute conditions* 

 coincident with various seasons of cholera-prevalence, which must be compared with 

 one another. The rainfall and soil of one place may be such that maximum rainfall 

 effectually interferes with soil-ventilation, whilst in another they are so different that 

 little or no effect is produced. 



Rainfall, however, may influence the local prevalence of cholera by means of its 

 influence on the soil in various ways. It does not only act on the ventilation of 

 the soil, it also acts on its conditions of moisture, and, indeed, it is only in doing 

 so that it affects the ventilation. If the development of the cause of cholera in a 

 locality be dependent on local conditions of soil, as there is much reason to believe 

 is the case, it is surely conceivable that one of these conditions is a certain degree 

 of moisture. Grranted this, and there can be no difficulty in assuming that given 

 localities may fail to produce cholera either on account of the soil being too damp 

 or too dry, and that the conditions ensuring development might be provided in one 

 case by diminishing, in the other by increasing, the amount of moisture present. 

 Looked at under this light, the apparently contrasted conditions of rainfall coincident 

 with maximum prevalence of cholera, in stations such as Calcutta and Lahore, 

 practically disappear. As in the case of the humidity, the contrast is one between 

 local relative conditions only, not between the final result of the action of these 

 conditions, or even between the absolute conditions themselves. 



In regard to water-level little need be said, as it has been already indicated that, 

 in this country, data of rainfall are more generally applicable as indices of conditions 

 of local soil-moisture than fluctuations in water-level can be. 



Calcutta and Lahore have been selected as typical examples of localities in the 

 endemic and non-endemic areas, regarding which we have relatively satisfactory data 

 for comparison. The remarks made regarding them are, however, generally applicable 

 to the other stations within these areas. Calcutta and Lahore are no doubt extreme 

 examples of the characteristics of these areas, both in regard to seasonal distribution 

 of cholera-prevalence and to the coincident physical phenomena, and the phenomena 

 of every locality must be scrutinized in detail for themselves. Lahore is peculiarly 

 distinguished by the smallness of its rainfall, and many other stations within the 

 non-endemic area may be pointed out where the season of maximum cholera is 

 coincident with considerable rainfall. The evidence of the influence of soil-ventilation 

 on prevalence is certainly not so manifest in these cases in regard to Calcutta, 



