Page:Popular Science Monthly Volume 26.djvu/527

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(March and April), and its minimum is attained during the wettest and hot month of August, so that the curve of cholera falls while that of rain rises. The rise and fall of endemic cholera is constant, while the prevalence of cholera in its epidemic form is very variable. In the Punjaub the rain-winds (monsoons) bring with them the cholera. This apparent contradiction of the experience above mentioned may be explained by considering that the monsoons bring to the Punjaub the necessary moisture which is believed to be requisite for the development of an epidemic of cholera. Rain falls in the Punjaub at the same time as in Lower Bengal, but in smaller quantity. The average rainfall at Lahore is twenty-two inches, as compared with sixty-two inches at Calcutta; so that in Lahore for the greatest part of the year the soil is too dry for cholera. That these claims are substantial will be granted by a study of the rainfall and related circumstances in other parts of India. This is well seen at Madras, as indicated by Table II. At Madras the average rainfall is about forty-eight inches,

and is therefore midway between those of Lahore (twenty-two inches) and Calcutta (sixty-two inches), but the quantity of rain in the several months is different. The greatest quantity of rain falls in November, and Madras is not under the influence of the southwest, but of the northeast monsoon, and the rainy season extends from July to December. As Madras lies farther south than Calcutta, and the quantity of rain is twenty-five per cent less, it is plain that the desiccation during the dry and hot season would be much greater there than in Calcutta. And this state of affairs is reflected in the vegetation. April, May, and June are like winter months; the leaves begin to fall; the verdure fades away, and the sap descends to the roots of the trees, not because of the cold, but on account of the excessive dryness. It is at this period that cholera is at its minimum. In July, when the rain begins to fall, cholera increases, and reaches it maximum in August. The rain continues, but the cholera decreases, owing to the excessive dampness of the soil; so that in November a second minimum in the number of cases of cholera is met with.