Page:Popular Science Monthly Volume 26.djvu/530

514 town of Munich, so the numbers concerning cholera obtained from the barracks and prisons of India may be taken as a good sample of the rate of mortality from cholera in the cities themselves. Bryden has given support, in his work on the time and space distribution of cholera in India, to the reports of barracks and prisons. That cholera should attain its greatest frequency in North Germany during the months of August, September, and October, and that winter should seldom see an epidemic, may be explained by the temperature which prevails in the air and earth. In the class of ectogenous infectious diseases, to which cholera belongs, the temperature, as in all organic processes, has a decided, though it can not be the chief, influence. That cholera is not very dependent on temperature is evidenced by the possibility of the occurrence of an epidemic of cholera during the winter. Why cholera spreads more during the summer and early autumn, as compared with winter and spring, must depend on other causes than temperature.

It is clear, in my opinion, that the soil and the moisture of the soil play a principal part. The dampness of the soil is, under certain conditions, clearly related to the subsoil-water, "Grundwasser." Epidemics of cholera abound during the time that the "Grundwasser" is falling, when the earth is comparatively dry. By "Grundwasser" is to be understood that condition of dampness of a porous soil when all the pores are filled with water. If water and air together fill up the interstices, then the soil is called simply damp. I have so long and so often spoken on the influence of the rise and fall of the ground-water on the frequency of typhoid fever and cholera, that I imagine a great many scientists credit me with the view that subsoil-water is highly harmful. But such is not the case. The subsoil-water is merely an indication of what is going on, and has no more to do with the actual processes than a dial and the hands have in the going of a clock. The fall of the ground-water by pumping away, or the rise of ground-water by the damming of a stream, has not the least effect on typhoid fever or cholera in the neighborhood. The observation of the level of the surface of the water in springs as an indication of the state of the subsoil-water is of no value from an etiological point of view, unless the spring be independent of the nearest water-course, and unless at the time of the observation the real state of the spring is a true reflex of the condition of the subsoil-water in its neighborhood. When the information, however, is obtained properly from springs free from objection, then the condition of the ground-water gives the state of moisture and of exchange in the overlying layers much more accurately than an observation made on the atmospheric downfall (rain and dew). Rain may fall for a week without causing any rise in the subsoil-water, and again a rise may occur when there has been no fall of rain for some time. The perusal of Professor Franz Hofmann's work, published in the "Archives of Hygiene," on the movement of subsoil-water,