Page:Popular Science Monthly Volume 20.djvu/485

Rh occupy the pores, and one in which the pores are quite filled with water excluding the air. In the former case we may speak of soilmoisture (Bodenfeuchtigkeit), in the latter of ground-water (Grundwasser). In many cases, but not universally, the transition from soilmoisture to ground-water is very plainly indicated.

The coincidence of the existence of ground-water with the prevalence of typhoid has been a constantly recognized fact since 1856. It was discovered by Buhl, has been tested by Seidel according to the law of probabilities, and has been followed out and confirmed down to the present day by Port for the garrison at Munich, and by Ziemssen for the avenues of the civil hospitals; by whom it is shown that, when the amount of ground-water is above the average, fewer, when it is below the average, more, cases of typhus appear. The same law was discovered by Virchow for the fluctuations of typhus in Berlin. The lowest degree of saturation exists in Berlin during the later summer and the fall, in Munich during the winter, and the seasons of typhus are correspondingly different in the two cities: the latter part of summer and the fall in Berlin, winter in Munich. When the summer is unusually dry at Munich, summer epidemics prevail there also.

That the cause of these conditions is not the ground-water in and of itself, but the moisture in the overlying strata and the processes dependent upon it, is very plainly shown by two facts: first, that some typhus-centers have a porous soil but no ground-water; and, second, that the level of the ground-water may be raised and depressed by artificial means, as by damming, pumping, or draining, without any notable influence being produced upon the prevalence of typhus. The first case occurs in places where on account of the steepness of the descent, or of any other cause, no ground-water collects on the lower impervious strata; the second case, where the level of the groundwater is within the mark of the flood-height of a river. In Munich, for example, it was necessary on one occasion, in order to carry on some excavations in a part of the city near the Isar, to leave a gate in the river open for several months, so that the water sunk more than a metre. No increase of typhus was observed; and when the gate was closed, and the water was allowed to rise again to its original height, the typhus did not diminish. A coincidence with the prevalence of typhus appears only when the variations in the level of the ground-water proceed from the saturation or the drying-up of the strata lying above it. I have always regarded the state of the groundwater only as the best and plainly visible sign or index of the rhythm of the soil-moisture in the overlying strata.

Macpherson, Lewis, and Cunningham have shown beyond a doubt that the cholera fluctuates in its home in India, like abdominal typhus with us, inversely with the annual amount of rain and of saturation of the soil. It behaves in the same way with us. The remarkable division of the cholera into a summer and a winter epidemic, which was