forest-culture, under the superintendence of Prof. Ebermayer, of Aschaffenburg. He has published his first year's observations in a work on "The Influence of Forests on the Air and Soil, and their Climatic and Hygienic Importance," which may be recommended to every one who wishes to study the subject.
Modern hygiene has observed that certain variations in the moisture of the soil have a great influence on the origin and spread of certain epidemic diseases, as for instance cholera and typhoid fever—that these diseases do not become epidemic when the moisture in the soil is not above or below a certain level, and has remained so for a time. These variations can be measured with greater accuracy by the ground-water of the soil than by the rainfall, because in the latter case we have to determine how much water penetrates the ground, how much runs off the surface, and how much evaporates at once. The amount of moisture in the soil of a forest is subject to considerably less variation than that outside. Ebermayer has deduced the following result from his meteorological observations on forestry: "If from the soil of an open space 100 parts of water evaporate, then from the soil of a forest free from underwood 38 parts would evaporate, and from a soil covered with underwood only 15 parts would evaporate." This simple fact explains clearly why the cutting down of wood over tracts of country is always followed by the drying up of wells and springs.
In India, the home of cholera, much importance has been attached in recent times to plantations as preventives of it. It has been always observed that the villages in wooded districts suffer less than those in treeless plains. Many instances of this are given in the reports of Dr. Bryden, President of the Statistical Office in Calcutta, and Dr. Murray, Inspector of Hospitals. For instance, Bryden compares the district of the Mahanadda, one of the northern tributaries of the Ganges, the almost treeless district of Rajpoor, with the forest district of Sambalpoor. It is stated that in the villages in the plain of Rajpoor, sixty or seventy per cent, of the inhabitants are sometimes swept away by cholera in three or four days, while the wooded district of Sambalpoor is often free from it, or it is much less severe. The district commissioner, who had to make a tour in the district on account of the occurrence of cholera, reports, among other things, as follows:
- "Die physikalischen Wirkungen des Waldes auf Luft und Boden und seine klimatologische und hygienische Bedeutung."
- "Epidemic Cholera in the Bengal Presidency," 1869, p. 225.