426 THE POPULAR SCIENCE MONTHLY. 



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 i-ecomrnended to 

 every one who wishes to study the subject. 



Modern hygiene has observed that certain variations in the moist- 

 ure of the soil have a great influence on the origin and spread of cer- 

 tain 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 consider- 

 ably 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 evapo- 

 rate, 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 al- 

 ways 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 2 cotn- 

 pares the district of the Mahanadda, one of the northern tributaries of 

 the Ganges, the almost treeless district of Rajpoor, with the forest dis- 

 trict of Sambalpoor. It is stated that in the villages in the plain of Raj- 

 poor, 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 dis- 

 trict commissioner, who had to make a tour in the district on account 

 of the occurrence of cholera, reports, among other things, as follows : 



"The road to Sambalpoor runs for sixty or seventy miles through the forest, 

 which round Petorah and Jenkfluss is very dense. Now, it is a remarkable fact, 

 but it is a fact, nevertheless, that on this route, traversed daily by hundreds of 

 travelers, vehicles, and baggage-trains, the cholera rarely appears in this extent 

 of sixty miles, and when it does appear it is in a mild form ; but when we come 

 to the road from Arang, westward to Chicholee Bungalow, which runs for about 



1 " Die physikalischen Wirkungen des Waldes auf Luft und Boden und seine klimatolo- 

 gische und hygienische Bedeutung." 



2 " Epidemic Cholera in the Bengal Presidency," 1869, p. 225. 



