38 ELEMENTARY FORESTRY. 
This is certain—that since summer and winter drouth, that is, 
rapid evaporation due to continuous dry winds, is the bane of 
the farmer on the plains, rationally disposed timber belts will 
do much to increase available water supply by reducing evap- 
oration. 
Evaporation, of course, goes on much less rapidly within 
than without the forest. How great this difference is in this 
section we have no exact figures to tell, but it is certain that 
it is much more than in Bavaria, where the following result 
was obtained:* In an experiment which was carried on to 
determine the amount evaporated from April to October*it was 
found that from a certain area without the forest 40.8 centi- 
meters were evaporated, within pine wood 15.9 centimeters and 
within deciduous woods 6.2 centimeters. This shows that the 
evaporation was six and one-half times as great in the open 
field as in deciduous woods. 
Transpiration. Another factor by which forests dissipate 
water supplies and which has been referred to (page 17) is 
transpiration. The quantity of water so used is as variable as 
the amount of precipitation, and in fact within certain limits 
depends largely upon it; that is to say, a plant will transpire 
in proportion to the amount of water which is at its disposal. 
Transpiration is also dependent on the stage of development of 
the plant, on the nature of its leaves and amount of foliage, on 
temperature, humidity and circulation of the air, on intensity 
of the sunlight, and on temperature and structure of the soil 
and on other meteorological conditions. Rain and dew reduce 
transpiration; wind increases it. 
The amount of transpiration depends considerably upon the 
thickness of the leaves; therefore the suriace of the foliage is 
not a reliable measure, but should be compared with the weight. 
In some European experiments carried on during the period 
of vegetation, the amount of water transpired by the different 
species per pound of dry matter in the leaves was as follows: 
