and the Storage of Water. 15- 
on its surface.* The mean daily evaporation from off the sur- 
face of a reservoir or other body of water is considered to vary 
from the 12th to the 16th of an inch.f Mr. Humber considers 
that the loss to be allowed for in a reservoir from this cause 
may be calculated at the rate of one inch of rainfall over the 
whole of the gathering ground. ;{: 
Evaporation proceeds more rapidly off soils covered with 
grass and similar vegetation than off those that are bare. From 
experiments made at Vienna, it was found that the proportion 
of percolation (two feet deep) through ground covered with turf, 
as compared with that bare of vegetation at the same place, 
varied as follows : — 
In May 
25*2 per cent, less through turf. 
June . . 53*1 
July . . 23-4 
Aug. . . 29-2 
Sept. . . 12'7 „ 
And as the result of these experiments, the conclusion arrived 
at was that in the summer half-year forest soil was the moistest ; 
bare, open ground less moist ; turf the driest.§ 
Percolation is therefore diminished when there is vegetation, 
and especially when the growth extends through the whole of 
the year, as in grass. Dr. Gilbert computes that for every ton 
of really dry substance grown, a depth of 3 inches of rain would 
be evaporated through the vegetation.§ Trees, whether planted 
singly, or in woodlands and forests, have a material effect in 
checking evaporation ; their influence upon the disposal of the 
rainfall being thus described by Mr. Steinmetz, in his popular 
book on ' Meteorology :' |1 — " Trees and forests contribute to the 
formation of springs and Avater-courses, not only by means of 
the humidity which they produce and the condensation of vapour 
by refrigeration, but also by reason of the obstacles which they 
present to the evaporation of the water in the soil itself, and by 
means of the roots which, by dividing the soil like so many 
perforations, render it more permeable and facilitate filtration. 
Certain it is that the clearance of forests and the consequent 
drying up or draining of marshes and bogs have caused a 
material alteration, not only in the entire face of the country, 
but in the supply of water to the rivers formerly derived from 
• Beardmore, ' Manual of Hydrology.' 
t Burnell, ' llyili aulic Engineering.' 
X ' Water Supply of Cities and Towns.' By W. Humber. Crosby Lockwood 
and Co., 1876. 
§ " Kainfcdl and Evaporation," ' Trans. Instit. Civil Engineers,' vol. xlv. 
II ' Sunshine and Showers, their Influence throughout Creation.' By A. 
Steinmetz. Reeve and Co., 18G7. 
