Factors Influencing Plant-Life on Sandy Sea-shores. 39 
the reservation indicated. The artificial conditions under which 
measurements of the rate of percolation are usually conducted, 
whether it be after the methods of Deherain, Whitney, Welitschowsky 
or others, make it important that in giving the results the details of 
the method should be furnished. 
To obtain the absolute value of the rate of percolation is hardly 
possible, as we cannot control the lateral translocation, which 
depends largely on the topography of the surface, a circumstance 
which is rarely taken into consideration in the various devices for 
determination of the power of the soil for permitting the downward 
flow of water. 
Seelheim 1 has given the following general laws for the rate of 
percolation, based on a great number of careful experiments. 
The quantity of water passed through the sand is :— 
(1) proportional to the pressure ; 
(2) inversely proportional to the thickness of the layer; 
(3) proportional to the area of the layer; 
(4) proportional to the square of the radius of the sand 
grains ; 
and with increase in temperature the rate of percolation increases. 
The Water-Content of the Soil. 
Only a few decimeters below the surface sand-formations are 
always moist, even on the top of the highest dunes. To account 
for this remarkable fact it was long assumed, and is still held by 
some observers, that the acting force was the capillarity. Andresen 
has doubted this, and he maintained that the humidity can be 
explained only by evaporation from below. He states the mean 
minimum of water contained by the sand of the dunes of Jutland, 
0-3 m. below the surface after a long drought, at 2 per cent., and the 
maximum after a rainy month at 4 per cent. At greater depth the 
quantity is larger. 2 The hygroscopicity of the sand of the coast of 
Jutland he found to be 33 per cent, by measure, or 21’5 per cent, 
by weight. The annual precipitation on that coast is 27 inches, 
and as the evaporation is about the same, he argues that rainwater 
does not penetrate far beneath the surface of the dunes. 
Before proceeding to my own observations in this matter, some 
interesting data by Kerner 3 will be referred to. He found that the 
1 Die Durchlassigkeit des Bodens fur Wasser. Forsch. a. d. Geb. 
d. Agric.-Physik. Bd. 3, 1880. 
2 Andresen: (l.c., p. 106). 
3 Die Aufforstung des ungarischen Tieflandes. Monatsschr. f. 
Forstwesen, 1865. 
