Chapter VI. 
MOISTURE OF THE SOIL. 
IN GENERAL. 
The soil receives its water supply either by natural rainfall or by 
irrigation. The plant in successive generations of cultivation adapts 
itself to the ordinary supply of water, but in order to perpetuate its 
kind it must have sufficient during the growing season to serve it as 
a medium for extracting from the soil and air the nutritious sub- 
stances needed by it for its own development. The water really 
available to the plant is principally that which is left in the soil close 
to the roots after the surface drainage has carried off a large per cent 
of the original rainfall and after the evaporation by the dry wind 
has taken 20 per cent of the remainder from the surface soil and after 
a further large per cent of the remainder has by percolation or 
seepage slowly settled down beyond the reach of the roots of the 
plant. Thus it happens that the roots rarely have left for their use 
20 per cent of the original rainfall, and this is the so-called “ useful 
remainder.” Generally this remainder is best expressed as a per- 
centage of what the soil would hold were it completely saturated. 
Therefore its absolute quantity will vary with the character of dif- 
ferent soils 
EVAPORATION FROM THE SURFACE OF FRESH WATER. 
MONTSOURIS DATA FROM DESCROIX. 
An approximate idea of the relation between the velocity of the 
wind, its temperature, and its dryness, on the one hand, and its power 
to evaporate water on the other, may be obtained by collating the 
data given by Descroix in his article on “ The climatology of Paris,” 
in the Montsouris Annuaire, 1890, page 121. From the mass of data 
given by him I select the averages taken according to the direction 
of the wind, or wind roses, for the three summer months June, July, 
and August, 1889, as these are the months during which crops are 
liable to suffer the most severely from droughts and dry winds. I 
give them in the following table: 
(104) 
