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moist atmosphere. Man naturally observes first the latter feature, 
which is so important to him, and then associates it with the budding 
of the plant, but he recognizes his mistake when he considers that the 
plant is firmly established in the earth and that its nourishment and 
growth must depend primarily on the condition of the soil and roots. 
TEMPERATURE AND MOISTURE OF THE SOIL. 
The temperature of the soil a short distance below the immediate 
surface does not depend, by way of cause and effect, primarily on the 
temperature of the air. It is not warmed or cooled appreciably by 
conduction of atmospheric heat, but by direct absorption or loss of 
the radiation that falls upon it. To a slight extent (perhaps 5 per 
cent) this sunshine is reflected from the surface particles of the 
ground according to the laws of simple reflection; the remainder is 
absorbed by the surface and warms it. This warmed surface layer 
immediately radiates back a small quantity (10 per cent) as long 
waves into the atmosphere and through that into space, since the 
atmosphere does not absorb these long waves, but it gives up a larger 
part, perhaps 50 per cent, by conduction to the adjacent lowest 
layer of air, which being thus warmed quickly rises and by convection 
distributes this 50 per cent of heat throughout the atmosphere, whence 
it is eventually radiated back into space. The remaining 40 per cent 
of the solar heat is by conduction carried downward through the solid 
earth; a large portion is consumed in the evaporation of soil water 
and returns to the atmosphere with the aqueous vapor; the rest goes 
on downward, warming up the soil until it arrives at a layer 30 to 50 
feet below the earth’s surface, where the gradient of temperature 
just in front of it is the same as that just behind it. Here the heat 
would accumulate and push its way still deeper were it not that by 
this time, in most cases, the diurnal and annual changes of tempera- 
ture at the earth’s surface, where this heat wave started, have brought 
about a deficiency just below the earth’s surface; consequently the 
heat that had reached the depth of 30 or 50 feet now finds the tem- 
perature gradient just above it beginning to reverse, wherefore this 
heat begins to flow back, upward, and outward. In this manner the 
temperature of the ground increases downward to a depth of a few 
yards during certain months and then upward during other months, 
in diurnal and annual fluctuations interspersed with irregular 
changes, depending on cloud and wind and rain, all of which are easily 
recognized by examining any system of curves representing the earth 
temperatures at different depths throughout the year. 
The ground is warmed by the air only in case the temperature of 
the surface soil is lower than that of the air, and, although this 
happens frequently, yet the quantity of heat thereby communicated 
