RELATIONS OF THE SOIL TO HEAT. 197 
with the deepening of the color, until in case of humus it 
lacks but a few degrees of reaching the warmth of a sur- 
face of lamp-black. 
According to the observations of Dickinson, made at 
. Abbot’s Hill, Hertfordshire, England, and continued 
through eight years, 90 per cent of the water falling be- 
tween April lst and October Ist evaporates from the sur- 
face of the soil, only 10 per cent finding its way into 
drains laid three and four feet deep. The total quantity 
of water that fell during this time amounted to about 
2,900,000 Ibs. per acre; of this more than 2,600,000 evap- 
orated from the surface. It has been calculated that to 
evaporate artificially this enormous mass of water, more 
than seventy-five tons of coal must be consumed. 
Thorough draining, by loosening the soil and causing a 
rapid removal from below of the surplus water, has a most 
decided influence, especially in spring time, in warming 
the soil and bringing it into a suitable condition for the 
support of vegetation. 
It is plain, then, that even if we knew with accuracy 
what are the physical characters of a surface soil, and if 
we were able to estimate correctly the influence of these 
characters on its fertility, still we must investigate those 
circumstances which affect its wetness or dryness, whether 
they be an impervious subsoil, or springs coming to the 
surface, or the amount and frequency of rain-falls, taken 
in connection with other meteorological causes. We can- 
not decide that a clay is too wet or a sand too dry, until 
we know its situation and the climate it is subjected to. 
The great deserts of the globe do not owe their barren- 
ness to necessary .poverty of soil, but to meteorological 
influences—to the continued prevalence of parching winds, 
and the absence of mountains, to condense the atmospheric 
water and establish a system of rivers and streams. This 
is not the place to enter into a discussion of the causes 
that may determine or modify climate; but to illustrate 
