Causes of Fertility or Barrenness of Soils. 
199 
mometers placed in the soil at various depths. In the natural 
peat, the depth of which was 30 feet, he found that during the 
whole time of his experiments (nearly three years), with only one 
exception, the temperature from 12 inches below the surface to 
the bottom of the bog was uniformly 46°. The exception took 
place during the winter of 1836, when the thermometer nearest the 
surface fell for a few days to 44°. Into the cultivated portion, 
which had been thoroughly drained, ploughed, and dug deeply, 
five thermometers were inserted at the following depths — 7, 13, 
19, 25, and 31 inches. The experiment only extended over 12 
days, the examination being made twice a-day, at 9 A.M. and 
2 P.M. ; the result was a very different temperature for each 
depth, greatest and most variable at the surface, least and most 
regular at the greatest depth, where the temperature only varied 
during the whole time from 46° to 48° ; while that at 7 inches 
varied from 52° to 66°, was always highest at 2 P.M., and ap- 
peared to vary according to the temperature of the air. The 
second bulb, 13 inches from the surface, ranged from 50° to 57° ; 
the third, at 19 inches, 48-4° to 52-8°; and the fourth, at 25 
inches, from 47° to 50 2°. These experiments speak for them- 
selves, and will afford a ready explanation of the otherwise mar- 
vellous effects which often follow thorough drainage. The in- 
crease and variety of temperature could alone be due to the 
removal of excess of water, and consequent permeating influence 
of rain-fall, atmosphere, and sun's rays. 
Loamy soils and sands are warm, from their dryness, porosity, 
and colour. Chalks and limestones are cooler, their light colour 
rendering them bad absorbents and good reflectors. All soils 
radiate the heat received from the sun's rays back again into the 
atmosphere as soon as the latter cools down to a lower tempera- 
ture, but in different degrees depending upon the state of me- 
chanical division, colour, &c. ; that soil ought to be warmest 
which absorbs most readily and radiates most slowly, and this 
is the case with sands. The formation of dew is connected with 
this radiating power ; the soil, giving up its heat, becomes cold, 
and lowers the temperature of the stratum of air immediately 
above it, causing a portion of the moisture which it contained in 
a state of vapour to be deposited as dew. But this very deposit 
of dew, being a conversion of steam or vapour back into water, is 
attended with the liberation of considerable heat, which probably 
helps to make up for the loss by radiation. We only see dew 
after still, clear nights, because the clouds radiate back and so keep 
up the temperature of the soil, and wind acts in the same way. 
We can easily test the relative powers of different soils to re- 
tain heat by subjecting portions to a strong heat, noting how 
soon they lose it again. Sand by such means has been found to 
retain heat for the longest period ; clays stand at about 65 to 70 
