186 HOW CROPS FEED. 
fully one-third of the resistance of a clay or loam to the 
plow thus disappearing, according to Schiibler’s experi- 
ments. 
Tillage, when carried on with the soil in a wet condi- 
tion, to some extent neutralizes the effects of frost, espe- - 
cially in tenacious soils. , 
Fall-plowing of stiff soils has been recommended, in 
_order to expose them to the disintegrating effects of frost. 
§ 9. 
RELATIONS OF THE SOIL TO HEAT. 
The relations of the soil to heat are of the utmost im- 
portance in affecting its fertility. The distribution of 
plants is, in general, determined by differences of mean 
temperature. In the same climate and locality, however, 
we find the farmer distinguishing between cold arid warm 
soils. 
The Temperature of the Soil varies to a certain depth 
with that of the air; yet its changes occur more slowly, 
are confined to a considerably narrower range, and dimin- 
ish downward in rapidity and amount, until at a certain 
depth a point is reached where the temperature is invari- 
able. 
In summer the temperature of the soil is higher in day- 
time than that of the air; at night the temperature of the 
surface rapidly falls, especially when the sky is clear. 
In temperate climates, at a depth of three feet, the tem- 
perature remains unchanged from day to night; at a depth 
of 20 feet the annual temperature varies but a degree or 
two; at 75 feet below the surface, the thermometer re- 
mains perfectly stationary. In the vaults of the Paris 
Observatory, 80 feet deep, the temperature is 50° Fahren- 
heit. In tropical regions the point of nearly unvarying 
temperature is reached at a depth of one foot. 
