[Sidenote: Three principal sources of heat.]
The earth’s crust receives its supply of heat from three principal sources, viz.:
2. From the interior of the earth through conduction; and
3. From various physical and chemical processes which take place in it and create heat.
[Sidenote: Dark soils absorb more heat than light-coloured soils.]
[Sidenote: Capacity for heat is greater in damp and fine-grained soils.]
[Sidenote: Evaporation and condensation of aqueous vapour produce the greatest effect in fine-grained soils.]
Dealing with the upper layers of the crust, it may be said that, besides the intensity of the sun’s rays, the temperature also depends on a variety of properties possessed by various kinds of soil, amongst which latter may be mentioned the absorption of heat, which is much greater in dark than in light-coloured soils; the heat conductivity and the capacity for heat, which lead to higher temperatures in damp and fine-grained soils; and finally the evaporation and condensation of aqueous vapour, which tend to prevent extremes of heat and cold and which likewise produce the greatest effects in fine-grained soils.
[Sidenote: A fine-grained damp soil does not get so hot, but retains the heat better.]
It follows from these observations that a coarse-grained, dark coloured and dry soil will show the highest and lowest temperatures, whereas a fine-grained damp soil does not get so hot but retains the heat better.
[Sidenote: The temperature of the surface of the soil may exceed that of the air.]
It ought to be pointed out in this place that a variety of circumstances may bring about very high temperatures on the surface of the ground which considerably exceed the average temperatures of the air at the same time.
[Sidenote: Laws regulating the subsoil temperatures.]
Concerning the laws that have been deduced from careful and long continued observations of subsoil temperatures, it will not be necessary at this point to deal minutely with them; it must on the contrary suffice to summarise only the more important ones.
With the distance from the surface of the ground,
1. The differences of temperature become less,
3. The variations of short durations gradually disappear.
[Sidenote: Subsoil temperatures 18 in. below surface.]