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.
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,
2. The temperatures are retarded, and
3. The variations of short durations gradually disappear.
Subsoil temperatures 18 in. below surface.
Subsoil temperatures at depths of 4 ft. 6 in. and 9 ft.