[Sidenote: Subsoil temperatures 18 in. below surface.]
[Sidenote: Subsoil temperatures at depths of 4 ft. 6 in. and 9 ft.]
At a depth of 18 inches below the surface the daily fluctuations are
hardly observable, the temperature differences of various days become
obscured, the differences between the monthly mean temperatures are
less by several degrees, and the yearly fluctuation amounts only to
about 10° C. At a depth of 4 feet 6 inches the latter is only 4° C.,
and at a depth of 9 feet it is only 1°C.
[Sidenote: Subsoil temperatures at depths from 9 ft. to 33 ft.]
Between 9 and 33 feet, according to the yearly mean of the surface,
the yearly fluctuation ceases and the temperature remains the same
throughout the year.
Below this point an increase of temperature is observable towards the
earth’s centre, which amounts to about 1° C. for every 40 feet.
[Sidenote: Retardation of temperatures with increase in depth.]
Concerning the retardation of the temperatures with an increase in
depth below the surface, it is interesting to point out that this,
according to Fodor, amounts to about three weeks for every yard, so
that the yearly maximum at a depth of 1 yard will take place in
August, at a depth of 2 yards in the beginning of September, and at a
depth of 4 yards in October. This is on the assumption that the
maximum temperature of the atmospheric air is reached in July.
[Sidenote: Frost depth about 3 ft.]
The depth to which frost under ordinary conditions penetrates is about
3 feet, but there are cases on record where water pipes at depths of
from 4 to 5 feet have been frozen up during long continued severe
frost.