T he general principles regulating soil temperature are well known. Measurements in deep mines and wells indicate the existence of a temperature gradient in the earth's crust from the interior outwards, which causes a flow of heat to the surface at a sensibly constant rate. Extensive measurements were carried out by Forbes1 and by William Thomson (afterwards Lord Kelvin) who worked up the former's experimental values, and later by a Committee of the British Association, which concluded that an average of “41·4 gramme-degrees of heat escape annually through a sq. cm. of a horizontal section of the earth's substance.” For our present purpose this source of heat is negligible.
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Keen et al. (1921) studied this question.