4 degrees. Although in Artesian wells there
are sometimes slight errors from the lateral permeation of water, these
errors are less injurious to the accuracy of conclusions than those
resulting from currents of cold air, which are almost always present in
mines. The general result of Reich's great work on the temperature of the
mines in the Saxony mining districts gives a somewhat slower increase of the
terrestrial heat, or 1 degree to 76.3 feet. (Reich, 'Beob. uber die
Temperatur des Gesteins in verschielen en Tiefen', 1834, s. 134.) Phillips,
however, found (Pogg., 'Annalen', bd. xxxiv., s. 191), in a shaft of the
coal-mine of Monk-wearmouth, near Newcastle, in which, as I have already
remarked, excavations are going on at a depth of about 1500 feet below the
level of the sea, an increase of 1 degree to 59.06 feet, a result almost
identical with that found by Arago in the Puits de Grenell.
We must distinguish in our globe three different modes for the transmission
of heat. The first is periodic, and affects the temperature of the
terrestrial strata according as the heat penetrates from above downward or
from below upward, being influenced by the different positions of the Sun
and the seasons of the year. The second is likewise an effect of the Sun,
although extremely slow: a portion of the heat that has penetrated into the
equatorial regions moves in the interior of the globe toward the poles,
where it escapes into the atmosphere and the remoter regions of space.
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