UNDERGROUND TEMPERATURES. In the process of cooling the earth still re tains a vast volume of heat. This heat is not appreciable at the surface where air tempera tures prevail but it is manifest in deep mines and borings which show a general increase of temperature with depth. The rate of increase is by no means regular but varies from place to place and in some cases it changes with depth. In general it ranges from 40 to 80 feet to 1° F. and the average indicates that at a depth of 25 miles the temperature is about 2,200° F. or above the melting point of iron and rocks. It is difficult to conceive how great the maximum may be farther down. This heat is believed to be a residual of that which was present when the earth was in molten and gaseous conditions. Much has been lost by the cooling-off process that is still in operation. Volcanoes may be evidence of the general internal heat although it is more likely that they are manifestations of some local heat conditions probably developed at moderate depths.
A large number of observations have been made of underground temperatures in deep mines, wells,.bore holes and tunnels in various parts of the world. Many of these have been taken with great accuracy and others are ap proximations. Some representative ones are given at the end of this article.
Rates of increase of earth temperature are calculated from the difference between the mean annual air temperature and the observed tem perature at the given depth, it being assumed that the temperature for the first 50 feet below the surface is that of the mean annual air tem perature. It has been found, however, that the latter figure is somewhat too low, the discrepancy varying with latitude and various local condi tions. The difference is about 11/2° in a humid climate like that of England and 21/2° to 3N° in a dry climate such as that of northern Af nca_ In some cases the temperature at some shallow depth is directly determined and used for the comparison. In many instances temperatures are taken of various depths and the results plotted in a curve which is independent of the surface temperature. Many temperatures have been obtained of flows from deep artesian wells and also of flows from pumped wells. In these
it is necessary to know the depth of the source of the water and the flow must be large in volume and long continued. The thermometers used are of several kinds but ordinarily self registering maximum mercury thermometers or thermo-electric instruments are lowered in the borings or sunk in holes in deep mines. In deep holes, convection, gas expansion and heat from recent boring vitiate the result. The influx of deeper waters, proximity of recent volcanic rocks, lodes in which mineralization is in progrtss and planes or zones of movement have great influence, the last especially in fa cilitating rise of deep seated underground water. In mines, especially coal mines, oxidation ma terially increases the heat. In the upper part of the Anzin coal mines, in France, the tem perature is 15° F. above the normal.
One of the most striking features of the rates of underground temperature increases as observed in borings and mines is their varia tion from place to place. Undoubtedly some of these variations are due to imperfect ob servations but most of them are related to local causes of which the nature is not known. At tempts have been made to show that rates are lower in the mountains, higher in petroleum areas and coal fields and various other general conditions but in most cases these relations are not borne out when many observations are as sembled. There are undoubtedly in the earth many factors which influence the rate of tem perature increase such as variation in conduc tivity of various rocks, underground tension. mineralization and movement of underground water. Local volcanic action of course is a most powerful agent but only present in certain areas where its influence generally is clearly apparent. Variation in radioactivity has been suggested as a factor, and the influence of bodies of cold water such as Lake Superior and former presence of glacial ice as causes of local diminution of earth temperature. Positive evidence is lacking along all these lines, however, and the results of extended special investigations must be awaited before the true explanation is attained.