Volcanoes, which eject molten and white-hot lavas, and thermal springs, which pour out floods of hot and even boiling water, plainly indicate that the interior of the earth is highly heated, at least along certain lines. Direct observations tend to prove that this high temperature is universally diffused through the earth's mass. For a short distance below the surface of the ground the temperature varies, like that of the air, though not so greatly, between day and night. Farther down, the daily variation ceases, but there is a seasonal variation, also with a less extreme range than the seasonal differences of the air-temperatures. The almost constant temperature of deep cellars and ice-houses is a familiar fact. At a still greater depth is reached a level where the temperature remains the same throughout the year, and is but slightly in excess of the average annual temperature of the air above ground at a given locality. Evidently the temperature at the level of no variation is determined by the solar heat and other climatic factors, and its depth depends upon the range of temperature changes in the air.

In the tropics, with their uniform degree of heat, the level of no variation is only three or four feet below the surface, and much the same is true of the polar regions, where the ground is permanently frozen to a depth of several hundred feet, but in the temperate zones this level is much deeper; generally speaking, the depth increases with the latitude and at New York is about fifty feet, but the level again rises toward the surface as the polar regions are approached.

Even at the level of no variation the inherent heat of the earth makes itself apparent, and below this level the temperature increases with the depth, though at very different rates in different places. Thus, in Great Britain the rate of increment varies between 1° F. for every 30 feet of descent and i° for every 90 feet. Increasing heat with increasing depth is observed in all deep borings, tunnels and mines, and often has completely checked any further penetration. The levels at which the great tunnels under the Alps were placed were determined chiefly by considerations of temperature, as it was necessary to avoid a degree of heat in which men could not work.

The deepest borings in the world are those in Prussia, some of them considerably exceeding a mile in depth. Observations made in these borings give an average increment of about i° F. for every 60 feet. Should this rate be continued regularly, it would reach, at a depth of 35 miles, a heat sufficient to melt almost any known rock, at atmospheric pressure. However, the available observations are much too superficial to permit the formulation of any general law, farther than the establishment of the fact of the universal increment of temperature with descent into the earth.