The mean heat of the earth, above the stratum of constant temperature, is determined from that of springs; and, if the spring be on elevated ground, the temperature is reduced by computation to what it would be at the level of the sea, assuming that the heat of the soil varies according to the same law as the heat of the atmosphere, which is about 1° of Fahrenheit’s thermometer for every 333·7 feet. From a comparison of the temperature of numerous springs with that of the air, Sir David Brewster concludes that there is a particular line passing nearly through Berlin, at which the temperature of springs and that of the atmosphere coincide; that in approaching the arctic circle the temperature of springs is always higher than that of the air, while, proceeding towards the equator, it is lower. Since the warmth of the superficial strata of the earth decreases from the equator to the poles, there are many places in both hemispheres where the ground has the same mean temperature. If lines were drawn through all those points in the upper strata of the globe which have the same mean annual temperature, they would be nearly parallel to the equator between the tropics, and would become more and more irregular and sinuous towards the poles. These are called isogeothermal lines. A variety of local circumstances disturb their parallelism, even between the tropics. The temperature of the ground at the equator is lower on the coasts and islands than in the interior of continents; the warmest part is in the interior of Africa; but it is obviously affected by the nature of the soil, especially if it be volcanic.