It is found, counting from the invariable layer, that the temperature increases about one degree of Fahrenheit’s scale for every fifty feet in depth. Thus, a well having been sunk at Rudersdorff to a depth of 630 feet, the water at the bottom was found to be 67 degrees, while the mean temperature was 50 degrees. In a coal mine at Newcastle, which reaches to a depth of 1584 feet, the mean temperature of the surface being 48 degrees, the thermometer was found to stand at 73 degrees in the lowest part of the mine, and hence the elevation of temperature was 25 degrees. Observations elsewhere vary between these limits; but the general result is, that the rise is one degree for about every fifty feet, as above stated. When we consider the great magnitude of the earth, and observe the rapidity with which the increase of temperature occurs, it will strike every person that we in reality inhabit a mere pellicle or skin, which has formed by cooling upon the surface, whilst all the internal mass of our globe may still be in the same state of igneous fusion and tumultuous action of elements, from which its present mineral constitution on the surface has resulted. For although it has cooled so far that at the surface all traces of its central fires have disappeared, yet at a mile and a half below the surface the temperature is such as should boil water: at a depth of five miles, lead would melt. Thirty miles below the surface, cast iron, and all those rocks which are generally the product of volcanoes in action, as trap and basalt, would fuse; and hence we may consider those terrific phenomena which have so frequently desolated some of the most beautiful districts of the earth, as being minute apertures or cracks in the thin coating of our planet, and giving vent from time to time to some small portions of the internal fires which work beneath. Additional evidence of the existence of this central heat may be derived from the peculiarity of springs. Those springs which carry off and are supplied with water from the surface, change their temperature with the season, being in winter cold, but in summer warm. Others, deriving their waters from a deeper layer of soil, as from the stratum of constant heat, are always the same, and, possessing the mean temperature of the place, feel warm in winter and cold in summer. Such springs exist in every country, and are very useful in ascertaining the mean temperature, for in place of watching a thermometer for a year, and taking averages, it is only necessary to select with proper caution such a deeply supplied spring, and by observing the temperature of its waters, the mean temperature of the place is found.