If we dig down still farther, the earth, though having lost the heating power of the sun, becomes sensibly warmer. The greater the depth to which we descend, the higher is the temperature found to be. Thus, where deep sinkings have been made for mines or wells, the air or water at the bottom is found to be much higher in temperature than at the invariable layer which gives the mean temperature of the place. This observation was first made in the case of the deep mines in Cornwall, and, although for some time ascribed to the presence of the workmen and the burning lamps, has since been verified by observations in all parts of Europe, and such agreement found, that the law connecting the temperature with the depth has been at least approximately determined.
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.
A certain quantity of the water which is absorbed by the ground after rain must penetrate to a great depth, must descend, in fact, until at 1½ miles it is boiled and driven up again to find some outlet as a spring. In rising up, however, it is for the most part cooled; but having charged itself with various saline and metallic bodies, under the most favourable circumstances of high temperature and pressure, it issues as a hot mineral spring or spa. On getting into the air, it generally abandons a great part of what it had dissolved, and forms in many cases enormous depositions of solid rock.
A company in Paris have formed the idea of using the water thus heated by the powers below, for the purposes of public baths. The neighbourhood of Paris is peculiarly fitted for what are termed Artesian wells, in which the water often rises considerably above the surface of the ground. Under the auspices of this company, a well has been sunk already to the depth of 1600 feet, and water obtained at 77 degrees; but to obtain natural hot water at a temperature of 100 degrees, which would be required for bathing purposes, an additional depth of probably as much more will be required. It is said the projectors are not now sanguine of its pecuniary success.
The Secret of Success in Life.—In no department of life do men rise to eminence who have not undergone a long and diligent preparation; for whatever be the difference in the mental power of individuals, it is the cultivation of the mind alone that leads to distinction. John Hunter was as remarkable for his industry as for his talents, of which his museum alone forms a most extraordinary proof; and if we look around and contemplate the history of those men whose talents and acquirements we must esteem, we find that their superiority of knowledge has been the result of great labour and diligence. It is an ill-founded notion to say that merit in the long-run is neglected. It is sometimes joined to circumstances that may have a little influence in counteracting it, as an unfortunate manner and temper; but generally it meets with its due reward. The world are not fools—every person of merit has the best chance of success; and who would be ambitious of public approbation, if it had not the power of discriminating?—Physic and Physicians.
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