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diminishes as the square of the distance increases, the light of Jupiter is proportionally greater than that of the moon, consequently Jupiter's atmosphere reflects more light than the dark volcanic soil of the moon; thus Professor Secchi observes photography may in time reveal the quality of the materials of which the celestial bodies are formed.
The effect of the earth's atmosphere on lunar heat is remarkable. Professor Forbes proved that the direct light of the full moon is incapable of raising a thermometer the one three thousandth part of a Centigrade degree, at least in England; but at an elevation of 8870 feet on the Peak of Teneriffe, Mr. Piazzi Smyth found a very sensible heat from the moon, although she was then 19° south of the equator; so it is no doubt absorbed by our atmosphere at lower levels.
Some exceedingly interesting experiments might be made by means of a telescope having a prism attached to its objective extremity, and furnished with a micrometer, because by it the difference of the illumination of objects might be determined with extreme accuracy-as for example, the comparative intensity between the bright and dark parts of the moon, the comparative intensity of the solar light reflected by the moon, and the lumière cendré, or the light of the earth reflected on the moon, whence a comparison might be made between the light of the sun and that of the earth. Hence also it might be known whether the terrestrial hemispheres successively visible from the moon are more or less luminous, according as they contain more land or water, and at the same time it might be possible to appreciate the more or less cloudy or clear state of our atmosphere, so that in time we might ultimately find in the lumière cendré of the moon data upon the mean diaphaneity of different terrestrial hemispheres which are of different temperatures.
It is found by experience that heat is developed in opaque and translucent substances by their absorption of solar light, but that the sun's rays do not sensibly alter the temperature of perfectly transparent bodies through which they pass. As the température of the pellucid planetary space can be but little affected by the passage of the sun's light and heat, neither can it be sensibly raised by the heat now radiated from the earth.
Doubtless the radiation of all the bodies in the universe maintains the ethereal medium at a higher temperature than it would
otherwise have, and must eventually increase it, but by a quantity so evanescent that it is hardly possible to conceive a time when a change will become perceptible.
The temperature of space being so low as 239° Fahrenheit, it becomes a matter of no small interest to ascertain whether the earth may not be ultimately reduced by radiation to the temperature of the surrounding medium; what the sources of heat are; and whether they be sufficient to compensate the loss, and to maintain the earth in a state fit for the support of animal and vegetable life in time to come. All observations that have been made under the surface of the ground concur in proving that there is a stratum at the depth of from 40 to 100 feet throughout the whole earth where the temperature is invariable at all times and seasons, and which differs but little from the mean annual temperature of the country above. According to M. Boussingault, that stratum at the equator is at the depth of little more than a foot in places sheltered from the direct rays of the sun; but in our climates it is at a much greater depth. In the course of more than half a century the temperature of the earth at the depth of 90 feet, in the cellars of the Observatory at Paris, has never been above or below 53° of Fahrenheit's thermometer, which is only 20 above the mean annual temperature at Paris. This zone, unaffected by the sun's rays from above, or by the internal heat from below, serves as an origin whence the effects of the external heat are estimated on one side, and the internal temperature of the globe on the other.
As early as the year 1740 M. Gensanne discovered in the leadmines of Giromagny, in the Vosges mountains, three leagues from Béfort, that the heat of the ground increases with the depth below the zone of constant temperature. A vast number of observations have been made since that time, in the mines of Europe and America, by MM. Saussure, Daubuisson, Humboldt, Cordier, Fox, Reich, and others, which agree, without an exception, in proving that the temperature of the earth becomes higher in descending towards its centre. The greatest depth that has been attained is in the silver-mine of Guanaxato, in Mexico, where M. de Humboldt found a temperature of 98° at the depth of 285 fathoms, the mean annual temperature of the country being only 61°. Next to that is the Dalcoath copper-mine, in Cornwall, where Mr. Fox's thermometer stood at 68° in a hole in the rock at the
depth of 230 fathoms, and at 82° in water at the depth of 240 fathoms, the mean annual temperature at the surface being about 50°. But it is needless to multiply examples, all of which concur in showing that there is a very great difference between the temperature in the interior of the earth and at its surface. Mr. Fox's observations on the temperature of springs which rise at profound depths in mines afford the strongest testimony. He found considerable streams flowing into some of the Cornish mines at the temperature of 80° or 90°, which is about 30° or 40° above that of the surface, and also ascertained that nearly 2,000,000 gallons of water are daily pumped from the bottom of the Poldice mine, which is 176 fathoms deep at 90° or 100°. As this is higher than the warmth of the human body, Mr. Fox justly observes that it amounts to a proof that the increased temperature cannot proceed from the persons of the workmen employed in the mines. Neither can the warmth of mines be attributed to the condensation of the currents of air which ventilate them. Mr. Fox, whose opinion is of high authority in these matters, states that, even in the deepest mines, the condensation of the air would not raise the temperature more than 50 or 60; and that, if the heat could be attributed to this cause, the seasons would sensibly affect the temperature of mines, which it appears they do not where the depth is great. Besides, the Cornish mines are generally ventilated by numerous shafts opening into the galleries from the surface or from a higher level. The air circulates freely in these, descending in some shafts and ascending in others. In all cases Mr. Fox found that the upward currents are of a higher temperature than the descending currents; so much so, that in winter the moisture is often frozen in the latter to a considerable depth; the circulation of air, therefore, tends to cool the mine instead of increasing the heat. Mr. Fox has also removed the objections arising from the comparatively low temperature of the water in the shafts of abandoned mines, by showing that observations in them, from a variety of circumstances which he enumerates, are too discordant to furnish any conclusion as to the actual heat of the earth. The high temperature of mines might be attributed to the effects of the fires, candles, and gunpowder used by the miners, did not a similar increase obtain in deep wells, and in borings to great depths in search of water, where no such causes
of disturbance occur. In a well dug with a view to discover salt in the canton of Berne, and long deserted, M. de Saussure had the most complete evidence of increasing heat. The same has been confirmed by the temperature of many wells, both in France and England, especially by the Artesian wells, so named from a peculiar method of raising water first resorted to in Artois, and since become very general. An Artesian well consists of a shaft a few inches in diameter, bored into the earth till a spring is found. To prevent the water being carried off by the adjacent strata, a tube is let down which exactly fills the bore from top to bottom, in which the water rises pure to the surface. It is clear the water could not rise unless it had previously descended from high ground through the interior of the earth to the bottom of the well. It partakes of the temperature of the strata through which it passes, and in every instance has been warmer in proportion to the depth of the well; but it is evident that the law of increase cannot be obtained in this manner. Perhaps the most satisfactory experiments on record are those made by MM. Auguste de la Rive and F. Marcet during the year 1833, in a boring for water about a league from Geneva, at a place 318 feet above the level of the lake. The depth of the bore was 727 feet, and the diameter only between four and five inches. No spring was ever found; but the shaft filled with mud, from the moisture of the ground mixing with the earth displaced in boring, which was peculiarly favourable for the experiments, as the temperature at each depth may be considered to be that of the particular stratum. In this case, where none of the ordinary causes of disturbance could exist, and where every precaution was employed by scientific and experienced observers, the temperature was found to increase regularly and uniformly with the depth at the rate of about 1° of Fahrenheit for every 52 feet. Professor Reich of Freyberg has found that the mean of a great number of observations both in mines and wells is 1° of Fahrenheit for every 55 feet of depth; and from M. Arago's observations in the Grenelle Artesian well at Paris, the increase is 1° of Fahrenheit for every 45 feet. Though there can be no doubt as to the increase of temperature in penetrating the crust of the earth, there is still much uncertainty as to the law of increase, which varies with the nature of the soil and other local circumstances; but, on an average, it has been
estimated at the rate of 1° for every 50 or 60 feet, which corresponds with the observations of MM. Marcet and De la Rive. In consequence of the rapid increase of internal heat, thermal springs, or such as are independent of volcanic action, rising from a great depth, must necessarily be very rare and of a high temperature; and it is actually found that none are so low as 68° of Fahrenheit; that of Chaudes Aigues, in Auvergne, is about 136°. In many places warm water from Artesian wells will probably come into use for domestic purposes, and it is even now employed in manufactories near Stutgardt, in Alsace, &c.
It is hardly to be expected that at present any information with regard to the actual internal temperature of the earth should be obtained from that of the ocean, on account of the mobility of fluids, by which the colder masses sink downwards, while those that are warmer rise to the surface. Nevertheless, it may be stated that the temperature of the sea decreases with the depth between the tropics; while, on the contrary, all our northern navigators found that the temperature increases with the depth in the polar seas. The change takes place about the 70th parallel of latitude. Some ages hence, however, it may be known whether the earth has arrived at a permanent state as to heat, by comparing secular observations of the temperature of the ocean if made at a great distance from the land.
Should the earth's temperature increase at the rate of 10 for every 50 feet, it is clear that at the depth of 200 miles the hardest substances must be in a state of fusion, and our globe must in that case either be encompassed by a stratum of melted lava at that depth, or it must be a ball of liquid fire 7600 miles in diameter, enclosed in a thin coating of solid matter; for 200 miles are nothing when compared with the size of the earth. No doubt the form of the earth as determined by the pendulum and arcs of the meridian, as well as by the motions of the moon, indicates original fluidity and subsequent consolidation and reduction of temperature by radiation; but whether the law of increasing temperature is uniform at still greater depths than those already attained by man, it is impossible to say. At all events, internal fluidity is not inconsistent with the present state of the earth's surface, since earthy matter is as bad a conductor of heat as lava, which often retains its heat at a very