large stones, and raise them to the height of 3 or 4 ells, with a noise like the explosion of a great gun. From the foregoing history, we may take occasion to reflect a little on the variety of exhalations prepared in and flying out from the vast subterraneous magazines and repositories, as to their qualities and effects, some being cold and dry, resembling air or wind, as those near Peroul, and in the caverns of mountains, especially those of Æolus, and other hills of Italy, as also in mines; others are inflammable, and of a bituminous nature, though not actually warm, as those near Wigan in Lancashire; there are also many steams very hot, sulphureous, and saline, more especially those in the natural stoves, sweating vaults, grots, baths, and the volcanos near Naples, Bajæ, Cuma, and Puzzuolo, as also in some of the subterraneous works at Rome; others there are of an arsenical and such like noxious qualities, as in the Grotta del Cane, on the bank of the Lago Agnano; in several mines, and in poisonous springs and lakes. Now these various steams meeting with, and running through waters, must cause a great variety of phænomena and effects in them." agency of volcanos, There are many hot equable, and which Many of these depend obviously upon the and are immediately connected with them. springs, however, whose temperature is too occur at too great a distance from any known volcanos to be produced by them. Thus the hot-spring at Bath," observes Dr. Thomson*, has continued at a temperature higher than that of the air for a period not less than 2000 years; yet it is so far from any volcano, that we cannot, without a very violent and improbable extension of volcanic fires, ascribe it to their energy. There are various decompositions of mineral bodies, which generate considerable heat. These decompositions are usually brought about by means of water; or, to speak more properly, water is itself the substance which is decomposed, and which generates heat by its decomposition. Thus, for example, there are varieties of pyrites, which are converted into sulphate of iron, by the contact of water, and such a change is accompanied by an evolution of heat. Were we to suppose the Bath spring to flow through a bed of such pyrites, its heat might be occasioned by such a decomposition. Such, probably, is * Ilistory of the Royal Society, b. I. ch. iii. the way in which those mineral springs, that contain sulphureted hydrogen gas, receive their impregnation. But we are pretty certain, that such a supposition will not apply to Bath water: first, because it does not contain the notable quantity of sulphate of iron, which would be necessary upon such a supposition; and, secondly, because instead of sulphureted hydrogen gas, which would infallibly result from such a decomposition of pyrites, there is an evolution of azotic gas. This evolution of azotic gas, however, is a decisive proof that the heat of Bath waters is owing to some decomposition or other, which takes place within the surface of the earth; though, from our imperfect acquaintance with the nature of the mineral strata, through which the water flows, we cannot give any satisfactory information about what that decomposition actually is." EDITOR. 2. On the Temperature of the Earth below the Surface, in regard to Springs and Hills, and especially those of Jamaica. By John Hunter, M.D. F.R.S. "The great difference, says Dr. H. between the temperature of the open air, and that of deep caverns or mines, has long been taken notice of, both as matter of curiosity and surprize. After thermo. meters were brought to a tolerable degree of perfection, and meteorological registers were kept with accuracy, it became a problem, to determine what was the cause of this difference between the heat of the air and the heat of the earth; for it was soon found that the temperature of mines and caverns did not depend on any thing peculiar to them; but that a certain depth under ground, whether in a cave, a mine, or a well, was sufficient to produce a very sen. sible difference in the heat. In observations of this kind, there was perhaps nothing more striking, than that the heat in such caves was nearly the same in summer and winter; and this even in changeable climates, that admitted of great variation between the extremes of heat in summer, and cold in winter. There is an example of this in the cave of the Royal Observatory at Paris. The explanations, which have been attempted of this phænomenon, have turned chiefly on a supposition, that there was an internal source of heat in the earth itself, totally independent of the influence of the sun. M. de Mairan has bestowed much labour on this subject, and by obser * Vid. Martine's Essays, p. 319. vation and calculation is led to conclude, that of the 1026° of heat, by Reaumur's scale, which he finds to be the heat of summer at Paris, 34°.02 only proceed from the sun, and the remaining 991°.98 from the earth, by emanations of heat from the centre *. The proportion therefore of heat derived from this latter source is to that of the sun, as 29.16 to 1. It must be evident that an hypothesis of this kind, which renders the influence of the sun of small account, is directly contrary to the general experience and conviction of mankind. Without entering however into any discussion of the data from whence M. de Mairan draws his conclusions, it will be more satisfactory to consider what would be the effect of the operation of those laws of heat with which we are acquainted. And first, it is well known, that heat in all bodies has a tendency to diffuse itself equally through every part of them, till they become of the same temperature. Again, bodies of a large mass are both cooled and heated slowly. Besides the mass of matter, there are two other considerations of much importance in the slow or quick transmission of heat through bodies; these are their different conducting powers, and their being in a state of solidity or fluidity. The conducting powers of heat are well known to be very various in different bodies; nor are they hitherto reducible to any law, depending either on the density or chemical properties of matter. Metals of all kinds are good conductors of heat, while glass, a heavy, solid, homogeneous body, is an extremely bad conductor, even when a metallic calx enters largely into its composition, as in flint-glass. A state of fluidity greatly promotes the diffusion of heat; for a body in a fluid state, by the particles moving readily among each other from their different densities or other causes, mixes the warm and cold parts together, which occasions a quick commu nication of heat. To apply these observations to the present subject; the surface of the earth being exposed to the great heats of summer, and the colds of winter, or more properly the low degree of heat of winter, will receive a larger proportion of heat in the former season, and a smaller in the latter; and being further of a large mass, and of a porous and spongy substance, and therefore not quickly sensible to small variations of heat, it will become of a mean temperature at a certain depth, between the heat of summer and the cold of winter, provided it contain no internal source of * Memoir de l'Acad. des Sciences, An. 1719 et 1765. heat within itself. This conclusion is strictly agreeable to the experiments and observations hitherto made, in heating and cooling bo dies, or in mixing portions of matter of the same kind of different temperatures. Water, though in a large mass, follows in some degree the heat and cold of our summer and winter, from the mobility of its parts occasioning a more speedy diffusion of heat. Air is quickly susceptible of heat, and from the expansions produced in it, and consequent motions in the whole mass, the temperature is soon rendered uniform. The changes in the heat of the air are what we have measured, and we are to be understood to speak of them, when we talk of the temperature of summer and of winter. It may be asked then, is the heat of the sun first communicated to the air, and thence to the earth? No, the air is susceptible of a very small degree of heat from the rays of the sun passing through it; for it is well known that they produce no heat in a transparent medium, and consequently, that the air is only so far heated as it differs from a me. dium that is perfectly transparent. The heat produced by the rays of the sun bears a proportion to their number, their duration, and their angle of incidence; and it takes place at the points where they strike an opaque and non-reflecting surface. The surface of the earth may therefore be considered as the place from which the heat proceeds, which is communicated to the air above, and the earth below. That this is really the case, is evident from the superior degree of heat produced by the action of the rays of the sɛn on au opaque body, which will often be heated to 150° of Fahrenheit, while the temperature of the air is not above 90° t. It may seeu, therefore, that to measure the heat communicated to the earth, it should be done at the surface, where the action of the rays immediately takes place. But though the heat be produced at the surface, it is communicated freely to the air as well as the earth; and though the apparent intensity of heat be greater in the earth, from the rays of light acting for a longer time on the same parts of inatter, yet, there is little doubt that much the greater part is carried off by the air, which as it is heated flies off, and allows a fresh portion of cold air to come in contact with the heated surface. But still it * De Luc Modifications de l'Atmosphere, vol. I. p. 285. 1 is immaterial, whether the heat of the sun be excited more in the earth or in the air; for whichever has the larger proportion will in the end communicate a part to the other, and so restore the balance. The same observation applies to such causes of cold as may operate at the surface of the earth, as evaporation, &c. The air therefore, near the surface of the earth, will show by a thermometer in the shade nearly, if not exactly, the same degree of heat that the sun communicates to our terrestrial globe; and if a mean of the heats thus shown be taken for the year round, and we penetrate into the earth to that depth, that it is no longer affected either by the daily, monthly, or annual variations of heat, the temperature at such depth should be equal to the annual mean above mentioned. To ascertain this with the utmost precision, it must be obvious that numerous observations should be made every day, corresponding to the fre. quent changes of temperature, which are known to happen in the course of 24 hours in all climates; and on these a daily mean should be taken, and the annual mean deduced from them. This has not yet been done, but where we have observations from which a mean temperature can be deduced with any degree of certainty, it will be found not to differ greatly from the heat of deep caves, or wells in the same climate. 4 For obtaining the temperature of the earth, the best observations are probably to be collected from wells of a considerable depth, and in which there is not much water. Springs issuing from the earth, though indicating the temperature of the ground from which they proceed, are not so much to be depended on as wells; for the course of the spring may be derived from high grounds in the neighbourhood, and it will thence be colder; it may run so near the sur. face as to be liable to variations of heat and cold from summer and winter; or it may be exposed to local causes of heat in the bowels of the earth. Wells seem also better than deep caverns, for the apertures to such are often large, and may admit enough of the external air to occasion some change in their temperature. Wells are not however to be met with in all places, and in that case we must remain satisfied with the temperature of the springs. The following observations were made in the island of Jamaica, where there are flat lands in many parts towards the coast, but all the interior part of the country is mountainous. The heat is greatest in the low lands, and decreases as you ascend the mountains. The |