26 June 1 13 July 223 20 25 28 56 13 58 IN a former Number we gave some account, from Professor Leslie's paper May 18 on Climate, of the curious experiments which had been instituted at Raith, on the temperature of the ground at various depths, with the view of illustrating the progress of the sun's heat, from the surface towards the centre of the earth. Thermometers, we saw, with long stems, had been sunk to various depths, and the temperatures which they indicated had been observed from time to time. The register of these observations we exhibited in the form of curve lines, and we endeavoured to point out some of the conclusions which might be drawn from it. It was remarked how curiously each of the thermometers, even to the small depth of eight feet, contracted its range of heat throughout the year, the deepest never rising so high in summer, nor falling so low in winter, as any of those nearer the surface. We noticed the slow progress of the heat downwards, as it was indicated by the effect of the depth in retarding the thermometer's arrival at its maximum and minimum, the extreme heat of July, and the extreme cold of January, never in the least affecting the eight feet deep thermometer till the months of October and March, so that the intervening three months must have been spent in travelling over this very inconsiderable space. We pointed out also the curious circumstance of the thermometers, however they might differ atother times,—of their all agreeing with each other at two stated periods of the year, and we endeavoured to explain the reason of these remarkable coincidences. In short, it seemed impossible to devise March 1 a simpler, more striking, or more accurate method than the observation of 48 49 494 51 511 514 51 50 50 22 56 50 501 51 513 45 50 Nov. Jan Feb. 1123 such thermometers for exhibiting the April 1 VOL. IV. May 34 40 8 42 6811 81861 6281 881|8885 14 44 43 44 31 49 48 36 showed, to the inconstancy of the supply of heat, joined to the constant tendency of this fluid to diffuse itself equally ` among bodies-to move from the hotter to the colder, till a uniform temperature is spread throughout, so that, if the supply is at any time much enfeebled, as in winter, the accumulation of the preceding summer returns to make up the deficiency. When the sun, therefore, in spring gives rise at the surface of the earth to a continued flow of heat downwards, this is met by an opposite current flowing upwards from the supply of the preceding year; their joint effect soon produces an equilibrium throughout the mass, and this equilibrium takes place, according to the above observations, about the months of April or May, when the thermometers, from the predominating influence of spring, are all somewhat below the mean temperature of the place. That these opposite motions of the fluid of heat near the surface of the earth really take place there can be no doubt, and the variations of these thermometers place it in a very striking point of view. In December 1815 the thermometer stood as follows: Dec. 1. 8 Ft. 4.70 10. 20. 29. The observations of 1818 are entirely conformable to those of the preceding years, and illustrate, in a similar inanner, all the above appearances; but, to show more clearly the peculiar effect of each year, compared with the others, we have combined the whole series into one plate, (see Plate.) The dark waving line which occupies nearly the whole compass of the plate represents the gradation of the thermometer which was sunk one foot in the ground, and the other lines, as explained in the plate itself. It is remarkable in these years how greatly all the lines, but especially those of the deeper thermometers, how much more they sink below than rise above the line of the mean temperature; and this shows, in a striking manner, the low mean temperature of these years, not even excepting 1818, of which the summer was so very hot. It is not improbable, therefore, that, in the present year, the lines will rise conderably above that of the mean temperature, from the warmth of the preceding summer, the mildness of the winter, and perhaps also on account of the additional supply which a series of cold years naturally draws up from the great reservoir below. The fluctuations to which the thermometers are occasionally subject occur, it will be seen, mostly at the extremes either of winter or summer; and this is what might be expected, for, in the intervals between their change of direction, before they have begun decidedly to rise on the one hand, or fall on the other, the least Jan. 1. circumstance, and, therefore, the ordinary fluctuations of the climate, is fully sufficient to turn the balance. Accordingly, after a few oscillations about the middle of summer or of winter, the lines then hold a pretty steady course from the one extreme to the other. In regard to the coincidence in our climate of all the lines at the beginning and end of summer, this must be ascribed, as we formerly Here the ground was evidently warmer in proportion to the depth; and, as all the thermometers were also on the decline, it is obvious that the heat surface; and nothing, therefore, can was flowing from the deepest to the show more clearly the reality of the current which the coldness of the at mosphere was thus raising up from below. In January 1816 they stood as follows: 12. 28. Feb. 12. 27. January the two feet thermometer became stationary, and continued so till March 10, when it also began to rise. On the 28th January the four feet one became stationary, and on March 10 began to rise; and, lastly, the eight feet thermometer became stationary on January 28, and on March 21 began to rise. Here, therefore, we trace clearly the beginning, the gradual progress, and the growing intensity of the stream of heat, which the now increasing influence of the sun had begun to send downwards from the surface; a uniform temperature spreads through the mass about the middle of May, and shortly afterwards the deepest thermometer, formerly the hottest, becomes the coldest, and the arrangement of the heat of all the three is exactly reversed, as they stood on May 26 1 Ft. 48° 2 Ft. 46° 8 Ft. 44° 4 Ft. 444° All the thermometers now continued rising till the end of summer, when the intermediate ones became the highest, and in a short time they all coincided, so that the extreme heat of summer, as it penetrated below the surface, seemed thus also partly to return-the wave of heat in its progress seemed gradually, at the same time, to subside, and to spread its influence equally over the superficial stratum of the ground. Thus, in July 1817, the thermometer stood as follows: Here the decline of the first thermometer, the extreme heat of the intermediate ones, and the gradual approach of them all to an equality, are apparent; and, if the thermometers had been deeper, the dispersion of the summer's heat, as it descended, would no doubt have been still more distinctly exhibited. Such, in our climate at least, is the slow and irregular, but, on the whole, the steady progress of the sun's heat towards the interior of the earth, and such are the curious and interesting facts exhibited even by this single register of observations. It is of great consequence, therefore, we should think, that experiments of a similar kind be made in different latitudes and in different situations. The circumstance of the thermometers all coinciding at the beginning and end of summer, may be of great use on many occasions, in approximating with expedition to the mean temperatures of places, and, therefore, at the same time, to their respective elevations above the level of the sea. But the period of these coincidences will undoubtedly vary in different latitudes, and it would be interesting, therefore, to know the extent of the variation. An approximation to the relative height of places might also be obtained in a manner somewhat different, and which might often prove useful to the traveller, in case a more exact method were beyond his reach. So slowly, it appears, does the heat and cold penetrate into the ground, that the vicissitudes of day and night are quite insensible at the depth of 1 foot. depth, will remain several days, and A thermometer, even at this small often whole weeks together, at the same degree of heat. It is obvious, then, that by thus observing the tem perature of the ground at the bottom and the top of a mountain, a pretty correct idea might be formed of its height, even though several days should elapse between the observations; and this method was often practised by the celebrated Saussure. It would be important, therefore, to obtain observations of this kind on heights already known, in order to discover, by actual experiment, how far they might in other cases be trusted. It may be remarked, indeed, in general, that observations on the temperature of the ground are, in most cases, far more valuable than those on the temperature of air, of which such numbers have been made to but little purpose. The former being more exempt from temporary vicissitudes, give us more expeditiously the medium of all. Collecting and retaining the variable impressions of heat, the earth shows at once the total sum of the whole; while the air being subject to every the slightest inequality, the observations on its temperature require to be made at regular intervals, and for a great length of time, before any information of consequence can be drawn from them. This con |