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The next morning they found the paper precisely where they had left it, but with about one-fifth of one of the leaves gone, and the broken margin of the remainder apparently nibbled. There was nothing to prevent the whole from being taken off, and it was noted that, though left in a precarious position, it had not fallen down. The broken leaf was then torn off and preserved, whilst the unbroken one was allowed to remain as a further experiment. The next morning no trace of it was to be seen. That evening a rat-trap was set at the spot, and very near it another leaf of paper was placed, having on it a small stone, which it was supposed a rat, but not a smaller animal, might be capable of moving. The next morning the paper was found where it had been put, but very much nibbled, whilst the trap and the grease with which it was baited appeared to have not been touched. Before leaving work, the men baited the trap with a tempting end of candle, and placed it on a leaf of paper ; whilst another leaf, weighted with a lump of earth, was placed near. On the following morning both pieces of paper were found to be considerably eaten or torn; and it was noted that the injury done to the former was within the margin of the trap placed on it, whilst the trap itself, as well as its bait, remained unaffected, further than that there were on it a few spindle-shaped fæces about a quarter of an inch long. There can bo no doubt that some animal, probably smaller than a rat, carried off the missing leaf to a recess in the Cavern, where it may serve to make its nest comfortable, and perhaps hereafter to puzzle a cavern searcher who may discover it.
R.S., G. T. s; Prof. J. CLERK
ELKIE, F.R.S., EWART, F.R.S., D
Fourth Report of the Committee for the purpose of investigating the
rate of Increase of Underground Temperature downwards in various Localities of Dry Land and under Water. Drawn up by Prof. EVERETT, at the request of the Committee, consisting of Sir Wm. Thomson, F.R.S., Sir CHARLES LYELL, Bart., F.R.S., Prof. J. CLERK MAXWELL, F.R.S., Prof. PHILLIPS, F.R.S., G. J. Symons, F.M.S., Dr. Balfour STEWART, F.R.S., Prof. RAMSAY, F.R.S., Prof. A. GEIKIE, F.R.S., JAMES GLAISHER, F.R.S., Rev. Dr. GRAHAM, E. W. BINNEY, F.R.S., GEORGE Maw, F.G.S., W. PENGELLY, F.R.S., S. J. MACKIE, F.G.S., EDWARD Hull, F.R.S., and Prof.
EVERETT, D.C.L. (Secretary). In last year's Report, the intention was expressed of boring down at the bottom of Rosebridge Colliery, if the Association would provide the necessary funds. The circumstances were exceptionally inviting, and the Association very liberally granted the sum asked. The Secretary thereupon paid two visits to Rosebridge, descended and to some extent explored the colliery, in company with Mr. Bryham, and, after a careful study of the plans and sections, agreed upon a particular spot where the bore was to be sunk. Tracings of the plans and sections were kindly sent by Mr. Bryham, who in every way cooperated most cordially, and gave much valuable assistance in arranging the scheme of operations. Several weeks elapsed, which were occupied in making and testing a very large spirit thermometer, suitable for reading in the bad light of a mine, and capable of being read, by estimation, to the hundredth of a degree, from 90° to 110° F.; and on the 7th November the Secretary wrote to Mr. Bryham requesting him to commence operations. Unfortunately, during this brief interval, circumstances had changed. In a neighbouring pit, where the workings were in the same seam of coal as at Rosebridge, though less deep by 200 yards, a considerable quantity of water was found in sinking into the strata underlying this seam. This was a very unexpected circumstance; and as any irruption of water at the bottom of Rosebridge pit, which is now quite dry, would be a most serious affair, Mr. Bryham was afraid to risk the experiment of boring down. Subsequent reflection has only confirmed him in the opinion that such a step would be hazardous, and the Committee have accordingly been most reluctantly compelled to renounce the plan. Mr. Bryham's final refusal was received on the 28th February.
Professor Ansted read a paper last year, in the Geological Section of the Association, upon the Alpine tunnel, commonly called the Mont-Cenis tunnel, and in that paper some interesting statements were made regarding its temperature. Since that time, Professor Ansted has interchanged very numerous letters with the Secretary, and bas furnished much valuable information, gathered from Prof. Sismonda, of Turin, and from M. Borelli, the resident engineer of the tunnel. Observations which appear to be reliable have been made in bore-holes in the sides of the tunnel, and the temperatures thus observed have beon compared with the estimated mean temperature at the surface overhead, which in the highest part is a mile above the tunnel, or 2905 metres above sea-level. It is directly under this highest part that the highest temperature is found in the walls of the tunnel, namely 29o.5 C., or 85°.1 F., which is 9° F. lower than the temperature found at the bottom of the Rosebridge shaft at the depth of only 815 yards. But though the tunnel is at more than double this depth from the crest of the mountain over it, we must bear in mind that the surface-temperatures are very different. In a paper published by the engineer of the tunnel, M. F. Giordano, the mean temperature of the air at the crest of the mountain (Mont Frejus) is calculated to be -20.6 C., or 270.3 F. Assuming this estimate to be correct, we have a difference of 570.8 F. between the deepest part of the tunnel and the air at the surface vertically over it; assuming further, as we did in the case of Rosebridge in last year's Report, that the surface of the hill itself has a mean temperature 1° F. lower than that of the air above it, we have a difference of 560.8 F., and the thickness of rock between is 1610 metres, or 5280 feet (exactly a mile). This gives, by simple division, a rate of increase of 1° F. for 93 feet; but a very large correction must be applied for the convexity of the ground; for it is evident that a point in the ground vertically under a steep crest is more exposed to the cooling influence of the air than a point at the same depth beneath an extensive level surface. No correction for convexity would be needed if the temperature of the air decreased upwards as fast as the temperature of the internal rock; but this is very far from being the case, the decrease being about 3 times more rapid in the rock than in the air. To form an approximate notion of the amount of this correction, we must determine, as well as we can, the forms of the successive isothermal surfaces in the interior of the mountain. The tendency is for all corners and bends to be eased off as we descend, so that each succeeding isothermal surface is flatter than the one above it. Accordingly, if we have a mountain rising out of a plain, without any change of material, the isothermals will be further apart in a vertical through the crest of the mountain than under the plain on either side; they will also be further apart at the highest part of this vertical, that is close under the crest, than at a lower level in the same vertical. It would be absurd to pretend to fix the amount of the correction with accuracy; but it seems pot unreasonable to estimate that, in the present case, the numer of isothermals cut through by a vertical line descending from the crest of the ridge to the tunnel itself is about seven-eighths of the number which would be cut through in sinking through an equal distance in level ground, other circumstances being the same. Instead of 1° in 93 feet, we should thus have 1° in 7 of 93, that is, in 81 feet.
This is a slow rate of increase, and is about the same as Mr. Fairbairn found at Dukenfield. The rocks penetrated by the tunnel consist of highly metamorphosed material, and are described as belonging to the Jurassic series. No fossils have been found in them. For two-thirds of the length of the tunnel, beginning from the Italian end, they are remarkably uniform, and it is in this part that the observations have been taken. The following account of them has been given by Prof. Ansted (Pop. Sci. Review, Oct. 1870, p. 351):-" The rocks on which the observations have been made are absolutely the same, geologically and otherwise, from the entrance to the tunnel, on the Italian side, for a distance of nearly 10,000 yards. They are not faulted to any extent, though highly inclined, contorted, and subjected to slight slips and slides. They contain little water and no mineral veins. They consist, to a very large extent indeed, of silica, either as quartz or in the form of silicates, chiefly of alumina, and the small quantity of lime they contain is a crystalline carbonate.”
This uniformity of material is very favourable to conduction, and the high inclination of the strata (in which respect these rocks resemble those at Dukenfield) also appears to promote either conduction proper or aqueous convection, which resembles conduction in its effects. As regards Mons. Giordano's estimate of the mean air-temperature at the crest, it is obtained in the following way :-The hill of San Theodule is 430 metres higher, and the city of Turin is 2650 metres lower than the crest; the temperature of the former has been determined by one year's observations to be – 5o.1 C., and that of the latter is 120.5 C. If a decrease of 1° C. for every 174 metres of elevation be assumed (1° F. for 317 feet), we obtain, either by comparison with San Theodule or with Turin, the same determination —20.6 for the air-temperature at the crest of the ridge over the tunnel.
This mode of estimating the temperature appears very fair, though of course subject to much uncertainty; and there is another element of uncertainty in the difference which may exist between the air-temperature and the rock-temperature at the summit.
These two elements of uncertainty would be eliminated if a boring of from 50 to 100 feet were sunk at the summit, and observations of temperature taken in it. The uncertain correction for convexity would still remain to be applied. It would therefore be desirable also to sink a boring, of about the same depth, in the plateau which extends for about a quarter of the length of the tunnel, beginning near the Italian end, its height above the tunnel being about a third of a mile.
In November last, when very little information had reached this country respecting the temperature-observations in the tunnel, an urgent appeal was addressed, jointly by your Committee and by the Geographical Society (of which Prof. Ansted is Foreign Secretary), to M. Sismonda, requesting him to use his influence with the Italian authorities to secure a series of accurate observations of the temperature in the sides of the tunnel, before time had been allowed for this temperature to undergo sensible change from its original
value. It was also suggested that the mean temperature of the surface overhead should be examined by boring.
M. Sismonda speedily replied, stating that he fully recognized the importance of such experiments, and had already made arrangements with the Government at Turin, and with the contractors for the railway works, to have them carried out as fully and fairly as possible. Had the communication reached him at a time of year when he could have travelled without great inconvenience, he would have gone to the spot himself ; but as that was now impossible, the Government Commissioner for the works, M. Salvatori, had undertaken to see the experiments carried through by employés under his orders. M. Sismonda further stated that, from the commencement of the tunnel, the Academy of Sciences of Turin had instituted a series of scientific observations in it, in which observations of temperature were included. The results of these observations he promised to forward as soon as they were completed and tabulated.
On the receipt of the final refusal to bore down at the bottom of Rosebridge Colliery, inquiries were instituted as to the feasibility of executing a similar operation in the deepest part of the Alpine tunnel. The contractors have, however, declined to grant permission, as the operation would involve additional encumbrance of the very narrow space in which their works are proceeding. It appears that a length of a mile or more in the deepest part of the tunnel has not yet been opened out to the full width, so that opportunity may yet be given to excavate a lateral heading and bore down, if the Association encourage the plan.
Mr. G. J. Symons has repeated his observations in the Kentish Town well, at every fiftieth foot of depth, from 350 to 1100 feet, which is the lowest point attainable. As the water begins at the depth of 210 feet, all these observations may be regarded as unaffected by the influence of the external air, and they have now been sufficiently numerous at each depth to render further verification needless. The following are the results finally adopted, and they do not differ materially from those first published (Report for 1869).
The numbers in the last column are the quotients of those in the two preceding, and denote the average number of feet of descent for 1° F. of increase, as deduced from comparing the temperature at each depth of observation with the temperature at the lowest depth. The earlier numbers in this column of course carry more weight than the later ones. The amount of steadiness in the increase of temperature of the water is best seen by inspecting the third column, which shows that the freest interchange of heat occurs at about the depth of 600 feet. This must be due to springs. The soil, from the depth of 569 to that of 702 feet, is described as “ light-grey chalk, with a few thin beds of chalk-marl subordinate.” The soil consists in general of chalk and marl, from 325 to 910 feet, and below this of sandy marl, sand, and clay (see list of strata in last year's Report, p. 41). The mean rate of increase in the former is a degree in 56 feet, and in the latter a degree in 49 feet. The mean rate of increase from the surface of the ground to the lowest depth reached is certainly very nearly 1° F. in 54 feet.
Mr. David Burns, of H.M. Geological Survey, has furnished observations taken in the W. B. lead-mines, at and near Allenheads, Northumberland, by the kind permission of Thomas Sopwith, Esq., F.R.S., and with the valuable assistance of Mr. Ridley, Underground Surveyor, who continued the observations after Mr. Burns had left.
The mineral for which these mines are worked is galena. There are very extensive old workings at a lower level than the present workings, and filled with water, which is kept down by pumping; but the quantity daily pumped out is very small in comparison with the whole, so that the change of water is slow.
From the offices of the lead-mines a small windlass with a supply of fine brass wire was obtained, which enabled the thermometer to be lowered steadily and quickly.
The first observations were taken in Gin-Hill shaft, 3rd June, 1871. The observers proceeded as far down in the works as they were able, and took their station in a level leading from the shaft, 290 feet from the surface of the ground, and 38 feet above the surface of the water in the shaft. The following observations were then made :Depth under
The mean temperature at the shaft mouth for the year ending 31st May 1871, was 44°3, as derived from daily observations of maximum and minimum thermometers, without applying a correction for diurnal range. Adding 1° to this, to obtain the probable mean temperature of the surface of the ground, and taking the temperature at 400 feet of depth as 512.3, Mr. Burns computes that the rate of increase downwards is 6° in 400 feet, or 1° in 66.6 feet. The data for this calculation are obviously in many respects very uncertain.
On the 21st June Mr. Ridley took observations in another shaft in the same workings, called the High Underground Engine Shaft. It is sunk