From tide-observations made at considerable intervals, and reduced in the manner followed above, some approximation to the secular changes (caused by the widening or narrowing, the deepening or filling up of rivers) can to some extent be estimated, and embankments or other protections carried out at those places where the tide is making encroachments, and a surer foundation afforded for the reclamation of land. There can be no doubt that in all these operations every advance towards more perfect knowledge of the tidal movements will be accompanied by an economy of time and labour which in the aggregate must be very considerable. 40. It may here be mentioned that there now appears to be a considerable difference in the spring-range of the tide in the Thames at the London Docks (amounting to 13 inches in twenty-five years), caused, it is supposed, by the removal of obstructions, extensive dredging, and the construction of the embankment. In the Admiralty Tide-tables the heights of high water have been augmented by the apportioned amount, but no correction has been applied to the times of high water, which are also probably different. It is to be hoped that the Lords Commissioners of the Admiralty may be pleased to direct that new determinations of the tides in the Thames and other places be made with a view of obtaining the requisite foundation for more extensive tide-tables than those now published, and supplying to the mariner more accurate and complete knowledge of the tides along our coasts. 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 Professor EVERETT, at the request of the Committee, consisting of Sir WILLIAM THOMSON, LL.D., F.R.S., Mr. E. W. BINNEY, F.R.S., F.G.S., Principal FORBES, LL.D., F.R.S., Mr. ARCHIBALD GEIKIE, F.R.S., F.G.S., Mr. JAMES GLAISHER, F.R.S., Rev. Dr. GRAHAM, Mr. FLEEMING JENKIN, C.E., F.R.S., Sir CHARLES LYELL, Bart., LL.D., F.R.S., Mr. J. CLERK MAXWELL, Mr. GEORGE MAW, F.L.S., F.G.S., Prof. PHILLIPS, LL.D., F.R.S., Mr. PENGELLY, F.R.S., F.G.S., Professor RAMSAY, F.R.S., F.G.S., Mr. BALFOUR STEWART, LL.D., F.R.S., Mr. G. J. SYMONS, Professor JAMES THOMSON, C.E., Professor YOUNG, M.D., F.R.S.E., and Professor EVERETT, D.C.L., F.R.S.E., Secretary. THE following Circular, issued to the Committee in November 1867, explains the plan of procedure proposed, and the progress made in the investigation up to that time : SIR,-No Meeting of the Committee has yet been held; and as the Members are scattered over the country, it appears undesirable to wait for a Meeting. I therefore take this opportunity of laying before you the suggestions of Sir W. Thomson and myself with regard to the system of observation which should be adopted, and shall be glad to receive any suggestions of your own in reply. The object of investigation is the rate at which, in various localities, the temperature of the earth increases in going downwards, at depths sufficiently great to render the annual range of temperature insensible. The annual range of temperature diminishes in the ratio of 2 or 3 to 1 for every 10 feet of descent, and becomes reduced to a tenth of a degree Fahr. at depths of from 50 to 80 feet in this climate,-these results being derived from observations extending to 25 feet of depth, made at Greenwich and at three localities in Edinburgh. A great boring or excavation, such as a mine, necessarily produces much disturbance of the normal temperature in its neighbourhood, and therefore, while observations in mines ought not to be neglected, we think the efforts of the Committee should be chiefly directed towards finding the temperatures at various depths in smaller borings, such as are usually made preliminary to mining-operations. As these are often carried to depths of from 300 to 600 feet, they will furnish very measurable differences of temperature at different depths in the same boring. It is suggested that observations should be made at every 50th foot of depth. A method which has been used by Ångström will probably be found most convenient. It consists in enclosing a thermometer in a large glass bottle of water, letting it down to the point where the temperature is to be taken, leaving it in that position long enough to ensure that its temperature shall not differ sensibly from that of the soil nearest to it, then drawing it quickly up and reading off the thermometer before time is allowed for any sensible variation in its temperature. It would probably be found advisable to use two pistons or plugs (two bags of sand might answer the purpose), one above and the other below the bottle, to check currents of air or of water. The thermoelectric method might also be followed with great advantage. Two wires, one of iron and the other of copper, insulated by gutta percha or some other covering as in submarine cables, and connected at their ends, might be let down, so as to bring their lower junction to the point where the temperature is to be taken, their upper junction being immersed in a basin of water, and the circuit completed through a galvanometer. The temperature of the water in the basin might then be altered till the galvanometer gave zero indication. An extremely accurate determination of the temperature at various depths could in this way be obtained with great ease and expedition, when the apparatus had once been prepared; but the method by water-bottle, though requiring more time for the observations, will probably be in general preferred on account of its simplicity. Currents of water in a boring will render it unsuitable for our purpose; but water free from currents will but little affect the accuracy of observation. Every Member of the Committee is requested to find out borings, in his own neighbourhood or elsewhere, that would be suitable for the investigation, and also to state whether he could undertake to make the observations himself, the thermometers or other apparatus required being supplied by the Committee. Sir Wm. Thomson has already ordered from Casella two thermometers suitable for the water-bottle method, and expects to have them almost immediately for trial; they are spirit thermometers, with Fahrenheit scales, and are to be accurate to a tenth of a degree. Your reply at your earliest convenience, with any suggestions you may have to offer, will oblige Your obedient Servant, Queen's College, Belfast, Nov. 16, 1867. J. D. EVERETT, Secretary to the Committee. The only observations yet taken in answer to the above invitation were made near Glasgow by Sir W. Thomson, from whose recent paper on Geological Time (Trans. Geol. Soc. Glasgow, vol. iii. part 1) the following paragraph (§ 29) may now be quoted. "All sound naturalists agree that we cannot derive accurate knowledge of underground temperature from mines. But every bore that is made for the purpose of testing minerals gives an opportunity of observation. If a bore is made, and is left for two or three days, it will take the temperature of the surrounding strata. Let down a thermometer into it, take proper means for ascertaining its indications, draw it up, and you have the measure of the temperature at each depth. There are most abundant opportunities for geothermic surveys in this locality, by the numerous bores made with a view to testing minerals, and which have been left, either for a time or permanently, without being made the centre of a shaft. Through the kindness of Mr. Campbell, of Blythswood, several bores in the neighbourhood of his house have been put at the disposal of the Committee of the British Association, to which I have referred. In one of these bores very accurate observations have been made, showing an increase of temperature downwards, but which is not exactly the same in all the strata, the difference being no doubt due to different thermal conductivities of their different substances. I need not specify minutely the numbers; but I may say, in a general way, that the average increase is almost exactly of a degree Fahrenheit per foot of descent; which agrees with the estimate generally admitted as a rough average for the rate of increase of underground temperature in other localities. "Another bore has been put at the disposal of the Committee, and the investigation of it is to be commenced immediately, so that I hope in the course of a few days some accurate results will be got. It has been selected because the mining engineer states in his report that the coal has been very much burned or charred, showing the effect of heat; and it becomes an interesting question, Are there any remains of that heat that charred the coal in ancient times, or has it passed off so long ago that the strata are now not sensibly warmer on account of it?" The following report of these observations has been sent by Sir W. Thomson to the Secretary of the Committee:— The operations were commenced in December last, with a spirit thermometer by Casella, having a stem 14 in. long, divided to tenths of a degree, and ranging from 39° to 61° Fahr.; it has a bulb 3 in. long by 1 in. diameter, is enclosed in an outer tube, nearly filled with spirit, and hermetically sealed; a protecting case of strong tin covered the whole; and to this case a stout copper wire was attached for lowering it in the bore. Before commencing the operations, the thermometer, thus mounted, was left in a vessel of water until the temperature remained unchanged; it was then plunged into another vessel whose temperature differed by about 10° Fahr. from the former, and an interval of more than a minute elapsed before any change of reading could be observed; it was also ascertained that it could be raised from a depth of 60 fathoms and a reading taken within this time. The following observations were then made at Blythswood, about five miles west of Glasgow, during December and January, in a bore (No. 1) 60 fathoms deep, and filled with water to a constant depth of about 2 fathoms from the surface : 1 In all the observations taken with this thermometer, it was allowed to remain a day or two at least at each depth before being raised. The thermometer was next removed to a second bore, at a short distance from the former, which was originally 95 fathoms deep, but had become filled up with sediment 45 fathoms, leaving only 50 fathoms free. Observations were made from early in March till the middle of April; the results from the greatest depths were pretty constant, but towards the surface they varied from time to time, occasioned in all probability by a constant flow of surface-water into the pipe, which found an escape at some unknown depth. The weather was generally wet, and there was always more or less entering the pipe, affected by the temperature of the air. It is considered that the thermometer acted very satisfactorily at the depths at which it was used; but it is doubtful if its indications could be relied on for considerably greater depths, on account of the time taken to wind it up; and the speed could not be safely increased, particularly if the bore has rough projections on its sides. An instance of this occurred in the middle of April. While raising the thermometer with considerable velocity, it stuck, and would neither move up nor down; it had to be left till means were got to detach it gently from the obstruction; and when brought to the surface, the upper part of its strong casing was half torn away, but the thermometer itself quite safe. With the view of obviating this difficulty, and of observing more rapidly, trials were being made at bore No. 1 with a thermo-electric junction of insulated copper and iron wire, let down to various depths, and the other junction placed in a vessel of water, which was heated or cooled, till a galvanometer showed no current, when the temperature was taken. Small discrepancies existed between the results thus obtained and those previously got directly with the thermometer; but there has not been time hitherto to ascertain their cause. The chief difficulties to be overcome were the want of a steady stand for the galvanometer (one of Sir Wm. Thomson's delicate mirror galvanometers), and the means of obtaining sufficient darkness to be able to read it. A third method has been tried, which promises to give satisfactory results. Two of Phillips's maximum thermometers were furnished by Casella; but unfortunately on the second day of trial they met with an accident, and had to be returned to London to be repaired, and have only just now been re ceived. They will, it is expected, be tried soon at several bores in the neighbourhood of Glasgow, one of which is 174 fathoms deep, and at about half that depth passes through 18 fathoms of greenstone. Several small hardy maximum thermometers, suited for rough work, are being constructed by Casella, and are to be had from him by any of the Members of Committee, or by parties recommended by them. Members of Committee are earnestly requested to cooperate energetically in the work. The arrangements now made for the supply of suitable thermometers will allow useful results to be easily obtained in great abundance, from every variety of situation where available bores exist, at small expense, and without much expenditure of time. Changes of the Moon's Surface. By Baron VON MÄDLER. THE earliest idea of the heavenly bodies was that they were simply accessories of the earth, and existing only on her account. The course of astronomical discovery compelled these notions to give place to another view, that these heavenly bodies were independent spheres; from this naturally sprang a desire to learn more of these distant worlds. This desire can be accomplished in part by means of the sight which the telescope gives of the surface of the heavenly bodies; and our own satellite in particular presents such a "wealth of objects" that the most diligent efforts of the observer of the present day will fall far short of the representation of them all; and much, very much, must remain for his successors to accomplish. But though Riccioli more than 200 years ago attached to his Grimaldi's Almanack' of the Moon the superscription Nec homines vivere, nec plantæ crescere possunt,' and all subsequent observers must agree with him, yet there were not a few who sought to maintain a contrary opinion. In short, inhabitants of the moon have been sought for-Selenites, as Helvetius first called them. Gruithuysen hoped to see them when they might happen to be passing in a great mass through some mountain defile (wood-roads was his own word); and if they themselves could not be perceived, he was reluctant to abandon the hope that at least their buildings and similar works might be observed. But it is superfluous to dwell on a subject which rests upon such chimerical notions. The changes which we meet with in the moon have been brought about by natural power far greater and more marked than all the artificial works we have been able to execute, and these changes are still working; and since it cannot be doubted that with our present instruments it would be possible to perceive from the moon events such as some volcanic eruptions, and the raising of a new island out of the depths of the sea, so may we take a hint to discover, if possible, such events in the moon, and to explain analogously anything seen or supposed to be seen there. First, let us consider the volcanoes of which Sir W. Herschel speaks. That renowned inquirer expresses himself with all possible caution; he plainly states that he makes use of the word only because there must be some term of designation, and that he aimed at nothing less than a definite explanation. Notwithstanding this caution, however, many authors have spoken of burning volcanoes in the moon as an undoubted fact, and with a reference to Herschel's remarks. Herschel was the only one at that time who possessed such resources, and it had long been usual to receive all his observations without the proof which no |