before it at a distance of six or eight feet: an object glass three or four feet focal length, might be placed between both, so as to produce exactly at the lower end of the pendulum of the clock an image of the lower end of the other pendulum. Then the coincidences of both might be accurately observed by a telescope placed at a convenient distance. Similar contrivances had been described in an account of some pendulum experiments made at Königsberg; and the accuracy of the method was such, that the relative rate of both pendulums might be ascertained with sufficient accuracy in a very short time: in from ten to twenty minutes. The rate of the pendulum was to be tried at different temperatures, being placed in a box, having an opening at the lower end covered with glass, and so fastened to the wall that the pendulum could swing within it. In the construction of the pendulum, attention should be paid to one thing, which seemed to have been much overlooked. It often happened that thermometers affixed to the top and bottom of a clock case did not agree, whence it was evident that the compensation, acting only below, would not compensate for the variation of the whole rod. He should prefer, on this account, the gridiron to the mercurial pendulum, especially if the rods began as low as possible below the point of suspension, and were carried on to the centre of gravity of the lens. He should prefer the several rods to be of equal diameter, and to be coated uniformly. Supposing the spring perfectly regulated, as well with respect to heat as the arc, only one cause would interfere with regular vibration times. This was the effect of the part of the elasticity of the air which depends on the variation of the height of the barometer; the other part depending upon the variations of the thermometer, is comprised in the adjustment for the compensation for heat. There was a possibility of compensating the former, by fastening a barometer tube to the pendulum, and it would not be difficult to find the suitable diameter of the tube; but he was aware that this complication of the pendulum would be rather inconvenient. At all events, the variations of the barometer were not very great, especially if the specific gravity of the pendulum be made as great as possible. He submitted these hints to those celebrated artists, whose works had greatly contributed to the promotion of astronomical purposes, and the determination of the longitude.

Sir Thomas Brisbane said, that Dr. Robinson, of Armagh, had ascertained that a variation of the height of the barometer, amounting to one inch, produced a difference of 0".27 per diem of rate in in the clock. He observed, that the place at which the crutch impelled the pendulum, was a matter of more consequence than was usually supposed.-Mr. Dent denied that comparisons could be safely instituted between pendulums, and those connected with the mechanism of a clock. In reply to a remark made by Mr. Clare, Professor Stevelly said, that cycloidal motion was laid aside on account of the shifting of the position of the centre of oscillation resulting from it. But, he thought, the discussion of compensatious was too wide a departure from the important subject brought before the

Section by Professor Bessel.-Sir John Herschel acknowledged the justice of this remark The question before the Section was, whether the methods proposed really compensated for the unequal arcs of vibration of the pendulum, or not. Fortunately the method of coincidences reduced the labour of subjecting this to the test of actual trial that which, a few years since, would have required days, or even months, to deterinine, could, by this method, be now ascertained in little more than twenty minutes.

Mr. Follet Osler "On the Application of the Principle of the Vernier to the Subdividing of Time."-Professor Stevelly said, Mr. Osler's idea was, to have a pendulum which should make, say ten swings, in the time that the principal pendulum made eleven, furnished with a small dial, and so placed that the coincidences, or want of coincidence, could be observed. The strokes of such a pendulum being counted, the time of every observed stroke of it, reckoned back from its coincidence with the principal, or seconds pendulum, would, it is obvious, be found in tenths of a second.

Sir David Brewster then made a communication " On a new Property of the Rays of the Spectrum, with Observations on the Explanation of it given by the Astronomer Royal, on the principles of the Undulatory Theory."-If we cover half the pupil of the eye with a thin plate of any transparent body, and thus view a prismatic spectrum, so that the rays which pass by the plate interfere with those which pass through it, the spectrum is seen crossed with beautiful black and nearly equidistant bands, whose breadth, generally speaking, increased with the thinness of the plate. If the edge dividing the ray were directed to the red end of the spectrum, then fringes were seen; but no such fringes appeared when it was turned to the violet end of the spectrum. One peculiarity of these fringes, not before noticed, was that they had not the forms of bands, but rather the appearance of screws, or dotted black lines, or as if they were formed by the shadow of a plate of metal perforated by small openings. This, which appeared to be a new property of light, and to indicate a polarity in the simple rays of light, when separated from each other by refraction, he had commented on at the meetings of the association at Liverpool and Bristol; and Mr. Airy, the Astronomer Royal, had given a paper and two publications on the subject, in which he endeavoured to account for this upon the undulatory theory, arguing that the appearance and magnitude of the fringe depended upon the diameter of the pupil, or of the objectglass. Sir D. Brewster said, he had repeated all his experiments under every variety of form, varying the diameter of the pupil from its greatest expansion to its greatest contraction, and the diameter of the object-glass from four inches to a quarter of an inch, and the fringe remained utterly unaffected by these variations. He further found, that these fringes varied in magnitude with the distance of the eye from the refracting body, and not with the magnitude of the pupil. He stated several other results, all of which, he thought, could not be explained on the principles of the undulatory theory.

Sir W. Hamilton observed, that the warmest advocate of the wave theory of light must be gratified with these valuable experiments of Sir David Brewster; even though they should require the wave theory, in its present form, to be abandoned; and yet it was probable they might suggest the very modifications which will adapt it to the enlargement of our knowledge.-Sir John Herschel said it appeared to him, that the wave theory was now placed in the same position in which gravitation was frequently placed in its infancy, when difficulties arose which could not be surmounted in the then state of mathematical knowledge. As soon as that knowledge was acquired, the theory was triumphant, or rather it was rendered more powerful by the very obstacles which it had been found capable of surmounting. The undulatory theory had already achieved so much, and even gone so far in predicting phenomena quite unlooked for, as in the extraordinary phenomena of conical refraction discovered by Professor Lloyd and Sir W. Hamilton, and in Fresnel's experiments, by which circular polarization was produced artificially, that he must protest against putting it on trial for life or death the moment a new fact was discovered which seemed inconsistent with it. The facts just brought forward by Sir D. Brewster were most extraordinary, and deserved the deepest consideration; but it was necessary to suspend our judgment until further inquiry, and perhaps new facts, threw more light on this very difficult subject. The screw-like appearance and dotted lines described by Sir D. Brewster, appeared to be one of the most extraordinary facts connected with the spectrum. But the spectrum might be said to be a world within itself, of which we know as nothing, compared with what remained to be known.-Sir D. Brewster did not wish to put the undulatory theory on its trial for life or death, but upon one count of the indictment; for he conceived it entirely failed in explaining those facts which he had brought before the Section.— Prof. M'Cullagh said, we as yet knew so little of the undulatory theory, that it would be premature to pronounce that it either could or could not explain every fact. It had long been his settled conviction, that it would be a foundation for physical principles. The only physical principle, in fact, which we had connected with it was that of interference, for he put no faith in Fresnel's mechanical theory of refraction, which seemed to be discovered by some kind of mathematical deduction, and then explained by principles invented to suit it. These were facts which he had long thought contradicted the fundamental principles of M. Cauchy as expressed in his fundamental equation. The well known fact of circular polarization, he conceived, contradicted it.--Prof. Lloyd agreed with Prof. M'Cullagh on the importance of establishing a sufficiency of physical principles, but he could not admit that the wave theory was so destitute of them as Prof. M'Cullagh represented it. Surely there was the principle of transversal undulations discovered by Dr. Young.-Prof. McCullagh thought that was rather a mathematical, than a physical theory, but, as he did not wish to differ about words,

he would admit it.- Prof. Lloyd said, it could be distinetly conceived as a physical principle. But if the theory was so destitute of physical principles, surely it was most wonderful that it was found to explain completely so many and such apparently unconnected facts, and this, he conceived, at least proved its parallelism with truth.-Sir W. Hamilton hoped it would not be supposed that the wave men were wavering, or that the undulatory theory was at all undulatory in their minds. The true practical question was, whether the dynamical explanations of M. Cauchy had a physical foundation, and, on that subject he knew no better authority than Prof. M'Cullagh. He was not so sanguine as to hope that, at a single bound, they should reach the physical conceptions connected with so subtle an element as light. He hoped Prof. McCullagh would publish his refutation of M. Cauchy's theory, particularly as it related to circular polarization.-The president said, that however difficult and abstruse the subject was which had been under discussion, the Section had had the opportunity of hearing the opinions of all the chief promoters of the science of optics in modern days. We were, perhaps, too impatient with reference to what this theory would or would not explain. We should recollect that it was a century and a half after Newton's publication of the theory of universal gravitation, before all the problems related to that theory, as, for instance, the problem of three bodies, could be brought entirely under its dominion. It was not more than fifty years since the undulatory theory was brought forward as anything more than a guess, and scarcely twenty since its principles were mathematically followed out to any considerable

extent.

Sir John Herschel then read a "Report on the great co-operative system of Magnetic and Meteorological Observations," which, three or four years ago, was commenced at the instance of the British Association. After noticing the vast increase of the surveys and observations, owing to the number of foreign establishments entering into the concerted plan, the Report referred to the Antarctic expedition, taking it up where the Report of last year left it, at Hobart Town, in 1840. Captain Ross observed the November term (for observations) in 1840, at the Auckland Islands. On leaving those islands, his adopted course led him between the two southern magnetic foci. It seems probable that he was still to the eastward of the present locality of the greatest intensity. The full import of the observations made in this voyage is not yet known, but it is understood that intensities have been observed by Captain Ross 2 times greater than the minimum observed by him near St. Helena, on the outward voyage; and that the general aspect of the intensity observations would appear to place the centre of the principal isodynamic oval in a latitude somewhat exceeding 50° south. nearest approach to the magnetic pole was in latitude 76° 12', long. 164° east, the dip being 88° 40'. The intensity was here found to be less than in 47° south. The Admiralty (who had rendered

The

every service to these inquiries) had placed under the care of Col. Sabine the observations made on board each ship, the results of which were most satisfactory, as regarded the practicability of making accurate observations at sea; for out of 647 observations of this kind, made between London and the Cape, on board the Erebus, one only had been declared doubtful; while the observations taken by both ships exhibited a steady accordance, that could not be accidental, and might well be called beautiful. From these it would appear, if earlier observations are to be relied on, that the line of least intensity, in successive meridians, is travelling rapidly northward. The term of November, 1840, had been kept (by Capt. Ross) at the Auckland Islands; those of May and June, 1841, at Van Diemen's Land; that of July, at Sydney: the four succeeding terms had been kept in New Zealand. From a letter from Capt. Ross, dated 22nd November, 1841, it appeared the expedition was to sail the day following to resume the investigation; it was his intention to traverse the isodynamical oval, surrounding the focus of greatest intensity, supposed to be in lat. 60° south, long. 235° east, commencing in long. 210°, and lat. 52° or 53° south; and steering thence directly to the edge of the ice-pack, to make, on reaching it, for the point where the first year's exploration of the new continent (of Victoria) had terminated, and to pursue that barrier; the working out of which intention might, of course, involve another winter, spent within the Antarctic zone. Should it be otherwise, we might expect ere long to hear of his arrival at Falkland Islands; but, in the other alternative, another year would elapse without any further tidings of the expedition. As to British and foreign observatories, the British and Indian stations, except that at Aden, as well as the chief continental ones, had long been in full activity. The Russian government has been pre-eminent in the aid given. Supported by M. Cancrin, Minister of Finance, as well as aided by the funds placed at his disposal by Prince Mentchikoff, M. Kupffer had brought into activity magnetic observatories at Kasan, Barnaoul, Nertschinsk, and Catharineberg. He had also effected the re-erection of observatories at Tiflis and Nicolayeff, and the erection of a new one at Moscow, under the care of Count Strogonoff, curator of the university of that city. These operations, conducted by every European power, had occupied much time; the original term granted by our own government and the East India Company expired in the current year, just when the arrangements were completed over a great portion of the world, and the fruits were beginning to be gathered in. Accordingly, application was made to government by the President and Council of the Royal Society, for their continuation for another period of three years, to terminate in 1845; and at the same time it was officially stated, on the part of the Russian government, that the observatories in that empire should be kept up as long as the British ones, Baron Brunow stating, that this extension was the shortest term adequate to obtain results to repay the outlay. The British government gave an unhesitating assent to the continuation of the present scheme