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raised to a height of 120 feet. In November, 1703, the entire structure was washed away, and in 1706 sanction was obtained for its being rebuilt, which was accordingly done, by Rudyerd, but which was destroyed by fire in 1755. The present tower, one of the artificial wonders of England, and built by Smeaton, is 100 feet high, and has given good proof of its capability of resisting the force of the waves. The Bell Rock lighthouse is a similar structure to the Eddystone; it was built by Stevenson, at a cost of £60,000. The most recent erection of this description is on the Skerryvore rock, which cost £90,700.
The author next alluded to the difficulty of constructing permanent lighthouses in exposed situations, and the advantages of them over floating lights, as well as the much smaller annual expenditure required to maintain an efficient light. The first floating light was the well-known Nore light-vessel, moored in 1734. In order to insure stability in a lighthouse, Mr. Findlay stated that it is necessary that the structure should be capable of affording resistance to a pressure of not less than 6,000 lbs. to each square foot of surface exposed to the action of the waves. This assertion was founded on experiments made by Mr. Alan Stevenson, who ascertained and registered the force of the waves at the Skerryvore rock, on March 25th, 1845, during a westerly gale, when it was found to be 6083 lbs. per square foot; this, the greatest force hitherto registered, was cited with many others. He next proceeded to point out the inapplicability of iron to the construction of lighthouses, where the metal was immersed in sea water, which has the effect of reducing it to a body similar in its chemical properties to blacklead; and instanced the effects produced on a cannon-ball taken from the Mary Rose, after having been sunk off Spithead, for a period of 150 years : the iron shot upon being exposed to the air gradually, became red hot, and then fell into a red powder, resembling burnt clay. The author next described the methods which have been suggested for overcoming the difficulty of exposing large surface to the action of the force of the waves, and also for obtaining a firmer foundation on a sand, and especially Mr. Alexander Mitchell's screw-pile lighthouse, erected on the Maplin Sand, and Dr. Potts's method of driving piles by atmospheric pressure, as applied at the South Calliper beacon on the Goodwin sands, in 1847, and to other beacons on various shoals at the mouth of the Thames, as on the Blyth sand, and on the shingles in the Prince's channel. Another plan for the erection of lighthouses has been carried into effect at the point of Ayr, by Mr. Walker; it consists in constructing hollow cylinders, which are filled with concrete and then sunk, and from them the piles rise. Capt. Sir S. Brown has also proposed a plan for the erection of lighthouses in deep water, upon bronze standards, and a modification of his plan was adopted by Capt. Bullock. The author further alluded to Mr. Bush's Light of all Nations, and to Mr. A. Gordon's iron lighthouses at Jamaica and the Bermudas, in which the cases are filled with a solid mass of concrete ; and alluded to the fact that Rennie had proposed iron for this purpose, as early as the year 1805, for the Bell Rock.
Having thus shown the different methods employed in the construction and erection of lighthouses, Mr. Findlay proceeds to remark on the various plans of illumination which have been employed : of these, the earliest was the coal fire and the Cordouan billets of oak. In 1752, the South Foreland lighthouse, previously illuminated with an open coal fire, was covered with a lantern, with large sash windows, and the fire was kept bright, by means of large bellows; the lantern was subsequently removed, and afterwards, at the commencement of the present century, fifteen large lenses, with separate
lamps were placed in it. In 1790, the only exception to the coal fire was the Eddystone lighthouse, which had a chandelier, with twenty-four wax candles, and the Liverpool lighthouses with oil lamps, and rude parabolic reAlectors. An interesting historical fact was then mentioned, viz. : that parabolic reflectors were used at the Liverpool lighthouses (built in 1763), as Mr. W.Hutchinson, in his “ Practical Seamanship," published that year, describes the apparatus then in use, the larger reflectors of wood, lined wiih small pieces of looking glass, the smaller of polished tin: this was the more curious, as it had been claimed by the French for M. Teulere, in 1783, and first used in Scotland, in 1786. The parabolic reflectors, of which some beautiful specimens were shown to the meeting, are now constructed upon the formula of the celebrated Capt. Huddart. Having explained the catoptric or reflecting principle of illumination, which received so great an improvement in the invention of the Argand lamp in 1780, or 85; several other lights were exhibited and described, viz. : the Drummond light, the Voltaic light, and the causes of their inapplicability.
The present mode of lighting is from lamps, constructed on a modification of the Argand principle. A first-order pneumatic lamp, with four concentric wicks, showing a most powerful light, was exhibited. The dioptric principle, in which the rays of light emanate from a central lamp, and are controlled and directed, by a series of lenses, placed before and around it, next occupied attention. The author claims the priority of its suggestion for an optician in London, as mentioned by Smeaton, who proposed, in 1759, to grind the panes of the Eddystone lighthouse in to a sphere of 15 feet diameter. The present form of lens, generally known as Fresnel's, was first suggested by the celebrated Buffon, to whom it is probable the catoptric system owes its origin. Sir David Brewster, in 1811, showed the practicability of constructing a lens of separate pieces, and this was first used in France, by Fresnel, and has since become universal in French lighthouses.
A comparative view of the catoptric and dioptric systems is afforded by the fixed lights of the South 'Foreland, the higher being from the dioptric principle, and the lower from Huddart's reflectors, which, to a distant observer appear equally bright, the only test of their efficiency. The cata-dioptric principle was illustrated by a beautiful fourth-order apparatus lent by Messrs. Wilkins, in which, above and below the light, a system of totally reflecting prismatic zones is arranged, the suggestion of Mr. A. Stevenson. Mr. Alesander Gordon's cata-dioptric system, a union of the reflector and refractor, was also described. Some particulars respecting the power of light in penetrating mist were also brought forward. During fogs, the attendants of lightvessels, sound a bell at intervals, or, as now used by the Trinity Board, a Chinese gong. Instead of this, Lieut. Sheringham, R.n., proposed, in 1842, to use a whistle, worked by bellows, and Mr. Gordon proposed to place the whistle in the focus of a parabolic reflector, to direct the sound. Mr. Findlay concluded his paper by suggesting the use of Mowbray's chemical whistle, which was exhibited and described.
BRITISH ASSOCIATION. PROCEEDINGS OF THE BRITISH ASSOCIATION ON THE CAUSE OF THE AURORA,
AND THE DECLINATION OF THE NEEDLE, by G. Rowell. As the trade winds are caused by the flowing of the denser air from the polar regions to the tropics, the superior trade winds in the higher regions of the air must be from the tropical to the coldest parts of the earth, to keep up the equilibrium of the air ; then, as it is proved by Forchhammer, that more vapour arises from tropical seas than falls there, and that more falls in polar regions than rises in those parts; and as it is proved by the experiments of Volta and others, that whenever evaporation takes place, positive electricity is carried off-it follows that there are electrical currents similar to the currents of air; the vapour, with its electricity, rising in the tropics (thus rendering those parts negatively charged) is carried thence by the superior trade winds to the colder parts of the earth, where the vapour falls; and its electricity escaping to the earth, renders those parts positively charged, whence the electricity rushes off along the earth's surface towards the more negative parts of the earth, and is again carried off by the rising vapour. Mr. Rowell ascribes the direction of the needle, to these currents of electricity from the positive to the negative parts of the earth, and the aurora to the interruption of these currents of electricity, by the dry and non-conducting state of the air in the frigid regions during severe frosts insulating the electricity of the clouds, where it accumulates till it flashes back through the higher and rarer air towards the more temperate regions, thus exhibiting the aurora, and at the same time causing a disturbance of the magnetic needle. The author thinks that many writers have fallen into errorin supposing the height of the aurora to be far above the limits of our atmosphere: wbich error may have arisen from some mistake in their observations, or from some other luminous me. teor being mistaken for the aurora; for as the observations of Parry, Franklin, Richardson, and others distinctly prove that the aurora does take place near the surface of the earth, and is in some way connected with the formation of clouds, the arches which are sometimes seen at such great altitudes may arise from totally different causes. He considers that the diurnal variation of the needle tells in favour of the opinion that the direction of the needle is dependent on evaporation; as very early in the morning, when to the eastward of our meridian, evaporation must be at a minimum, the declination is least: the declination then increases till about the time when the evaporation must be most rapid, and then decreases, till in the evening, it reaches its medium position; and the fact that the diurnal variation is more than double in summer what it is in winter, tells in favour of this view.
The cause of magnetic poles in this hemisphere he ascribes to the quantities of ice blocked up both in winter and summer in the high latitudes above the two continents, thus causing those parts to be the coldest in this hemisphere, and therefore, the magnetic poles; for, as the density of the air from the frigid regions is the cause of the trade winds, and as the density of the air increases with the degree of cold, it follows that more air must flow from the coldest parts of the earth towards the warmer regions than from any other parts, and, consequently, there must be the greatest flow of the superior currents of air from the warmer to those colder parts, thus bringing more vapour and electricity there than to any other parts in this hemisphere. Now if the greatest degree of cold be at the pole of the earth, and evaporation increased regularly thence to the equator, there would then be no declination of the needle, as the electricity would pass off from the coldest or positive parts towards the more negative parts of the earth in the direct lines of longitude; but as the magnetic poles are at a distance from the terrestial pole, and as those parts are more positively charged with electricity than other parts in the same latitude, the electricity must diverge eastward and westward of the direct lines of longitude in passing off to the more negative parts of the earth, and thus cause the declination of the needle.
The author contends that the fact that the aurora did not affect the
needle at Port Bowen, in 73° N. lat., whilst it had great effect at Fort Franklin, in 65° N., tells in favour of his views that the direction of the needle is owing to currents of electricity from the magnetic pole to the more negative parts of the earth, as the American magnetic pole is in 70° N.
Mr. Rowell exhibited a large diagram of the earth from the north pole to 40° N. lat., showing at one view the situation of the American magnetic pole according to Sir James Ross, and the Siberian pole according to Hansteen, the lines of equal intensity from Col. Sabine's maps, the lines of equal temperature from Humboldt, the direction of the needle shown by arrows, &c. By the diagram he showed that, in the meridian of the American pole, the lines of equal temperature descend to a much lower latitude than in any part of this hemisphere, which he ascribed to the polar seas there being land locked and causing a great accumulation of ice in those regions, both winter and summer, whilst the magnetic force, also, is the greatest in that meridian.
In the meridian of the Siberian pole, the Polar Sea is far more open, the temperature is higher, and the magnetic intensity less. In the neighbour, hood of Behring's Straits, where the Polar Sea is open to the Pacific Ocean, the intensity is still less; but in the meridian of London, or rather to the east of it, the line of equal temperature rises to a much higher latitude than in any other part of this hemisphere, the intensity of magnetism is the least, and the Polar Sea is there open from Greenland to Nova Zembla, and the ice formed in those regions is liable at all times to be broken up and dispersed by the storms of the Atlantic Ocean. He considers the magnetic poles not to be mere points in the earth, but extensive districts in the coldest parts and that even mountains, which from their elevation, are continually conducting electricity from the higher regions of the air, must have some local effect upon the needle. He contends that the opinion is erroneous which ascribes the changes of declination to a rotation of the magnetic poles round the pole of the earth; as we have no proof that the magnetic poles in this hemisphere were ever situated otherwise than in the high latitudes above the two continents; and that the change of declination may be fairly explained on the supposition that the American pole has increased in strength, or the Siberian pole has decreased in strength, and that the line of no variation where the influence of the two poles are equal, bas receded during the last two centuries from some point west of England to its present position eastward of St. Petersburgh, thus bringing parts which formerly had an eastward variation to be under the influence of the American pole. He suggests that any geological change which has made the Siberian Polar Sea more open would tend to weaken that magnetic pole; or any change which may have blocked up the American Polar Sea, would increase the strength of that pole. He concludes by again suggesting the experiment of raising electrical conductors, to the height of the clouds in the frigid regions during severe frosts, which he believes would cause the aurora, and also throw some light on terrestial magnetism.
ON THE ATMOSPHERE OF THE Moon, by J. Groby. WHETHER the Moon has an atmosphere or body of air similar to that which surrounds the Earth, has long been a fertile subject of dispute among philosophers, some affirming its existence, and others as strenuously denying it. Some, who take the negative side of the argument, have urged in defence of their opinion, the constant serenity of the Moon's surface, always undisturbed by clouds or vapours, and even the sellest of the numerous spots or macula which crowd her surface being at all times equally visible. This certainly would be a very strong argument against an atmosphere were the assertions on which it is fonnded to be relied upon. But this does not appear to be the case ; for, on the other hand, there are many astronomers who affirm that the Moon is not always equally clear and distinct, Hevelius says, “ that he has several times found, in skies perfectly clear, when even stars of the sixth and seventh magnitudes were conspicuous, yet at the same altitude of the Moon, and the same distance from the Earth, and with one and the same excellent telescope, the Moon and its maculæ do not appear equally lucid, clear, and perspicious at all times." He also adds. “ From the circunstances of the observation, it is evident that the reason of this phenomonon is not either in our air, in the telescope, in the moon, or in the spectator's eve, but must be looked for in something existing about the Moon, that is (1 presume) in the atmosphere.” Again, the existence of a lunar atmosphere has been denied, because the stars, in an occultation, when just about to disappear behind the body of the Moon, retain their full lustre till they seem to touch the very edge, and then vanish in a moment, which phenomenon (it is urged) could not bappen if the moon were encompassed with an atmosphere. Here, again, the evidence of different astronomers is at variance, one party affirming and the other denying the fact. Nay, even the same individual has sometimes observed both stars and planets to undergo a ehange, both in brightness, form, and colour, when close to the Moon's limb, while at other times he has perceived nothing of the kind. A third argument against the existence of an atmosphere (and the last I shall notice) is this. If (it is affirmed) the Moon were surrounded by an atmosphere, then the duration of eclipses and occultations ought to be diminished by means of its refractive power, and hence a celebrated French astronomer (in a memoir written expressly on this subject), has endeavoured to demonstrate that, if such an atmosphere did exist, and its horizontal refraction amounted to only 8", there could never be a total eclipse of the Sun. But (he continues) in the eclipse of that luminary, which happened in 1724, the total darkness continued for two minutes sixteen seconds. Many eminent astronomers concur with the one I have just quoted, in denying that anything like refraction can exist, or has ever been observed to exist about the Moon's atmosphere ; others equally eminent assert that they have observed the most unequivocal proots of it. Both Halley and Euler speak of the evident distortion observable in the Sun's limb in total and annular eclipses. The latter, in particular, says, in the eclipse of the Sun which happened in July, 1748, he observed that when the uncovered part of the Sun resembled the Moon in her quadrature, the horns of the solar crescent appeared to be bent outwards beyond the circle in which every other part of his disk was comprehended ; and when the eclipse became annular, the Sun's disk was dilated beyond the circle which formerly embraced it. This dilation was observed at Frankfort, and was estimated by "Euler at 25'. Here, then, we have one astronomer observing in a solar eclipse a refraction of 12", while another, from his observations, denies that any perceptible refraction does or can exist. From such contradictory evidence, it seems extremely difficult to draw anything like a satisfactory conclusion. The most probable one seems to be this: -That the Moon is surrounded by an atmosphere in some respects like our own, but much rarer; and that it is differently modified by the peculiar circumstances attached to it. For when we consider that from the slow motion of the Moon on its axis the principal part of its surface is exposed to the direct force of the Sun's rays for fourteen and a-half days and nights, without any intermission, and