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positions of the aurora have a considerable influence upon the direction of the magnetic needle, has been repeatedly confirmed during our residence at Bear Lake. It was also remarked that, from whatever point the flow of light, or, in other words, the motion of the aurora proceeded, if that motion was rapid, the nearest end of the needle was drawn towards that point, almost simultaneously with the commencement of the motion.
A careful review of the daily registers of the appearance of the aurora has led me to form the following general conclusions: 1st, That brilliant and active coruscations of the aurora borealis, cause a deflection of the needle almost invariably, if they appear through a bazy atmosphere, and if the prismatic colours are exhibited on the beams or arches. When, on the contrary, the atmosphere is clear, and the aurora presents a steady dense light, of a yellow colour, and without motion, the needle is often unaffected by its appearance.
2nd. That the aurora is generally most active when it seems to have emerged from a cloud near the earth.
3rd. When the aurora is very active, a haziness is very generally perceptible about the coruscations, though the other parts of the sky may be free from haze or cloud.
4th. That the nearest end of the needle is drawn towards the point from whence the motiou of the aurora proceeds, and that its deflections are greatest when the motion is most rapid, the effect being the same whether the motion flows along a low arch or one tints that crosses the zenith.
5th. That a low state of temperature seems favourable for the production of brilliant and active coruscations, it being seldom that we witnessed any that were much agitated, or that the prismatic were very apparent, when the temperature was above zero.
6th. That the coruscations were less frequently visible between the first quarter day and the full moon, than in any other period of the lumination, and that they were most numerous between the third quarter and the new moon.*
7th. That the appearance of the aurora was registered at Bear Lake, in 1825-26, 343 times, without any sound liaving been heard to attend its motione.
8th. The height of the aurora was not determined by actual observation, but its having been seen on several occasions to illuminate the under surface of some dense clouds, is conclusive that its elevation could not have been very great. When Dr. Richardson and Mr. Kendall made their excursion on Bear Lake, in the spring of 1826, the former saw the aurora very brilliant and active, display
1. The proportion of coruscations seen at these periods, from the month of October, 1825, to April, 1826, was 38 to 125. The moonlight being strong between the first quarter and the full moon at those hours when we more particularly watched for the aurora, may, perhaps, account for our not having seen its coruscations so often during this part of the lunation.
ing prismatic colours in a cloudless sky (on the 23rd April); while Mr. Kendall, who was watching at the time, by agreement, for its appearance, did not see any coruscation, though he was only twenty miles distant from Dr. Richardson.
9th. The gold-leaf electrometer, which was kept in the observatory, was never affected by the appearance of the aurora.
10th. On four occasions, the coruscations of the aurora were seen very distinctly before the day-light had disappeared, and we olten perceived the clouds in the day-time disposed in streams and arches, such as the aurora assumes.
The opinions I have ventured to advance above, are at variance with the conclusions drawn by Captains Parry and Foster, from their observations at Port Bowen : those officers inferring that the aurora does not influence the motion of the needle; but the discrepancy may be perhaps explained by the difference in activity and altitude of the aurora in the two places. I have stated that the needle is most affected when the aurora is very active, and displays the prismatic colours. Captains Parry and Foster have informed me, that the aurora seen at Port Bowen, was generally at a low altitude, without much motion in its parts, and never exhibiting the vivid prismatic colours, or the rapid streams of light, which are so frequently recorded in our registers, of its appearance at Fort Enterprise and Fort Franklin. At both these places, we as often witnessed the coruscations crossing the zenith, as at any other altitude, and under such a variety of forms, and in such rapid motion, as to baffle description,
From the difference in the appearance and activity of the aurora at Port Bowen, and Forts Enterprise and Franklin, an inference may be deduced that the parallel of 65° N., is more favourable for observing this phenomena, and its effect on the needle, than a higher northern latitude.
A Sketch of the Climate of the Mediterranean, with Remarks on its Medical Topography ; being the result of Five Years Observation. By the late WILLIAM BLACK, Esq., Surgeon, Royal Navy ; and communicated by Dr. Black, of Bolton, in Lancashire.*
The great basin of the Mediterranean, from its lying between countries differing so remarkably in their several localities and productions, has its general climate jinpressed with a mixed character, which it is as interesting to study, as it is important to analyse. Though the average climate for twelve months may be called equable, which is the character it has in England, yet there is, perhaps, no similar extent of water and coast where great climatoral vicissitudes are so plentifully produced by dif
• From the “ Edinburgh Philosophical Journal, Sept. 1828,"
ferences of situation and changes of wind. The father of meteorology, as well as of physic, in his treatise on airs, waters, and localities, has faithfully recorded the influence of winds and situation on the constitution of the atmosphere; and, from every observation which I have been enabled to make, it appears that amidst the wrecks and changes which the face of every country on the shores of this sea has experienced, the saine characteristic climate, general and particular, exists, as it did, upwards of twenty-two centuries ago; and that the observations of Hippocrates may still be considered the best synopsis of the meteorology of this part of the world.
Equable as the general climate has been remarked to be, yet, if one day is compared often with another, or one part even with another of the same day, the atmospheric vicissitude is sometimes very considerable; and particularly as respects the humidity of the air. Such changes are most sensibly felt on the shores of Europe, and on the south coasts of Greece and Turkey in Asia; and it is on a line, equally distant from Africa and Europe, that such variable states of the atmosphere are least perceptible. Malta is, therefore, thought to be most out of the sphere of this vicissitude, yet a great change of wind at this place is attended with very sensible changes of its climate; and it is by no means that desirable residence for an invalid which it is thought by many to be.
A moist or damp atmosphere is certainly to be avoided by the majority of invalids; and that of England is so much blamed in this respect, as to be accounted the chief cause of the pulmonary complaints prevalent in the kingdom. The moisture of the English atmosphere, except under the influence of rare localities, is perhaps less than that of Malta; for Humboldt has found, by hydrometrical observations, the superior humidity of the atmosphere as we approach the equator. Invalids who generally resort to Malta and Italy, are of relaxed fibres of body; and one argument against the salubrity of the last mentioned place for them, is, that, in removing from England, they avoid little, if any, atmospheric humidity; added to which, they remove to an increased temperature, which must still farther increase the relaxing effects derived from humidity. In corroboration of this, we every day see people who, by chronic disease, have been reduced to an enseebled and very relaxed state of body, sent from the Mediterranean to England with the happiest effect; while it is an established rule in the fleet, to remove every one im. mediately from the climate who betrays any incipient symtoms of phthisis. I have also seen chronic and syphilitic rheumatism deriving, particularly, the greatest benefit from a return to England. But, to resume the natural history of the subject : though the extensive surface of this midland sea, lying between the 31st and 45th degrees of north latitude, and embracing about 40° of longitude, has a general climate, constituted by the regular succession of seasons, like all other geographical surfaces which have a marked summer and winter; yet the several places bordering on and within its ample circuit, have climates peculiar to themselves. These peculiarities are compounded of the general Mediterranean climate at any given season of the year, and of the collateral influence of the winds prevailing at the time, conjoined with the nature of the land which surrounds the place, and over which these winds previously blow; whether the sea, and what extent of it, lies in the course of the winds; and whether it is situated on the north or south shore of the mainland or island. Before, however, poticing the few remarks which I have personally made on the particular topography of the climate, I shall first give a summary view of the great modifying, if not elementary, principles of Heator Temperature, Humidity, the Winds, and Electricity, as observed in the Mediterranean, for the space of more than five years.
Temperature. It will be seen, from the table annexed, that the average temperature of the year at noon is considerably above what is called temperate in England, being for three years very near 67o; and from the thermometer being registered always on board in an airy and shaded situation, it may correctly be inferred that the temperature on land is a few degrees higher. Equable and mild as this annual heat is, yet the changes from day to day, or from morning to night, are sometimes as great as they occur in England, during the same space of time. The average heat for the summer of three years never exceeded 81°, nor was it below 74°; and, in the winter months, it never descended below 54:6°, which is 2° above the mean annual temperature at Gosport, as observed by Dr. Burney.*
This extreme monthly temperature of 54:6° in February 1824, was attributed to the strong northerly winds which for ten days prevailed at Smyrna ; and as the average for the same season in the other two years was nearly two degrees higher, I should consider that they best expressed the corresponding temperature in the two years in which my daily register was not kept. The highest range observed at noon was 86°, which was off Algiers, in August, 1824, and the lowest was 41°, at Smyrna, in the evening, at eight, in January, 1827. The range of the summer months never exceeded 11°, while that of the other months was often as much as 250. For three months after the summer solstice, the heat on board was steady above 76°; and when the winds at this season are scanty, The thermometer is sometimes above 90° on sliore. If it were not that the great heats of summer exhaust the sources of humidity, the atmosphere would be felt the moistest during the greatest heat. We should, also, have the heaviest dews at night; but the reverberation from the heated surface of the earth often keeps the vapour suspended through the night, though clouds may be precipitated in the higher and cooler regions.
• From registering thermometers kept for several years at London, it appears, as calculated in the British Almanac for 1828, that the mean temperature of the year, by night and day, is 4904. The mean daily temperature of the year in the south of Scotland has been verified to be about 54%, and that of Devonshire to be a degree or two higher.-J. B.
Besides the characteristic temperature of the season, the heat at any place is moreover greatly affected by the winds at the time; thus, the westerly winds will not disturb much the regular increase or fall for the season, and the easterly but little; while the winds from the north, before the melting of the snows on the Apennines, and on the Chain of Pindus, in May and June, will lower the temperature many degrees on the south coasts of Italy and the Morea. The south and south-east winds will, on the other hand, as remarkably elevate the thermometer ; especially if they have blown steadily for a few days, and not over a widely intervening extent of sea. The effect of warm winds, immediately succeeding those from the north or a cold quarter, bas often been observed to be productive of severe catarrhs; and to elicit those affections, it seems necessary that the warm and moist winds should be preceeded by cold ones ; having some analogy to the circumstance of individuals catching cold, or a catarrh, not from being exposed to cold alone, but from coming into a warm room immediately after exposure to the cold air.
Humidity.—The hygrometrical condition of the atmosphere is an important object of attention in any climate, and it exerts a great modifying influence in that of the Mediterranean. This state of the air is very much affected by the direction of the winds, as well as by the temperature, at the time; it also nearly observes variations corresponding with the temperature, being generally, in its sensible qualities, drier as the air is warmer, and moister as it is cooler. An exception to this concomitancy, however, exists in the currents of air over an extent of sea being always moist, whether in summer or winter; though, it must also be added, that the Sirocco, if felt moist first on the northern shores of the Mediterranean, becomes drier if it continues for some days; and it sometimes will arrive there in all that arid state which is experienced on the coasts of Barbary and Egypt. Winds off land free from marshes, are dry in summer; and they are steadily moist, if they blow from snowy surfaces in the advanced part of the cold season. They are therefore moist, from moist places, in winter, under many changes of the wind; for the temperature never descends so low as to reduce the evaporation to a nullity, but ranges between those degrees on the scale where the dew point is very near the point of saturation.
At Modon, in the south of the Morea, the humidity in summer is much influenced by the prevailing winds. After the snow Jias melted on Pindus, the Olympus, and Mount Taygetus, the laud winds are dry, and the south winds are moist. If these last have blown for a length of time, they become drier, especially if they are of the Sirocco, and even if they have blown over the sea long in any direction; for it appears the longer winds blow over the sea, it does not get agitated, the evaporation becomes less, and it is much greater after rains or heavy dews, which seem to form a thin stratum of fresh water on the surface, liable to be instantly evaporated on the first increase of hygrometrical capacity. At Patrasso and Lapanto, the variations in the atmospherical humidity are very