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ries a vast mass of water which is below the normal off the 2000 fathoms. That the cold water should thus run upcoast of Portugal, into a region where it is above the nor- hill is quite conformable to what we see in other cases, in mal, with very little loss of heat by the way, except in its which a heavier under-stratum has a definite set towards a surface-film; and a little consideration will show that such slope; and whilst the existence of such a westerly set is, a movement must be much more effectual as a heater than ex hypothesi, a necessary consequence of the southerly move a corresponding movement of a thin stratum of much ment of the Arctic underflow, no other explanation of it warmer water. For the latter, when it passes beneath an has been suggested. We now see that the cold Labrador atmosphere much colder than itself, will soon be brought Current overlies a band of water as cold as itself; and the down to a like standard, not having warmer water from southward extension of this cold band, far beyond that of below to take its place when it has been cooled down; any definite current-movement, and its entrance into the whilst in the former, each surface-layer, when cooled Gulf of Mexico, through the Florida Channel, at the side below the temperature of the warmer stratum beneath, of and beneath the outflowing Gulf Stream, are thus acwill sink and be replaced by it. Now since the true Gulf counted for. Stream, when we last know it, has been so thinned out The remarkable accordance of so many facts of actual that it could not long retain any excess of temperature, it observation, in the Atlantic area, with the probabilities seems inconceivable that it should exert any decided effect deducible from a theory whose soundness can scarcely be on the temperature of the Faroes and the coast of Norway, disputed, seems now to justify the admission of the general unless (as supposed by Dr. Petermann and Professor Wy- (vertical) oceanic circulation sustained by opposition of ville Thomson) its thickness undergoes an increase from temperature as an accepted doctrine of terrestrial physics. less than 100 fathoms to 600. But since the course of Dr. Distribution of Organic Life.-All that will be attempted Petermann's isotherms shows that the northward flow ex- under this head will be to Indicate the general conditions tends across the whole breadth of the Atlantic between that seem, from recent researches, to have the greatest Newfoundland and the British Isles—a distance of about influence on the distribution of plants and animals through 2000 miles-we are required to believe that a rivulet (for this great oceanic basin. such it is by comparison) of 60 miles' breadth and 100 The distribution of marine plants seems mainly deterfathoms' depth (see section, fig. 5), of which the greater mined by light, temperature, and depth, -a further influence part turns southwards round the Azores, and of which the being exerted by the character of the shores. The diminuremainder is flowing due east when we last recognize it, is tion of light in its passage through sea-water is so rapid, able to impart a northerly movement to a stratum of 2000 that the quantity which penetrates to a depth of 250 or miles in breadth, and at least 600 fathoms' depth. On the 300 fathoms may be regarded as almost infinitesimal; and other hand, the eastward set of this stratum, considered as in conformity with this we find a very rapid diminution a northward indraught into the polar area, is readily ace of Algal life below the depth of 100 fathoms. The upper counted for by the excess of easterly momentum which it stratum is occupied for the most part by the larger and derives from the earth’s rotation, this being only half as coarser forms of the Fucaceae, or olive-green sea-weeds, rapid in lat. 60° as it is under the equator; and since there whilst the more delicate Ceramiacea, or red sea-weeds, freis a still more rapid reduction in the rate of this rotation quent deeper waters; and, as it appears from experiments in yet higher latitudes, the continually increasing excess made in aquaria that the latter do not flourish in full light, of easterly momentum will give to the northward flow a but gruw well in shadow, it may be concluded that their progressively stronger eastward set.
preference for a moderate depth is rather for reduced light On the other hand, the deficiency of easterly momentum and stillness than for depth per se. At a depth of 150 in the cold underflow coming from the pole towards the fathoms very few ordinary sea-weeds maintain their ground;
and below this we seldom find any Algæ, save the Corallines and Nullipores consolidated by calcareous deposit. The distribution of particular types over different parts of the Atlantic area appears to be mainly regulated by temperature; and this would seem to be remarkably the case with the floating Diatomacece, which, though they form green bands in the surface-water of polar seas, have not been encountered in like abundance in the Atlantic, and do not contribute largely, by the subsidence of their siliceous loricæ, to the composition of its bottom-deposit. Although it is the habit of the larger Algæ to grow from a base of attachment (their roots serving no other purpose, however, than that of anchorage), the enormous mass of Gulf-weed found in the Sargasso Sea seems quite independent of any such attachment. It was at one time supposed that this originally grew on the Bahama and Florida shores, and was torn thence by the powerful current of the Gulf Stream; but it seems certain that if such was its original source, the “Gulf-weed” now lives and propagates whilst freely floating on the ocean-surface, having become adapted by various modifications to its present mode of existence.
The distribution of the animals that habitually live in that upper stratum of the ocean whose degree of warmth varies with the latitude, seems mainily determined by tem. perature. Thus the “right whale" of Arctic seas, and its representative in the Antarctic, seem never to enter the inter-tropical area, generally keeping away from even the
temperate seas, whilst, on the other hand, the sperm-whale Horizontal Scale of Nautical Miles.
ranges through the parts of the ocean where the “right Fig. 6.-Section from Bermuda to Halifax.
whales" are never seen.
The distribution of fishes seems generally to follow the equator will tend to produce a lagging-behind, or westward same rule; as does also that of floating mollusks. Thus Bet of that underflow; and this has been shown by the the little Clio (a Pteropod mollusk), which is a principal “Challenger” temperature-soundings to be the case, the article of the food of the "right whales" in polar seas, 18 cold deep strata of the Western Atlantic surging upwards rarely met with in the Atlantic, where, however, other along the slope of the North American coast-line, as is pteropods, as Hyalæa, present themselves in abundance. shown in fig. 6, where we see not only the bathymetrical On the other hand, the warmer parts of its area swarm with isotherms of 60°, 55°, and 50°, but the yet deeper isotherms Salpa-chains, which are not frequent in higher latitudes; of 45° and 40°, successively rising to the surface as we and the few representatives of the Nautiloid Cephalopoda approach the land; while at a depth of only 83 fathoms a that were so abundant in Cretaceous seas, are now restricted temperature of 356 was encountered, which, at no great to tropical or sub-tropical areas. And the distribution of distance to the south, would only be found at a depth of the mollusks, echinoderms, and corals, which habitually
Vertical Scale of Fatboms.
live on the bottom, seems to be determined, within certain the condition of deep sea throughout the whole subsequent limits at least, by temperature rather than by depth. succession of geological changes.
The bathymetrical range to which animal life of any BIBLIOGRAPHY.-In addition to the ordinary sources of higher type than the Rhizopodal might extend, was until information, the following publications may be specially recently quite unknown; but the researches initiated by referred to for recent information in regard to the physical Prof. Wyville Thomson and Dr. Carpenter in 1868, and geography of the Atlantic:-“Reports of the Deep-Sea since prosecuted by the “Challenger”, expedition, have Explorations carried on in H.M. Steam-vessels ‘Lightning,'
. fully established the existence of a varied and abundant 'Porcupine,' and 'Shearwater,
,'” in Proceedings of the Royal fauna in ocean-depths ranging downwards to 2000 fathoms. Society for 1868, 1869, 1870, and 1872; “On the Gibraltar And these researches have further established that the Current, the Gulf Stream, and the General Oceanic Circuladistribution of this fauna is mainly determined by the tion," in the Journal of the Royal Geographical Society for temperature of the sea-bed; so that whilst in the channel 1871; and “Further Inquiries on Oceanic Circulation” between the north of Scotland and the Faroes there were (containing a summary of the “Challenger” Temperature found at the same depths, and within a few miles of each Survey of the Atlantic), in the same journal for 1874; other, two faunæ almost entirely distinct-one a boreal and Currents and Surface-Temperature of the North and South the other a warmer-temperate on sea-beds having respect- Atlantic, published by the
Meteorological Committee; and ively the temperatures of 30° and 43°, various types to The Depths of the Sea, by Prof. Wyville Thomson. which a low temperature is congenial are traceable con
(W. B. C.) tinuously along the whole abyssal sea-bed that intervenes ATLANTIS, ATALANTIS, or ATLANTICA, an island menbetween those northern and southern polar areas within tioned by Plato and other classical writers, concerning the which they present themselves at near the surface. And real existence of which many disputes have been raised. hence it becomes clear that, since glacial types are even in the Timæus, Critias relates how his grandfather Critias now being enibedded in the strata which are in process of had been told' by Solon some remarkable events in early formation beneath the equator, no inferences as to terres- Athenian history which he had learned from the Egyptian trial climate can be drawn from the character of marine priests at Sais, whose records went much further back deposits.
than the native accounts. “The most famous of all the One very remarkable feature which presents itself over Athenian exploits,” Solon had been told, “was the overthrow a large proportion of the Atlantic basın is the abundance of the island Atlantis. This was a continent lying over of the minute Globigerine and other Foraminifera, the against the pillars of Hercules, in extent greater than accumulation of whose shells and of their disintegrated Libya and Asia put together, and was the passage to other remains is giving rise to a calcareous deposit of unknown islands and to another continent, of which the Mediterthickness, that corresponds in all essential particulars to ranean Sea was only the harbor; and within the pillars Chalk. This deposit, in some parts of the North Atlantic, the empire of Atlantis reached to Egypt and Tyrrhenia. is replaced by an Arctic drift of fine sand, whilst in other This mighty power was arrayed against Egypt and Hellas parts there is a mixture of arenaceous and of calcareous and all the countries bordering on the Mediterranean, components, such as is found in certain beds of the Then did your city bravely, and won renown over the Cretaceous formation. Now on the surface of this deposit whole earth. For at the peril of her own existence, and there have been found so many living types, especially when the other Hellenes had deserted her, she repelled belonging to the groups of Echinoderms, Corals, Siliceous the invader, and of her own accord gave liberty to all the Sponges, and Foraminifera, which closely correspond with nations within the pillars. A little while afterwards there types hitherto regarded as characteristic of the Cretaceous was a great earthquake, and your warrior race all sank into epoch, that the question naturally suggests itself whether the earth; and the great island of Atlantis also disappeared the existing are not the lineal descendants of the fossil in the sea. This is the explanation of the shallows which types,--the differences they present being not greater than are found in that part of the Atlantic ocean.”—(Jowett's may be fairly attributed to the prolonged action of Introduction to the Timous.) Such is the main substance differences of temperature, food, pressure, &c. And when of the principal account of the island furnished by the these facts are taken in connection with those previously ancients, -an account which, if not entirely fictitious, bestated as to the probable remoteness of the period longs to the most nebulous region of history. The story when (if ever) the present sea-bed of the Atlantic was may embody some popular legend, and the legend may have dry land, the doctrine first put forth by Prof. Wyville rested on certain historical circumstances; but what these Thomson, that there has been a continuous formation of were it is (as the numerous theories advanced on the subject Globigerina-mud on the bottom of the Atlantic from may be held as proving)
impossible now to determine. the Cretaceous epoch to the present time,-or, in other ATLAS ("Atłas), in Greek Mythology, called sometimes words, that the formation of chalk on the sea-bed of the a son of Japetus and the nymph Asia, or of Uranus and Atlantic did not cease with the elevation of the European Gaia, and at other times traced to a different parentage, area, but has been going on through the whole Tertiary, but always known as the being who supported on his period,-must be admitted as (to say the least) a not shoulders the pillars on which the sky rested. He knew improbable hypothesis. That some considerable change the depths of the sea (Odyssey, vii. 245), and in the first took place at the conclusion of the Cretaceous epoch, by instance seems to have been a marine creation. The pillars which the temperature of the upper stratum was lowered, which he supported were thought to rest in the sea, so as to be no longer compatible with the existence of the immediately beyond the most western horizon. But by fishes and chambered cephalopods characteristic of the the time of Herodotus (iv. 184), a mountain is suggested Cretaceous fauna, may be fairly assumed from their disap- as best suited to hold up the heavens, and the name of pearance; but this would not so much affect the deeper Atlas is transferred to a hill in the N.W. of Africa. Then part of the basin, in which those lower types that seem the name is traced to a king of that district, rich in flocks more capable of adapting themselves to changes in external and herds, and owning the garden of the Hesperides. conditions would continue to hold their ground. That the Finally, Atlas was explained as the name of a primitive like conditions had prevailed also through long previous geo- astronomer. He was the father of the Pleiades and logical periods, may be surmised from the persistence, over Hyades. Perseus encountered him when he searched for various parts of the Atlantic sea-bed, of the Apiocrinite Medusa. Heracles took the burden of the sky from his type, which carries us back to the Oolitic formati and of shoulders, but cleverly contrived to replace it. Atlas bearthe Pentacrinus type, which has come down with very little ing up the heavens is mentioned as being represented on alteration from the Liassic; whilst many existing Tere early works of art, e.g., on the chest of Cypselus (Pausan., bratulido do not differ more from Oolitic types than the v. 18, 1), and on the throne of Apollo at Amyclæ Pausan., latter differ among each other. Going back still further, iii. 18, 7); and this subject occurs on several existing works we find in the persistence of certain Foraminiferal types of art. from the Carboniferous limestone to the present time, and ATLAS, a mountain-chain of Northern Africa, between in the character of its deep-sea beds, a strong indication the great desert of the Sahara and the Mediterranean. The that they originated in a Foraminiferal deposit, represent- range has been but partially explored, and geographers differ ing in all essential particulars that which is now going on; as to its extent, some considering it to reach from Cape Ghir while the persistence of the Lingula from the early on the Atlantic to Cape Bon, the north-east point of Tunis, Silurian strata to the present time suggests the question while others include under the name the whole mountain whether certain oceanic areas may not have remained in system between Cape Nun and the greater Syrtis. In this latter sense it forms the mountain-land of the countries inch of the earth's surface. Sir John Herschel has calof Marocco, Algeria, Tunis, and Tripoli. It is composed culated that the total weight of an atmosphere averaging of ranges and groups of mountains, enclosing well-watered 30 inches of pressure is about 113 trillions of pounds; and fertile valleys and plains, and having a general and that, making allowance for the space occupied by the direction from W. to E. The highest peaks are supposed land above the sea, the mass of such an atmosphere is about to attain an elevation of nearly 15,000 feet; and although notooo part of that of the earth itself. This enormous none of them reach the height of perpetual snow, some of pressure is exerted on the human frame in common with their loftiest summits are covered with snow during the all objects on the earth's surface, and it is calculated that greater part of the year. Mount Miltsin, 27 miles S.E. a man of the ordinary size sustains a pressure of about 14 of the city of Marocco, was ascertained by Captain Wash- tons; but as the pressure is exerted equally in all direcington to be 11,400 feet high. The greatest heights are tions, and permeates the whole body, no inconvenience in Marocco, from which point they appear to diminish in arises in consequence of it. elevation as they extend towards the E. These mountains, A pressure agreeing approximately with the average except the loftier summits, are, for the most part, covered atmospheric pressure at sea-level is often used as a unit with thick forests of pine, oak, cork, white poplar, wild of pressure. This unit is called an atmosphere, and is olive, and other trees. The inferior ranges seem to be employed in measuring pressures in steam-engines and principally composed of Secondary limestone, which, at a boilers. The value of this unit which has been adopted, greater elevation, is succeeded by micaceous schist and in the metrical system, is the pressure of 760 millimètres quartz-rock; and the higher chains are said to consist of (29:922 Eng. inches) of the mercurial column at 0° C. (32° granite, gneiss, mica-slate, and clay-slate. The Secondary Fahr.) at Paris, which amounts in that latitude to 1033 and Tertiary formations are frequently disturbed and kilogrammes on the square centimètre. In the English upraised by trap-rocks of comparatively modern date. system, an atmosphere is the pressure due to 29.905 inches Lead, iron, copper, antimony, sulphur, and rock-salt occur of the mercurial column at 32° Fahr. at London, amountfrequently; and in the Marocco portion of the range gold ing there to nearly 14} weight on the square inch. The and silver are said to exist. In the Algerian division are latter atmosphere is thus 0-99968 of that of the metrical mines of copper, lead, silver, and antimony. The lion, system. hyena, boar, and bear are common throughout the moun- As regards the distribution of atmospheric pressure over tains. None of the rivers which take their rise in the sys- the globe, there was little beyond conjecture, drawn from tem are of any great importance. The Tafilet is absorbed theoretical considerations and for the most part erroneous, in the sands; the Tensift and Draa flow into the Atlantic; till the publication in 1868 of Buchan's memoir “On the and about five or six find their way to the Mediterranean. Mean Pressure of the Atmosphere and the Prevailing Dr. Hooker has explored the botany of many parts of the Winds over the Globe.”? By the monthly isobaric charts range, and the travels of Rohlfs have added largely to our and copious tables which accompanied the memoir, this general knowledge of it.
important physical problem was first approximately solved. ATMOSPHERE is the name applied to the invisible Since then the British Admiralty has published charts elastic envelope which surrounds the earth, the gaseous showing the mean pressure of the atmosphere over the matter of which it is composed being usually distinguished ocean. The more important general conclusions regarding by the name of air. Storms and weather generally, solar the geographical distribution of atmospheric pressure are and terrestrial radiation, the disintegration of rocks, animal the following: and vegetable life, twilight, and the propagation of sound, There are two regions of high pressure, the one north are some of the more striking phenomena which are either and the other south of the equator, passing completely to a large extent or altogether dependent on the atmo- round the globe as broad belts of high pressure. They sphere. That air possesses weight may be shown by the enclose between them the low pressure of tropical regions, simple experiment of taking a hollow globe filled with through the centre of which runs a narrower belt of still air and weighing it; then removing the contained air lower pressure, towards which the north and south trades by means of an air-pump, and again weighing the globe, blow. The southern belt of high pressure lies nearly when it will be found to weigh less than at first. The parallel to the equator, and is of nearly uniform breadth difference of the two results is the weight of the air which throughout; but the belt north of the equator has a very has been removed. From Regnault's experiments, 100 irregular outline, and great differences in its breadth and in cubic inches of dry air, or air containing no aqueous vapor, its inclination to the equator—these irregularities being under a pressure of 30 English inches of mercury, and at due to the unequal distribution of land and water in the a temperature of 60° Fahr., weigh 31.03529 grains; and northern hemisphere. Taking a broad view of the subject, since 100 cubic inches of distilled water at the same pres- there are only three regions of low pressure—one round sure and temperature weigh 25,252) grains, it follows that each pole, bounded by or contained within the belts of air is 813.67 times lighter than water.
high pressure just referred to, and the equatorial belt of Air as an elastic fluid exerts pressure upon the earth or low pressure. The most remarkable of these, in so far as any substance on which it rests, the action of a boy's l" yet known, is the region of low pressure surrounding the sucker and of a water-pump being, familiar instances south pole, which appears to remain pretty constant showing the pressure of the atmosphere. When air is during the whole year. The depression round the north renoved from a water-pump, the water rises in the pump pole is divided into two distinct centres, at each of which only to a certain height; for as soon as the water has risen there is a diminution of pressure greatly lower than the to such a height that the weight of the column of water in average north polar depression. These two centres lie in the pump above the level of the surface of the water in the the north of the Atlantic and Pacific Oceans respectively. well just balances the pressure exerted by the atmosphere The distribution of pressure in the different months of the on the surface of the well
, it ceases to rise. If the pres- year differs widely from the annual average, particularly in sure of the atmosphere be increased, the water will rise January and July, the two extreme months. In January higher in the pump; but if diminished, the level of the the highest pressures are over the continents of the northwater will sink. The height to which the water rises ern hemisphere,—and the larger the continental mass within the pump thus varies with the pressure of the the greater the pressure,—and the lowest pressures are atmosphere, the height being generally about 34 feet. over the northern portions of the Atlantic and Pacific, Since a given volume of mercury weighed in vacuo at a South America and South Africa, and the Antarctic Ocean. temperature of 62° Fahr. is 13-569 times heavier than the In the centre of Asia the mean pressure of the atmosphere same volume of water, it follows that a column of mercury in this month is fully 30•400 inches, whereas in the North will rise in vacuo to a height 13.569 times less than a Atlantic, round Iceland, it is only 29.340 inches, or upcolumn of water, or about 30 inches. If we suppose, then, wards of an inch lower than in Central Asia. The area the height of the mercurial column to be 30 inches, which of high barometer is continued westwards throngh Central is probably near the average height of the barometer at and Southern Europe, the North Atlantic between 5° and sea-level, and its base equal to a square inch, it will contain 45o N. lat., North America, except the north and north30 cubic inches of mercury; and since one cubic inch of west, and the Pacific for some distance on either side of mercury contains 3426-7 grains, the weight of 30 cubic 15° Ñ. lat. It is thus an exaggerated form of the high belt inches will be nearly 14.7304 tb avoirdupois. Thus the pressure of the atmosphere is generally, at least in these
1 Trans. Roy. Soc. Edin., vol. xxv. p. 575.
9 Physical Charts of the Pacific, Allantic, and Indian (ceans, Lond latitudes, at sea-level equal to 14.7304 to on each square | 1872.
of annual mean pressure, spreading, however, over a much after passing through the north of Spain, out to sea at greater breadth in North America, and a still greater Coruña; over North America, except the north-east and breadth in Asia.
north-west. On the other hand, the July exceeds the In July, on the other hand, the mean pressure of Central January pressure generally over the whole of the southern Asia is only 29.468 inches, or nearly, an inch lower than hemisphere, over the northern part of the North Atlantic during January; or, putting this striking result in other and regions immediately adjoining the excess amounting words, about a thirtieth of the pressure of the atmo- in Iceland to 0.397 inch), and over the northern part of sphere is removed from this region during the hottest the North Pacific and surrounding regions. Thus the months of the year as compared with the winter season. pressure which is so largely removed from the Old and The lowest pressures of the northern hemisphere are now New Continents of the northern hemisphere in July is distributed over the continents, and the larger the con- transferred, partly to the southern hemisphere, and partly tinental mass the greater is the depression. At the same to the northern portions of the Atlantic and Pacific Oceans. time, the highest are over the ocean between 50° N. and Atmospheric pressure is more uniformly distributed over 50° $. lat., particularly over the North Atlantic and the the globe in April and October than in any of the other North Pacífic between 25° and 40° N. lat., and in the months. In May and November, being the months southern hemisphere over the belt of high mean annual immediately following, occur the great annual rise and pressure, which in this month reaches its maximum height. fall of temperature; and since these rapid changes take Pressure is high in South Africa and in Australia, just as place at very different rates, according to the relative in the winter of the northern hemisphere pressures are distribution of land and water in each region, a comparison high over the continents.
of the geographical distribution of May with that for the Over the ocean, if we except the higher latitudes, year brings out in strong relief the more prominent causes atmospheric pressure is more regular throughout the which influence climate, and some of the more striking year than over the land. In the ocean to westwards of results of these causes. This comparison shows a diminueach of the continents there occurs at all seasons an area tion of pressure in May over tropical and sub-tropical of high pressure, from 0.10 inch to 0.30 inch higher than regions, including nearly the whole of Asia, the southern what prevails on the coast westward of which it lies. The half of Europe, and the United States. An excess prevails distance of these spaces of high pressure is generally about over North America to the north of the Lakes, over Arctic 30° of longitude; and their longitudinal axes lie, roughly America, Greenland, the British Isles, and to the north of speaking, about the zones of the tropics. The maximum a line passing through the English Channel in a northis reached during the winter months, and these areas of easterly direction to the Arctic Sea. The excess in the high pressure are most prominently marked west of those southern hemisphere includes the southern half of South continents which have the greatest breadth in 30° lat.; and America and of Africa, the whole of Australia, and adjathe steepest barometric gradients are on their eastern sides. cent parts of the ocean. The influence of the land of the It is scarcely possible to over-estimate the importance of southern hemisphere, which in this month is colder than these regions of high and low mean pressures, from their the surrounding seas, brings about an excess of pressure; intimate bearing on atmospheric physics, but more partic on the other hand, the influence of land over those regions ularly from their vital connection with prevailing winds which are more immediately under the sun brings about a and the general circulation of the atmosphere. This rela- lower pressure, interesting examples of which occur in tion will be apprehended when it is considered that winds India, the Malayan Archipelago, and the Mediterranean, are simply the Howing away of the air from regions where Black, and Caspian Seas. In many cases the lines of there is a surplus (regions of high pressure) to where there pressure follow more or less closely the contours of the is a deficiency of air (regions of low pressure). Every coasts. Thus the diminution is greater over Italy and where over the globe this transference takes place in strict Turkey than over the Adriatic and Black Seas. The accordance with Buys-Ballot's “Law of the Winds," which greatest diminution occurs in Central Asia, where it ex. may be thus expressed :—The wind neither blows round ceeds 0.200 inch, and the greatest excess round Iceland, the space of lowest pressure in circles returning on them. where it exceeds 0-200 inch. It is to the position of Great selves, nor does it blow directly toward that space; but it Britain, with reference to the deficiency of pressure on the takes a direction intermediate, approaching, however, more one hand and the excess on the other, that the general nearly to the direction and course of circular curves than prevalence of east winds at this season is due. These eastof radii to a centre. More exactly, the angle is not a right erly winds prevail over the whole of Northern Europe, as angle, but from 45° to 80°. Keeping this relation between far south as a line drawn from Madrid and passing in a wind and the distribution of pressure in mind, the isobaric north-easterly direction through Geneva, Munich, &c. To lines give the proximate causes of the prevailing winds the south of this line the diminution of pressure is less, and over the globe, and through these the prominent features over this region the winds which are in excess are not of climates. As regards the ocean, the prevailing winds easterly, but southerly. Crossing the Mediterranean, and indicate the direction of the drift-currents and other sur. advancing on Africa, we approach another region of lower face-currents, and thereby the anomalous distribution of pressure, towards which easterly and north-easterly winds the temperature of the sea as seen in the Chili, Guinea, again acquire the ascendency, as at Malta, Algeria, &c. and other ocean currents, and the peculiarly marked This, in many cases great, variation of the pressure in climates of the coasts past which these currents flow, are the different months of the year must be kept carefully in explained; for observations have now proved that the view in deducing heights of places from observations made prevailing winds and surface-currents of all oceans are all by travellers of the pressure of atmosphere, by the baromebut absolutely coincident.
ter or the temperature of boiling water. In reducing the As regards the annual march of pressure through the observations, it is necessary to assume a sea-level pressure months of the year, curves representing it for the different if the place is at a considerable distance from any meteorregions of the earth differ from each other in every con- ological observatory. Previous to the publication of Buceivable way. It is only when the results are set down chan's Mean Pressure of the Atmosphere, it appears that a in their proper places on charts of the globe that the mean sea-level pressure of 29-92 or 30-00 inches was in subject can be well understood. When thus dealt with, such cases universally assumed. The mean pressure at many of the results are characterized by great beauty aná Barnaul, Siberia, being 29.536 inches in July, 30-293 simplicity. Thus, of all influences which determine the inches in January, and 29-954 inches for the year, it folbarometric fluctuation through the months, the most im- lows that, by the former method of calculating the heights, portant are the temperature, and through the temperature observations made in January to ascertain the height of The humidity. Comparing, then, the average pressure in Lake Balkash would make the lake 350 feet too high, and obJanuary with that in July, which two months give the servations made in July would make it 330 feet too low,greatest possible contrasts of temperature, the following is the difference of the two observations, each set being supthe broad result:
posed to be made under the most favorable circumstances, The January exceeds the July pressure over the whole and with the greatest accuracy, being 680 feet. This illusof Asia except Kamtchatka and the extreme north-east, tration will serve to account for many of the discrepancies the greatest excess being near the centre of the continent; met with in books regarding the heights of mountains and over Europe to south and east of a line drawn from the plateaus. White Sea south-westward to the Naze, thence southward Of the periodical variations of atmospheric pressure, the to the mouth of the Weser, then to Tours, Bordeaux, and I most marked is the daily variation, which in tropical and
3 3 3
9 10 9
Min. Hour. Max. Hour.
sub-tropical regions is one of the most regular of recurring served in the summer and winter months, thus showing phenomena. In higher latitudes the diurnal oscillation is that in the summer of both hemispheres the influence of masked by the frequent fluctuations to which the pressure the sun tends to lower the minimum at 3 to 4 P.M. to a is subjected. If, however, hourly observations be regularly greater extent than to raise the 9 to 10 A.M. maximum. made for some time, the diurnal oscillation will become ap- Decrease between Morning Maximum and Afternoon Minparent. The results show two maxima occurring from 9 to imum.-Of the four daily oscillations, this is the most im11 A.M. and 9 to 11 P.m., and two minima occurring from 3 portant. When the amounts at different places are ento 6 A.M. and 3 to 6 P.M. The following are the extreme tered on charts of the globe, it is seen that the amplitude variations for January, April, July, and October from the of this fluctuation is, speaking generally, greatest in the daily mean pressure at Calcutta, deduced from the observa- tropics, diminishing as we advance into higher latitudes; tions made during six years, viz., 1857–62:
greater over the land than over the sea, increasing greatly
on proceeding inland ; nearly always greater with a dry A.M.
than with a moist atmosphere; and generally, but by
no means always, it is greatest in the month of highest Min. Hour. Max. Hour. Min. Hour. Max. Hour.
temperature and greatest dryness combined. The regions
of largest amplitude include the East India Islands, East
Inch. ern Peninsula, India, Arabia, tropical Africa, and tropical January. -023 3 +.079 10 --053 4
10 South and Central America, where it either closely apApril...... -020
- 071 +.016 proaches or exceeds 0.100 inch. At Silchar, in Assam, it is July 019 +:040 --051 4
0.133 inch. In the tropical parts of the ocean the oscillaOctober... -·026 | +.064 --047
| +.0181 10
tion is from 0.020 to 0·030 inch less than on land. The in
fluence of the Mediterranean Sea in lessening the amount Similarly the maxima and minima at Vienna, with the over all regions bordering it is very strongly marked. The hour of their occurrence, are as follows:
line showing an oscillation of 0.050 inch crosses North America about lat. 44°, curves southward at some distance from the east coast to lat. 23°, then north-eastward along
the coast of Africa, passes eastwards near the north coast Min. Hour. Max. Hour. of that continent, thence strikes north wards, cutting the
eastern part of the Black Sea, and eastward across the Cas
pian to a point to northward of Peking, and then bends January. -.008 +018
+012 10 southward to the Loo Choo Islands. The line of 0·020 April......--003
inch cuts the N.W. of Spain and N.W. of France, and runs July .. +022
+.009 October... -·010|
northward through Great Britain as far the Tweed, thence 1 +020
to Christiania, then southwards to Copenhagen and to Cra
cow, the latitude of which it follows eastward through Asia. These two illustrations may be regarded as typical, to The more marked seasonal changes are these:- In India a large extent, of the diurnal barometric oscillations in the oscillations during the dry and wet seasons, or in Jantropical and temperate regions. At Calcutta the amounts uary and July, respectively, are-Bombay, 0:120 and 0.067 are large, and the dates of the occurrence of the maxima inch; Poonah, 0.133 and 0-059 inch; and Calcutta, 0-132 and minima very regular from 3 to 4 and 9 to 10 A.M. and and 0.091 inch. At Madras, where the rain-bringing chaP.M. respectively. On the other hand, the oscillations at racters of the monsoons are reversed, the numbers are 0·114 Vienna are much smaller and more variable in amount, and 0.115 inch, and at Roorkee, where rain falls all the and the dates of occurrence of the critical phases take place year round, 0.088 and 0·079. Again, at Aden, in Arabia, through a wider interval, viz., from 3 to 6 and 9 to 11 A.M. where the weather of July is peculiarly hot and dry, the and P.M. respectively.
oscillation in December is 0:106, but in July it rises to 0-137 Though the diurnal barometric oscillations are among inch. The point to be insisted on here is, that, whatever the best-marked of meteorological phenomena, at least in be the cause or causes to which the daily barometric oscillatropical and sub-tropical regions, yet none of these phe- tion is due, the absolute amount is largely dependent on nomena, except perhaps the electrical, could be named re- comparatively local influences. specting whose geographical distribution so little is really While illustrations similar to the above may be adduced known, whether as regards the amount of variation, the from many other parts of the globe, showing the influence hour of occurrence of the critical phases, or, particularly, in the same direction of prevailing dry or wet
, hot or cold the physical causes on which the observed differences de- seasons on the amplitude of the oscillation, the North Atpend. This arises chiefly from the want of a sufficient lantic and regions adjoining present an apparent exception number of ascertained facts ; and to remedy this deficiency, to the law which seems to be indicated by these results. observations have, in the preparation of this present article, The whole of the North Atlantic, particularly north of lat been collected and calculated from upwards of 250 places 20°, and the sea-boards which bound it, to which the Mediin different parts of the globe, and the data set down on terranean and its immediate sea-board may be added, are charts. The chief results of this inquiry are the following, strikingly characterized by a small summer oscillation; attention being entirely confined to the chief oscillation, and this diminution is most strongly marked along the viz., that occurring from the A.M. maximum to the P.M. eastern part of the ocean. Thus, in July, at Ponta Delminimum.
gada, in the Azores, the oscillation is only 0.06 inch ; at The A.m. Maximum.-In January this occurs from 9 to Angra do Heroisma, also in the Azores, 0·010 inch; at 10 in tropical and temperate regions as far as 50° N. lat.; Funchal, Madeira, 0:011 inch; at Oporto, 0.018; Lisbon, in higher latitudes the time of occurrence varies from 8 A.M. 0·030; and Lagos, 0.021 ; at Naples and Palerno, 0·008; to noon. In July it occurs from 9 to 10 everywhere only and at Malta, 0-020 inch. Now, with reference to this ex as far as about 40° N. lat.; the time at Tiflis (41° 42' N. tensive region, it is to be noted that the rainfall of July is lat.) being between 7 and 8 A.M. In higher latitudes the either zero or very small; and yet with this dry state of time varies from 8 to 11 A.M., the last hour being general the atmosphere and high temperature (the annual maxi. in north-western Europe.
mum occurring at the time), this oscillation is extraordiThe P.M. Minimum.-In January this occurs from 3 to 4 narily diminished, being exactly the reverse of what takes P.M. nearly everywhere over the globe, a few exceptions oc- place during the dry and wet seasons in India. The dimicurring in north-western Europe, the extremes being 2 P.m. nution on the western half of the Atlantic, though not so at Utrecht and 6 P.M. at St. Petersburg. It is quite differ- great, is also striking, the January and July oscillations beent in July, when the time from 3 to 4 P.M. is regularly ing 0.056 and 0:036 inch in Barbadoes, 0.080 and 0.056 at kept as far north as about 40° N. lat. In higher latitudes Jamaica, 0·082 and 0.054 at Havana, 0-053 and 0.024 in the hour is very generally 5, but at some places it is as the Bahamas, and 0.054 and 0:022 in Bermuda. Over the early as 4 P.M., and at others as late as 6 P.M.
whole of the region here indicated the rainfall of July is In the northern hemisphere, in summer, the afternoon largely in excess of that of January. The apparently ex. minimum falls to a greater extent below the mean of the ceptional character of this region is probably due to the day than the forenoon maximum rises above it, at 82 per circumstance, that at this time of the year the sun's rays cent. of the stations; but in winter the percentage is only fall perpendicularly over a more diversified surface of the 61. In the southern hemisphere the same relation is ob- earth, that is, on a greater extent of lanıl, than at any other