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hill is quito conformable to what we see in other cases, in which a heavier under-strstam has a definite set towards a slope, and whilst the existence of i j such a westerly J . set is, eje hyj>otheii, a necessary consequence of ■•* the southerly movement of the Arctic under- I flow, no other I explanation of 5 "1 it has been Bug- *1 a 0 0 gested. We now .: i»»t seo that the cold j , Labrador Cur-1 rent overlies a band of water as "*0 cold as itself; •••« and the south- m, ward extension of this cold band, far boyond that"" of any definite current - movement, and its entrance into the Gulf of Mexico, through the Florida Channel, at the side of and beneath the outflowing Gulf Stream, are thus accounted for.
The remarkable accordance of so many facta of actual observation,' in the Atlantic area, with the probabilities deducible from a theory whose soundness can scarcely be disputed, seems now to justify the admission of the general (vertical) oceanic circulation sustained by opposition of temperature as an accepted doctrine of terrestrial physics.
Dittribution of Organic Life.—All that will be attempted under this head will be to indicate the general conditions that seem, from recent researches, to have the greatest influence on the distribution of plants and animals through this groar oceanic basin.
The distribution of marine plants seems mainly determined by light, temperature, and depth,—a further influence being exerted by the character of the shores. The diminution of light in its passage through sea-water is so rapid, that the quantity which penetrates to a depth of 250 or 300 fathoms may be regarded as almost'infinitesimal; and in couformity with this we find a very rapid diminution of Algal life below the depth of 100 fathoms. The upper stratum is occupied for the most part by the larger ana coarser forms of the Fvcaeea, or olive-green sea-weeds, whilst the more delicate Ceramiacea, or red sea-weeds, frequent deeper waters; and, as it appears from experiments made in aquaria that the latter do not flourish in full light, but grow well in shadow, it may be concluded that their preference for a moderate depth is ratheitfor reduced light and stillness than for depth per te. At a depth of 150 fathoms very few ordinary sea-weeds maintain their ground; and below this we seldom find any Algse, save the Corallines and XuUiporcs consolidated by calcareous deposit The distribution of particular types over different parts of ihc Atlantic area appears to be mainly regulated by temperature; and this would seem to be remarkably the case with the .floating Diatomacea, which, though they form green band* 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 hrieat, to the composition of its bottom-deposit Although it ii the habit of the larger Alga to grow from a base of attachment (their root* serving no other purpose however,
than that of anchorage), the enormous mass of Gulf-v •«-to 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 mainly determined by temperature. Thus the "right whale" of Arctic seas, and its representative in the Antarctic, seems never to enter the inter-tropical area, generally keeping away from even the temperate seas, whilst, on the other hand, the sperm-whale ranges through the parts of the ocean where the " right whales " are never seen.
The distribution of fishes seems generally to follow the same rule; as does also that of floating molluska. Thus the little Clio (a Pteropod mollusk), which is a principal article of the food of the "right whales " in polar Beas, is rarely met with in the Atlantic, where, however, other pteropods, as Hyalcea, present themselves in abundance. On the other hand, the warmer parts of its area swarm with Salpa-chains, which are not frequent in higher latitudes; and the few representatives of the Nautiloid Cephalopoda, that were so abundant in Cretaceous seas, are now restricted to tropical or sub-tropical areas. And the distribution of the mollusks, echinoderms, and corals, which habitually live on the bottom, seems to be determined, within certain limits at least, by temperature rather than by depth.
The bathymetrical range to which animal life of any higher type than the Bhisopodal might extend, was until recently quite unknown; but the researches initiated by Prof. ,Wyville Thomson and Dr Carpenter in 1868, and since prosecuted by the ''Challenger" expedition, havo fully established the existence of a varied and abundant fauna in ocean-depths ranging downwards to 2000 fathoms. And these researches have further established that the distribution of this fauna is mainly determined by the temperature of the sea-bed; so that whilst in the channel between the north of Scotland and the Faroes there wero found at 'the same depths, and within a few miles of each other, two faunae almost entirely distinct—one a boreal and the other a warmer-temperate—-on sea-beds having respectively the temperatures of 30° and 43°, various types to which a low temperature is congenial are traceable continuously along the whole abyssal sea-bed that intervenes between those northern and southern polar areas within which they present themselves at or near the surface. And hence it becomes clear that, since glacial types are even now being embedded in the strata which are in process of formation beneath the equator, no inferences as to terrestrial climate can bo drawn from the character of marine deposits.
One very remarkable feature which presents itself over a large proportion of the Atlantic basin is the abundance of the minute Globigcrina and other Fornminifera, the accumulation of whose shells, and of their disintegrated remains, is giving rise to a calcareous deposit of unknown thickness, that corresponds in all essential particulars to Chalk. This deposit, in some parts of the North Atlantic, is replaced by an Arctic drift of fine sand, whilst in other parts there is a mixture of arenaceous and of calcareous components, Buch as is found in certain beds of the Cretaceous formation. Now on the surface of this deposit there have been found so many living types, especially belonging to the groups of Echinoderms, Corals, Siliceous Sponges, and Foraminifera, which closely correspond with types hitherto regarded as characteristic of the Cretaceous epoch, that the question naturally suggest* itaeli whether the existing are not the liuoul descendants ui" the fossil types,—the differences they present being not greater than ■ay be fairly attributed to the prolonged action of differences of temperature, food, pressure, <tc And when these facta are taken in connection with those previously stated as to the probable remoteness of the period when (if ever) the present Bea-bed of the Atlantic was dry land, the doctrine first put forth by Prof. Wyville Thomson, that there has been a continuous formation of Globigerina-mud on the bottom of the Atlantic from the Cretaceous epoch to the present time—or, in other words, that the formation of chalk on the sea-bed of the Atlantic did not cease with the eleration of the European area, but has been going on through the whole Tertiary period,—most be admitted as (to say the least) a not improbable hypothesis. That some considerable change took place at the conclusion of the Cretaceous epoch, by which the temperature of the upper stratum was lowered, so as to be no longer compatible with the existence of the fishes and chambered cephalopods characteristic of the Cretaceous fauna, may be fairly assumed from their_disappaarauce; but this would not so much affect the deeper part of the basin, in which those lower types that seem man capable of adapting themselves to changes in external conditions would continue to hold their ground. That the like conditions had prevailed also through long previous geological periods, may be surmised from the persistence, over various parts of the Atlantic sea-bed, of the Apiocrinite type, which carries us back to the Oolitic formation, and of
the Pentacrinut type, which has come down with very little alteration from the Liasaic; whilst many existing l'erebratulidas do not differ more from Oolitic types than the latter differ among each other. Going back still further, we find in the persistence of certain Foraminiferal types from the Carboniferous limestone to the present time, and in the character of its deep-sea beds, a strong indication that they originated in a Foraminiferal deposit, representing in all essential particulars that which is now going on; while the persistence of the Lingula from the early I Silurian strata to the present time suggests the question whether certain oceanic areas may not have remained in the condition of deep sea throughout the whole subsequent succession of geological changes.
Bibliography.—In addition to the ordinary sources of information, the following publications may be specially referred to for recent information in regard to the physical geography of the Atlantic:—" Reports of the Deep-Sea Explorations carried on in H.M. Steam-vessels 'Lightning,' 'Porcupine,' and ' Shearwater,'" in Proceedings of the Royal Society for 1868, 1869,1870, and 1872; "On the Gibraltar Current, the Gulf Stream, and the General Oceanic Circulation," in the Journal of the Royal Geographical Society for 1871; and "Further Inquiries on Oceanic Circulation" (containing a summary of the "Challenger" Temperaturo Survey of the Atlantic), in the same journal for 1871; Currenti and Surface-Temperature of the North and South Atlantic, published by the Meteorological Committee; and The Depth* of the Sea, by Prof. Wyville Thomson. (w.B.o.)
ATLANTIS, Atalaktis, or Atlantica, an island mentioned by Plato and other classical writers, concerning the real existence of which many disputes have been raised. Ia the Tinueue, Critias relates how his grandfather Critias had been told by Solon some remarkable events in early Athenian history which ho had learned from the Egyptian priests at Sais, whose records went much further back than the native accounts. "The most famous of all the Athenian exploits," Solon had been told,"wasthe overthrow of the island Atlantis. This was a continent lying over against the pillar* of Hercules, in extent greater than Libya and Asia put together, and was the passage to other islands and to another continent, of which the Mediterranean Sea was only the harbour; and within the pillars the empire of Atlantis reached to Egypt and Tyrrhenian This mighty power was arrayed against Egypt and Hellas aad all the countries bordering on the Mediterranean. Theu did your city bravely, and won renown over the whole earth. For at the peril of her own existence, and when the other Hellenes had deserted her, she repelled the invader, and of her own accord gave liberty to all the nations within the pillars. A little while afterwards there was a great earthquake, and your warrior race all sank into the earth . and the great island of Atlantis also disappeared ia the sea. This is the explanation of the shallows which are found in that part of the Atlantic ocean."—(Jowctt'a Introduction to the Timceus.) Such is the main substance of the principal account of the island furnished by the ancients,—an account which, if not entirely fictitious, belongs to the most nebulous region of history. The story may emb xly some populn r legend, and the legend may have rested on certain historical circumstances; but what these were it is (as the numerous theories advanced on the subject nay be held as proving) impossible now to determine.
ATLAS ("ataos), in Greek Mythology, called sometimes a son of Japetus and the nymph Asia, or of Uranus and Oaia, and at other times traced to u different parentage, hat alway» known as the being who supported on liis
shoulders the pillars on which the sky rested. He knew the depths of the sea (fidywey, vii. 245), and in the first instance seems to have been a marine creation. The pillars which he supported were thought to rest in the sea, immediately beyond the most western horizon. But by the time of Herodotus (iv. 181), a mountain is suggested as best suited to hold up tho heavens, and tho name of Atlas is transferred to a hill in the N.W. of Africa. Then the name is traced to a king of that district, rich in flocks and herds, and owning the garden of the Hesperidcs. Finally, Atlas was explained as the name of a primitive astronomer. He was the father of the Pleiades and Hyades. Perseus encountered him when he searched for Medusa. Heracles took the burden of the sky from his shoulders, but cleverly contrived to replace it Atlas bearing up the heavens is mentioned as being represented on early works of art,«.^.,on the chest of Cypselus (Pausan., v. 18,1), and on the throne of Apollo at Amyclas (Pausan., iii. 18, 7); and this subject occurs on several existing works of art
ATLAS, a mountain-chain of Northern Africa, between the great desert of the Sahara and the Mediterranean. Th.range has been but partially explored, and geographers differ as to its extent, some considering it to reach from Cape Qhir on the Atlantic to Cape Bon, the north-east point of Tunis, whilo others include under the name the whole mountain system between Capo Nun and the greater Syrtis. In this latter sense it forms the mountain-land of the countries I of Marocco, Algeria, Tunis, and Tripoli It \a composed I of ranges and groups of mountains, enclosing well-watered and fertile valleys and plains, and having a general direction from W. to E. Tho highest peaks are supposed to attain an elevation of nearly 15,000 feet; and although none of them reach the height of perpetual snow, some of their loftiest summits are covered with snow during the greater part of the year. Mount Miltsin, 27 miles S.E. of the city of Marocco, was ascertained by Captain Washington to be 11,400 feet high. The greatest heights aro in Maroco. fmin which point the) appear to dimini li in elevation as they extend towards the E. These mountains, except the loftier summits, are, for the most part, covered with Thick forests of pine, oak, cork, white poplar, wild olive, and other trees. The inferior ranges seem to be principally composed of Secondary limestone, which, at a greater elevation, is succeeded by micaceous schist and quartz-rock; and the higher chains are said to consist of granite, gneiss, mica-slate, and clay-slate. The Secondary and Tertiary formations are frequently disturbed and upraised by trap-rocks of comparatively modern date. Lead iron copper, antimony, sulphur, and rock-salt occur
frequently; and in the Morocco portion of the range gold and silver are said to exist. In the Algerian division are mines of copper, lead, silver, and antimony. The lion, hyena, boar, and bear are common throughout the mountains. None of the rivers which take their rise in the system are of any great importance. The Tafilet is absorbed in the sands; the Ten si ft and Draa flow into the Atlantic; and about five or six find their way to the Mediterranean. Dr Hooker has explored the botany of many parts of tha range, and the travels of Bohlfs have added largely to our general knowledge of it.
ATMOSPHERE is the name applied to the invisible elastic envelope which surrounds the earth, the gaseous matter of which it is composed being usually distinguished by the name of air. Storms and weather generally, solar and terrestrial radiation, the disintegration of rocks, animal and vegetable life, twilight, and the propagation of sound, are some of the more striking phenomena which are either (o a large extent or altogether dependent on the atmojphero. That air possesses weight may bo shown by the simple experiment of taking a hollow globe filled with air and' weighing it; then removing the contained air by means of an air-pump, and again weighing the globe, when it will be found to weigh less than at first. The difference of the two results is the weight of the air which lias been removed. From Reghault's experiments, 100 cubic inches of dry air, or air containing no aqueous vapour, under a pressure of 30 English inches of mercury, and at a temperature of GO0 Fahr., weigh 3V03S29 grains; and since 100 cubic inches of distilled water at the same pressure and temperature weigh 25,252| grains, it follows that air is 813 07 times lighter than water.
Air as an elastic fluid exerts pressure upon the earth or any substance on which it rests, the action of a boy's sucker and of a water-pump being familiar instances showing the pressure of the atmosphere. When air is removed from a water-pump, the water rises in the pump only to a certain height; for as soon as the water has risen to such a height that the weight of the column of water in the pump above the level of the surface of the water in the well just balances the pressure exerted by the atmosphere on the surface of the well, it ceases to rise. If the pressure of the atmosphere be increased, the water will rise higher in the pump; but if diminished, the level of the water will sink. The height to which the water rises within the pump thus varies with the pressure of the atmosphere, the height being generally about 31 feet Since a given volume of mercury weighed in vacuo at a temperature of 62° Fahr. is 13 569 times heavier than the same volume of water, it follows that a column of mercury will rise tn vacuo to a height 13 569 times less than a column of water, or about 30 inches. If we suppose, then, the height of the mercurial column to be 30 inches, which is probably near the average height of the barometer at sea-level, and its base equal to a square inch, it will contain 30 cubic inches of mercury; and since one cubic inch of mercury contains 34267 grains, the weight of 30 cubic inches will bo nearly 14-7304 £> avoirdupois. Thus the pleasure of the atmosphere is generally, at least in these latitudes, at sea-level equal to 14-7304 B> on each square inch of the earth's surface. Sir John Herschel has calculated that the total weight of an atmosphere averaging 30 inches of pressure is about 11} trillions of pounds; and that, making allowance for the space occupied by the land above the sea, the mass of such an atmosphere is about tiooeao P*rt of that of the earth itself. This enormous
pressure is exerted on the human frame in common with all objects on the earth's surface, and it is calculated that a man of the ordinary size sustains a pressure of about 1 4 tons ; but as the pressure is exerted equally in all directions, and permeates the whole body, no inconvenience arises in consoquence of it.
A pressure agreeing approximately with the average atmospheric pressure at sea-level is often .used as a unit of pressure. This unit is called an atmosphere, and is employed in measuring pressures in steam-engines and boilers. The value of this unit which has been adopted, in the metrical system, is the pressure of 760 millimetres (29-922 Eng. inches) of the mercurial column at 0° C. (32° Fahr.) at Paris, which amounts in that latitude to L033 kilogrammes on the square centimetre. In the English system, an atmosphere is the pressure due to 29"905 inches of the mercurial column at 32° Fahr. at London, amounting there to nearly 14J lb weight on the square inch. The latter atmosphere is thus 0 99968 of that of the metrical system.
As regards the distribution of atmospheric pressure over the globe, there was little beyond conjecture, drawn from theoretical considerations aud for the most part erroneous, till the publication in 1868 of Buchan's memoir "On the Mean Pressure of the Atmosphere and the Prevailing Winds over the Globe."1 By the monthly isobaric charts and copious tables which accompanied the memoir, this important physical problem was first approximately solved. Since then the British Admiralty has published charts showing the mean pressure of the atmosphere over the ocean.3 The more important general conclusions regarding the geographical distribution of atmospheric pressure are the following:—
There are two regions of high pressure, the one north and the other south of the equator, passing completely round the globe as broad belts of high pressure. They enclose between them the low pressure of tropical regions, through the centre of which runs a narrower belt of still lower pressure, towards which the north and south trades blow. The southern belt of high pressure lies nearly parallel to the equator, and is of nearly uniform breadth throughout; but the belt north of the equator has a very irregular outline, and great differences in its breadth and in its inclination to the equator,—these irregularities being due to the unequal distribution of land and water in thf northern hemisphere. Taking a broad view of the subject, there are only three regions of low prossure,—one round each pole, bounded by or contained within the belts of high pressure just referred to, and the equatorial belt of low pressure. The most remarkable of these, in so far as yet known, is the region of low pressure surrounding the south pole, which appears to remain pretty constant
1 Trans. Roy. Soc. Edin., vol. xxv. p. S7S.
'Physical Charts of the Pacific, Atlantic, and Indian Octant, Loot. isring the whole year. The depression round the north pole is divided into two distinct centres, at each of -which •here is a diminution of pressure greatly lower than the average north polar depression. These two centres lie in the ndrth, of the Atlantic and Pacific Oceans respectively. The distribution of pressure in the different months of the year differs widely from the annual average, particularly in January and July, the two extreme months. In January the highest pressures are over the continents of the northern hemisphere,—and the larger the continental mass tie greater the pressure,—and the lowest pressures are over the northern portions of the Atlantic and Pacific, South America and South Africa, and the Antarctic Ocean. In the centre of Asia the mean pressure of the atmosphere in this month is fully 30-400 inches, whereas iu the North Atlantic, round Iceland, it is only 29'340 inches, or upwards of an inch lower than in Central Asia. The area of high barometer is continued westwards through Central and Southern Europe, the North Atlantic between 5° and 45° N. 1st, North America, except the north and north-west, and the Pacific for some distance on either side of 16° X. 1st. It is thus an exaggerated form of the high belt of annual mean pressure, spreading, however, over a much greater breadth in North America, and a still greater breadth in Asia.
In July, on the other hand, the mean pressure of Central Asia is only 29168 inches, or nearly an inch lower than during January; or, putting this striking result in other words, about a thirtieth of the pressure of the atmosphere is removed from this region during the hottest months of the year as compared with the winter season. The lowest pressures of the northern hemisphere are now distributed over the continents, and the larger the continental mass the greater is the depression. At the same time, the highest are over the ocean between 50° N. and 50s S. lat, particularly over the North Atlantic and the North Pacific between 25° and 40° N. lat., and in the fouthern hemisphere over the belt of high mean annual pressure, -which in this month reaches its TnaTimnm height pressure is high in South Africa and in Australia, just as in the winter of the northern hemisphere pressures are high orei the continents.
Over the ocean, if we except the higher latitudes, atmospheric pressure is more regular throughout the year than over the land. In the ocean to westwards at each of the continents there occurs at all seasons an area of high pressure, from 0'10 inch to 0 30 inch higher than what prevails on the coast westward of which it lies. The distance of these spaces of high pressure is generally about 30° of longitude; and their longitudinal axes he, roughly speaking, about the zones of the tropics. The maximum it reached during the winter months, and these areas of high pressure are most prominently marked west of those continents which have the greatest breadth in 30° lat.; and the steepest barometric gradients are on their eastern sides. It is scarcely possible to over-estimate the importance of these regions of high and low mean pressures, from their uitim&te bearing on atmospheric physics, but more particularly from their vital connection with prevailing winds and the general circulation of the atmosphere. This relation will be apprehended when it is considered that winds are simply the flowing away of the air from regions where there is a surplus (regions of high pressure) to where there is a deficiency of air (regions of low pressure). Everywhere over the globe this transference takes place in strict accordance with Buys-Ballot's "Law of the Winds," which may be thus expressed:—The wind neither blows round the space of lowest pressure in circles returning on themselves, nor does it blow directly toward that space; but it takes a direction intermediate, approaching, however, more
nearly to the direction and course of circular curves than of radii to a centre. More exactly, the angle is not a right angle, but from 45° to 80°. Keeping this relation between wind and the distribution of pressure in mind, the isobaric lines give the proximate causes of the prevailing winds over the globe, and through these the prominent features of climates. As regards the ocean, the prevailing winds indicate the direction of the drift-currents and other surface-currents, and thereby the anomalous distribution of the temperature of the sea as seen in the Chili, Guinea, and other ocean currents, and the peculiarly marked climates of the coasts past which these currents flow, are explained; for observations have now proved that the prevailing winds and surface-currents of all oceans are all butabsolutely coincident.
As regards the annual march of pressure through the months of tho year, curves representing it for the different regions of the earth differ from each other in every conceivable way. It is only when the results are set down in their proper places on charts of the globe that the subject can be well understood. When thus dealt with, many of the results are characterised by great beauty and simplicity. Thus, of all influences which determine the barometric fluctuation through the months, the most important ore the temperature, and through the temperature the humidity. Comparing, then, the average pressure in January with that in July, which two months give the greatest possible contrasts of temperature, the following is the broad result:—
The January exceeds the July pressure over the whole of Asia except Kamtchatka and the extreme north-east, the greatest excess being near the centre of the continent; over Europe to south and east of a line drawn from the White Sea south-westward to the Haze, thence southward to the mouth of the Weser, then to Tours, Bordeaux, and after passing through the north of Spain, out to sea at Coruna; over North America, except the north-east and north-west On the other hand, the July exceeds tho January pressure generally over the whole of the southern hemisphere, over the northern part of the North Atlantic and regions immediately adjoining (the excess amounting in Iceland to 0'397 inch), and over the northern part of the North Pacific and surrounding regions. Thus the pressure which is so largely removed from the Old and New Continents of the northern hemisphere in July is transferred, partly to the southern hemisphere, and partly to the northern portions of the Atlantic and Pacific Oceans.
Atmospheric pressure is more uniformly distributed over the globe in April and October than in any of the other months. In May and November, being the months immediately following, occur tho great annual rise and fall of temperature; and since these rapid changes take place at very different rates, according to the relative distribution of land and water in each region, a comparison of the geographical distribution of May with that for t'jo year brings out in strong relief the more prominent causes which influence climate, and some of the more striking results of these causes. This comparison shows a diminution of pressure in May over tropical and sub-tropical regions, including nearly the whole of Asia, the southern half of Europe, and the United States. An excess prevails over North America to the north of the Lakes, over Arctic America, Greenland, the British Isles, and to the north of a lino passing through the English Channel in a northeasterly direction to the Arctic Sea. The excess in tie southern hemisphere includes the southern half of south America and of Africa, the whole of Australia, and adjacent parts of the ocean. The influence of the land of the southern hemisphere, which in this month is colder than the surrounding seas, brings about an excess of pressure; on the other hand, the influence of land over those regions which are more immediately under the sun brings about a lower pressure, interesting examples of which occur in India, the Malayan Archipelago, and the Mediterranean, Black, and Caspian Seas. In many cases the lines of pressure follow more or less closely the contours of the coasts. Thus the diminution is greater oyer Italy and Turkey than over the Adriatic and Black Seas. Tho greatest diminution occurs in Central Asia, where it exceeds 0-200 inch, and the greatest excess round Iceland, where it exceeds 0'200 inch. It is to the position of Great Britain, with reference to the deficiency of pressure on the one hand and the oxcess on tho other, that the general prevalence of east winds at this season is due. These easterly winds prevail over the whole of Northern Europe, as far south as a line drawn from Madrid and passing in a north-easterly direction through Geneva, Munich, &c. To the south of this line tho diminution of pressure is less, and over this region tho winds which are in excess are not easterly, but southerly. Crossing the Mediterranean, and advancing on Africa, we approach another region of lower pressure, towards which easterly and north-easterly winds again acquire the ascendency, as at Malta, Algeria, Ac.
This, in many cases great, variation of the pressure in the different months of the year must be kept carefully in view in deducing heights of places from observations made by travellers of the pressure of atmosphere, by the barometer Or the temperature of boiling water. In reducing the observations, it is necessary to assume a sealevel pressure if the place is at a considerable distance from any meteorological observatory. Previous to the publication of Buchau'tf Menu JPretture of the Almotphere, it appears that a mean sea-level pressure of 29-92 or 30 '00 inches was in such cases universally assumed. The mean pressure at Barnaul, Siberia, being 29 536 inches in July, 30 293 inches in January, and 29'954 inches for the year, it follows that, by the former method of calculating the heights, observations made in January to ascertain the height of Lake Balkash would make the lake 350 feet too high, and observations made in July would make it 330 feet too low,—the difference of the two observations, each set being supposed to be made under the most favourable circumstances, and with the greatest accuracy, being 680 feet This illustration will serve to account for many of the discrepancies met with in books regarding the heights of mountains and plateaus
Of the periodical variations of atmospheric pressure, the most marked is the daily variation, which in tropical and sub-tropical regions is one of the most regular of recurring phenomena. In higher latitudes the diurnal oscillation is masked by the frequent fluctuations to which the pressure is subjected. If, however, hourly observations be regularly made for some time, the hourly oscillation will become apparent. The results show two maxima occurring from 9 to 11 A.M. and 9 to 11 P.M., and two minima occurring from 3 to 6 A.m. and 3 to 6 P.m. The following are the extreme variations for January, April, July, and October from the doily mean pressure at Calcutta, deduced from the observations made during six years, viz., 1857-62:—
These two illustrations may be regarded as typical, to a large extent, of the diurnal barometric oscillations in tropical and temperate regions. At Calcutta the amounts are large, and the dates of the occurrence of the maxima and minima very regular from 3 to 4 and 9 to 10 A M. and P.m. respectively. On the other hand, the oscillations at Vienna are much smaller and more variable in amount, and the dates of occurrence of the critical phases take place through a wider interval, viz., from 3 to 6 and 9 to 11 A.M. and P.m. respectively.
Though the diurnal barometric oscillations are among the best-marked of meteorological phenomena, at least in tropical and sub-tropical regions, yet none of these phenomena, except perhaps the electrical, could be named respecting whose geographical distribution so little is really known, whether as regards the amount of variation, the hour of occurrence of the critical phases, or, particularly, the physical causes on which the observed differences depend. This arises chiefly from the want of a sufficient number of ascertained facts; and to remedy this deficiency, observations have, in the preparation of this present article, been collected and calculated from upwards of 250 places in different parts of the globe, and the data set down on charts. The chief results of this inquiry are the following, attention being entirely confined to the chief oscillation, viz., that occurring from the A.m. maximum to the P.m. minimum.
The A.m. Maximum.—In January this occurs from 9 to 10 in tropical and temperate regions as for as 50° N. lat; in higher latitudes the time of occurrence varies from 8 A.m. to noon. Tn July it occurs from 9 to 10 everywhere only as far as about 40° N. lat.; the time at Tiflis (41° 42' N. lat) being between 7 and 8 A.m. In higher latitudes the time varies from 8 to 11 A.m., the last hour being general in,north-western Europe.
The pjt Minimum.—In January this occurs from 3 to 4 P.m. nearly everywhere over the globe, a few exceptions occurring in north-western Europe, the extremes being 2 P.m. at Utrecht and 6 P.m. at St Petersburg. It is quite different in July, when the time from 3 to 4 P.m. is regularly kept as far north as about 40° N. lat In higher latitudes the hour is very generally 5, but at some places it is as early as 4 P.M., and at others as late as 6 P.M.
In the northern hemisphere, in summer, the afternoon minimum falls to a greater extent below the mean of the day than the forenoon maximum rises above it, at 82 percent, of the stations; but in winter the percentage is only 61. In the southern hemisphere the same relation is observed in the summer and winter months, thus showing that in the summer of both hemispheres the influence of the sun tends to lower the minimum at 3 to 4 P.m. to a greater extent than to raise the 9 to 10 A.m. maximum.
Decrease between Morning Maximum and Afternoon Minimum.—Of the four daily oscillations, this is the most important. When the amounts at different places are entered on charts of the globe, it is seen that the amplitude of this fluctuation is, speaking generally, greatest in the tropics, diminishing as we advance into higher latitudes; greater over the land than over the sea, increasing greatly on proceeding inland; nearly always greater with a dry than with a moist atmosphere; and generally, but by no means always, it is greatest in the month of highest