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the existing are not the lineal descendants of the fossil the Pentacrinus type, which has come down with very
little types, the differences they present being not greater than alteration from the Liassic; whilst many existing Teremay be fairly attributed to the prolonged action of bratulidæ do not differ more from Oolitic types than the differences of temperature, food, pressure, &c. And when latter differ among each other. Going back still further, these facts are taken in connection with those previously we find in the persistence of certain Foraminiferal types stated as to the probable remoteness of the period from the Carboniferous limestone to the present time, and when (if ever) the present sea-bed of the Atlantic was in the character of its deep-sea beds, a strong indication dry land, the doctrine first put forth by Prof. Wyville that they originated in a Foraminiferal deposit, representThomson, that there has been a continuous formation of ing in all essential particulars that which is now going on; Globigerina-mud on the bottom of the Atlantic from while the persistence of the Lingula from the early the Cretaceous epoch to the present time-or, in other | Silurian strata to the present time suggests the question words, that the formation of chalk on the sea-bed of the whether certain oceanic areas may not have remained in Atlantic did not cease with the elevation of the European the condition of deep sea throughout the whole subsequent area, but has been going on through the whole Tertiary succession of geological changes. period,-must be admitted as (to say the least) a not BIBLIOGRAPHY.—In addition to the ordinary sources of improbable hypothesis. That some considerable change information, the following publications may be specially took place at the conclusion of the Cretaceous epoch, by referred to for recent information in regard to the physical which the temperature of the upper stratum was lowered, geography of the Atlantic :-“Reports of the Deep-Sea so as to be no longer compatible with the existence of the Explorations carried on in H.M. Steam-vessels ‘Lightning,' fishes and chambered cephalopods characteristic of the Porcupine,' and 'Shearwater,'”in Proceedings of the Royal Cretaceous fauna, may be fairly assumed from their disap- Society for 1868, 1869, 1870, and 1872; “On the Gibraltar pearance; but this would not so much affect the deeper Current, the Gulf Stream, and the General Oceanic Circulapart of the basin, in which those lower types that seem tion," in the Journal of the Royal Geographical Society for more capable of adapting themselves to changes in external 1871; and “Further Inquiries on Oceanic Circulation" conditions would continue to hold their ground. That the containing a summary of the “ Challenger” Temperature like conditions had prevailed also through long previous geo- Survey of the Atlantic), in the same journal for 1874; logical periods, may be surmised from the persistence, over Currents and Surface-Temperature of the North and South various parts of the Atlantic sea-bed, of the Apiocrinite Atlantic, published by the Meteorological Committee ; and type, which carries us back to the Oolitic formation, and of The Depths of the Sea, by Prof. Wyville Thomson. (W.B.c.)
ATLANTIS, ATALANTIS, or ATLANTICA, an island men shoulders the pillars on which the sky rested. He knew tioned by Plato and other classical writers, concerning the the depths of the sea (Odyssey, vii. 245), and in the first real existence of which many disputes have been raised. instance seems to have been a marine creation. The pillars In the Timæus, Critias relates how his grandfather Critias which he supported were thought to rest in the sea, had been told by Solon some remarkable events in early immediately beyond the most western horizon. But by Athenian history which he had learned from the Egyptian the time of Herodotus (iv. 184), a mountain is suggested priests at Sais, whose records went much further back as best suited to hold up the heavens, and the name of than the native accounts. “The most famous of all the Atlas is transferred to a hill in the N. W. of Africa. Then Athenian exploits," Solon had been told,“was the overthrow the name is traced to a king of that district, rich in flocks of the island Atlantis. This was a continent lying over and herds, and owning the garden of the Hesperides. against the pillars of Hercules, in extent greater than Finally, Atlas was explained as the name of a primitive Libya and Asia put together, and was the passage to other astronomer. He was the father of the Pleiades and islands and to another continent, of which the Mediter- Hyades. Perseus encountered him when he searched for ranean Sea was only the harbour; and within the pillars Medusa. Heracles took the burden of the sky from his the empire of Atlantis reached to Egypt and Tyrrhenia. shoulders, but cleverly contrived to replace it. Atlas bearThis mighty power was arrayed against Egypt and Hellas ing up the heavens is mentioned as being represented on early and all the countries bordering on the Mediterranean. works of art, e.g., on the chest of Cypselus (Pausan., v. 18,1), Then did your city bravely, and won renown over the and on the throne of Apollo at Amyclæ (Pausan., iii. 18, 7); whole earth. For at the peril of her own existence, and and this subject occurs on several existing works of art. when the other Hellenes had deserted her, she repelled ATLAS, a mountain-chain of Northern Africa, between the invader, and of her own accord gave liberty to all the the great desert of the Sahara and the Mediterranean. The nations within the pillars. A little while afterwards there range has been but partially explored, and geographers differ was a great earthquake, and your warrior race all sank into as to its extent, some considering it to reach from Cape Ghir the earth; and the great island of Atlantis also disappeared on the Atlantic to Cape Bon, the north-east point of Tunis, in the sea. This is the explanation of the shallows which while others include under the name the whole mountain are found in that part of the Atlantic ocean.”——(Jowett's system between Cape Nun and the greater Syrtis. In Introduction to the Timous.) Such is the main substance this latter sense it forms the mountain-land of the countries of the principal account of the island furnished by the of Marocco, Algeria, Tunis, and Tripoli. It is composed ancients,—an account which, if not entirely fictitious, of ranges and groups of mountains, enclosing well-watered belongs to the most nebulous region of history. The story and fertile valleys and plains, and having a general may embody some popular legend, and the legend may have direction from W. to E. The highest peaks are supposed rested on certain historical circumstances; but what these to attain an elevation of nearly 15,000 feet; and although were it is (as the numerous theories advanced on the subject none of them reach the height of perpetual snow, some of may be held as proving) impossible now to determine. their loftiest summits are covered with snow during the
ATLAS ("Arias), in Greek Mythology, called sometimes greater part of the year. Mount Miltsin, 27 miles S.E. a son of Japetus and the nymph Asia, or of Uranus and of the city of Marocco, was ascertained by Captain WashGaia, and at other times traced to a different parentage, ington to be 11,400 feet high. The greatest heights are but always known as the being who supported on his in Marocco, from which point they appear to diminish in
elevation as they extend towards the E. These mountains, frequently; and in the Marocco portion of the range gold except the loftier summits, are, for the most part, covered and silver are said to exist. In the Algerian division are with thick forests of pine, oak, cork, white poplar, wild mines of copper, lead, silver, and antimony. The lion, olive, and other trees. The inferior ranges seem to be hyena, boar, and bear are common throughout the mounprincipally composed of Secondary limestone, which, at a tains. None of the rivers which take their rise in the sysgreater elevation, is succeeded by micaceous schist and tem are of any great importance. The Tafilet is absorbed quartz-rock; and the higher chains are said to consist of in the sands; the Tensift and Draa flow into the Atlantic; granite, gneiss, nica-slate, and clay-slate. The Secondary and about five or six find their way to the Mediterranean. and Tertiary formations are frequently disturbed and Dr Hooker has explored the botany of many parts of the upraised by trap-rocks of comparatively modern date. range, and the travels of Rohlfs have added largely to our Lead iron copper, antimony, sulphur, and rock-salt occur general knowledge of it.
ATMO S P H E RE TMOSPHERE is the name applied to the invisible elas- pressure is exerted on the human frame in common with
matter of which it is composed being usually distinguished a man of the ordinary size sustains a pressure of about 14 by the name of air. Storms and weather generally, solar tons ; but as the pressure is exerted equally in all direcand terrestrial radiation, the disintegration of rocks, animal tions, and permeates the whole body, no inconvenience and vegetable life, twilight, and the propagation of sound, arises in consequence of it. are some of the more striking phenomena which are either A pressure agreeing approximately with the average to a large extent or altogether dependent on the atmo- atmospheric pressure at sea-level is often used as a unit sphere. That air possesses weight may be shown by the of pressure. This unit is called an atmosphere, and is simple experiment of taking a hollow globe filled with employed in measuring pressures in steam-engines and air and weighing it; then removing the contained air boilers. The value of this unit which has been adopted, by means of an air-pump, and again weighing the globe, in the metrical system, is the pressure of 760 millimetres when it will be found to weigh less than at first. The (29.922 Eng. inches) of the mercurial column at 0° C. (32° difference of the two results is the weight of the air which Fahr.) at Paris, which amounts in that latitude to 1.033 has been removed. From Regnault's experiments, 100 kilogrammes on the square centimètre. In the English cubic inches of dry air, or air containing no aqueous vapour, system, an atmosphere is the pressure due to 29.905 under a pressure of 30 English inches of mercury, and at inches of the mercurial column at 32° Fahr. at London, a temperature of 60° Fahr., weigh 31:03529 grains; and amounting there to nearly 14j Ib weight on the square since 100 cubic inches of distilled water at the same inch. The latter atmosphere is thus 0:99968 of that of pressure and temperature weigh 25,252) grains, it the metrical system. follows that air is 813.67 times lighter than water.
As regards the distribution of atmospheric pressure over Air as an elastic fluid exerts pressure upon the earth or the globe, there was little beyond conjecture, drawn from any substance on which it rests, the action of a boy's theoretical considerations and for the most part erroneous, sucker and of a water-pump being familiar instances till the publication in 1868 of Buchan's memoir “On the showing the pressure of the atmosphere. When air is Mean Pressure of the Atmosphere and the Prevailing removed from a water-pump, the water rises in the pump Winds over the Globe.”? By the monthly isobaric charts only to a certain height; for as soon as the water has risen and copious tables which accompanied the memoir, this to such a height that the weight of the column of water in important physical problem was first approximately solved. the pump above the level of the surface of the water in the Since then the British Admiralty has published charts well just balances the pressure exerted by the atmosphere showing the mean pressure of the atmosphere over the on the surface of the well, it ceases to rise. If the ocean. The more important general conclusions regarding pressure of the atmosphere be increased, the water will rise the geographical distribution of atmospheric pressure are higher in the pump; but if diminished, the level of the the following :water will sink. The height to which the water rises There are two regions of high pressure, the one north within the pump thus varies with the pressure of the and the other south of the equator, passing completely atmosphere, the height being generally about 34 feet. round the globe as broad belts of high pressure. They Since a given volume of mercury weighed in vacuo at a enclose between them the low pressure of tropical regions, temperature of 62° Fahr. is 13.569 times heavier than the through the centre of which runs a narrower belt of still same volume of water, it follows that a column of mercury lower pressure, towards which the north and south trades will rise in vacuo to a height 13.569 times less than a blow. The southern belt of high pressure lies nearly column of water, or about 30 inches. If we suppose, then, parallel to the equator, and is of nearly uniform breadth the height of the mercurial column to be 30 inches, which throughout ; but the belt north of the equator has a very is probably near the average height of the barometer at irregular outline, and great differences in its breadth and in sea-level, and its base equal to a square inch, it will contain its inclination to the equator,—these irregularities being due 30 cubic inches of mercury; and since one cubic inch of to the unequal distribution of land and water in the mercury contains 3426.7 grains, the weight of 30 cubic northern hemisphere. Taking a broad view of the subject, inches will be nearly 14:7304 lb avoirdupois, Thus the there are only three regions of low pressure, one round pressure of the atmosphere is generally, at least in these each pole, bounded by or contained within the belts of latitudes, at sea-level equal to 14.7304 1h on each square high pressure just referred to, and the equatorial belt of inch of the earth's surface. Sir John Herschel has low pressure.
The most remarkable of these, in so far as calculated that the total weight of an atmosphere averaging yet known, is the region of low pressure surrounding the 30 inches of pressure is about 113 trillions of pounds; south pole, which appears to remain pretty constant and that, making allowance for the space occupied by the land above the sea, the mass of such an atmosphere is about
1 Trans. Roy. Soc. Edin., vol. xxv. p. 575.
9 Physical Charts of the Pacific, Atlantic, and Indian Oceans, Lond. Todooo part of that of the earth itself. This enormous
during the whole year. The depression round the north | nearly to the direction and course of circular curves than pole is divided into two distinct centres, at each of which of radii to a centre. More exactly, the angle is not a right there is a diminution of pressure greatly lower than the angle, but from 45° to 80°. Keeping this relation between average north polar depression. These two centres lie in wind and the distribution of pressure in mind, the isobaric the north of the Atlantic and Pacific Oceans respectively. lines give the proximate causes of the prevailing winds The distribution of pressure in the different months of the over the globe, and through these the prominent features year differs widely from the annual average, particularly in of climates.
of climates. As regards the ocean, the prevailing winds January and July, the two extreme months. In January indicate the direction of the drift-currents and other surthe highest pressures are over the continents of the nor face-currents, and thereby the anomalous distribution of thern hemisphere, -and the larger the continental mass the temperature of the sea as seen in the Chili, Guinea, and the greater the pressure,—and the lowest pressures are other ocean currents, and the peculiarly marked climates of over the northern portions of the Atlantic and Pacific, the coasts past which these currents flow, are explained; for South America and South Africa, and the Antarctic Ocean. observations have now proved that the prevailing winds and In the centre of Asia the mean pressure of the atmosphere surface-currents of all oceans are all but absolutely coincident. in this month is fully 30•400 inches, whereas in the North As regards the annual march of pressure through the Atlantic, round Iceland, it is only 29.340 inches, or months of the year, curves representing it for the different upwards of an inch lower than in Central Asia. The area regions of the earth differ from each other in every conof high barometer is continued westwards through Central ceivable way, It is only when the results are set down and Southern Europe, the North Atlantic between 5° and in their proper places on charts of the globe that the 45° N. lat., North America, except the north and north-west, subject can be well understood. When thus dealt with, and the Pacific for some distance on either side of 15° | many of the results are characterised by great beauty and N. lat. It is thus an exaggerated form of the high belt of simplicity. Thus, of all influences which determine the annual mean pressure, spreading, however, over a much barometric fluctuation through the months, the most imporgreater breadth in North America, and a still greater tant are the temperature, and through the temperature the breadth in Asia.
humidity. Comparing, then, the average pressure in In July, on the other hand, the mean pressure of Central January with that in July, which two months give the Asia is only 29.468 inches, or nearly an inch lower than greatest possible contrasts of temperature, the following is during January ; or, putting this striking result in other the broad result :words, about a thirtieth of the pressure of the atmo The January exceeds the July pressure over the whole sphere is removed from this region during the hottest of Asia except Kamtchatka and the extreme north-east, months of the year as compared with the winter season. the greatest excess being near the centre of the continent; The lowest pressures of the northern hemisphere are now over Europe to south and east of a line drawn from the distributed over the continents, and the larger the con White Sea south-westward to the Naze, thence southward tinental mass the greater is the depression. At the same to the mouth of the Weser, then to Tours, Bordeaux, and time, the highest are over the ocean between 50° N. and after passing through the north of Spain, out to sea at 50° S. lat., particularly over the North Atlantic and the Coruña ; over North America, except the north-east and North Pacific between 25° and 40° N. lat., and in the north-west. On the other hand, the July exceeds the southern hemisphere over the belt of high mean annual January pressure generally over the whole of the southern pressure, which in this month reaches its maximum height. hemisphere, over the northern part of the North Atlantic Pressure is high in South Africa and in Australia, just as in and regions immediately adjoining (the excess amounting the winter of the northern hemisphere pressures are high in Iceland to 0.397 inch), and over the northern part of over the continents.
the North Pacific and surrounding regions. Thus the presOver the ocean, if we except the higher latitudes, sure which is so largely removed from the Old and New atmospheric pressure is more regular throughout the Continents of the northern hemisphere in July is transyear than over the land. In the ocean to westwards ferred, partly to the southern hemisphere, and partly to the of each of the continents there occurs at all seasons an area northern portions of the Atlantic and Pacific Oceans. of high pressure, from 0.10 inch to 0.30 inch higher than Atmospheric pressure is more uniformly distributed over what prevails on the coast westward of which it lies. The the globe in April and October than in any of the other distance of these spaces of high pressure is generally about months. In May and November, being the months 30° of longitude ; and their longitudinal axes lie, roughly immediately following, occur the great annual rise and speaking, about the zones of the tropics. The maximum fall of temperature ; and since these rapid changes take is reached during the winter months, and these areas of place at very different rates, according to the relative high pressure are most prominently marked west of those distribution of land and water in each region, a comparison continents which have the greatest breadth in 30° lat.; and of the geographical distribution of May with that for the the steepest barometric gradients are on their eastern sides. year brings out in strong relief the more prominent causes It is scarcely possible to over-estimate the importance of which influence climate, and some of the more striking these regions of high and low mean pressures, from their results of these causes. This comparison shows a diminuintimate bearing on atmospheric physics, but more par- tion of pressure in May over tropical and sub-tropical ticularly from their vital connection with prevailing winds regions, including nearly the whole of Asia, the southern and the general circulation of the atmosphere. This rela- half of Europe, and the United States. An excess prevails tion will be apprehended when it is considered that winds over North America to the north of the Lakes, over Arctic are simply the flowing away of the air from regions where America, Greenland, the British Isles, and to the north of there is a surplus (regions of high pressure) to where there a line passing through the English Channel in a northis a deficiency of air (regions of low pressure). Every easterly direction to the Arctic Sea. The excess in the where over the globe this transference takes place in strict southern hemisphere includes the southern half of south accordance with Buys-Ballot's “Law of the Winds,” which America and of Africa, the whole of Australia, and adjacent may be thus expressed :—The wind neither blows round parts of the ocean. The influence of the land of the the space of lowest pressure in circles returning on them-southern hemisphere, which in this month is colder than selves, nor does it blow directly toward that space; but it the surrounding seas, brings about an excess of pressure ; takes a direction intermediate, approaching, however, more on the other hand, the influence of land over those regions
5 3 6
Inch. - .020 -027 - 028 - 015
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
Inch. coasts. Thus the diminution is greater over Italy and
+ .014 11 Turkey than over the Adriatic and Black Seas. The
+ .009 u greatest diminution occurs in Central Asia, where it exceeds
October - 010
4 +.008 10 0-200 inch, and the greatest excess round Iceland, where it exceeds 0.200 inch. It is to the position of Great Britain, These two illustrations may be regarded as typical, to with reference to the deficiency of pressure on the one a large extent, of the diurnal barometric oscillations in hand and the excess on the other, that the general prevalence tropical and temperate regions. At Calcutta the amounts of east winds at this season is due. These easterly winds are large, and the dates of the occurrence of the maxima prevail over the whole of Northern Europe, as far south as and minima very regular from 3 to 4 and 9 to 10 A.M. and a line drawn from Madrid and passing in a north-easterly P.M. respectively. On the other hand, the oscillations at direction through Geneva, Munich, &c. To the south of
To the south of Vienna are much smaller and more variable in amount, this line the diminution of pressure is less, and over this and the dates of occurrence of the critical phases take place region the winds which are in excess are not easterly, but through a wider interval, viz., from 3 to 6 and 9 to 11 southerly. Crossing the Mediterranean, and advancing on A.M. and P.m, respectively. Africa, we approach another region of lower pressure, Though the diurnal barometric oscillations are among towards which easterly and north-easterly winds again the best-marked of meteorological phenomena, at least in acquire the ascendency, as at Malta, Algeria, &c.
tropical and sub-tropical regions, yet none of these phenoThis, in many cases great, variation of the pressure
mena, except perhaps the electrical, could be named in the different months of the year must be kept carefully respecting whose geographical distribution so little is really in view in deducing heights of places from observations known, whether as regards the amount of variation, the made by travellers of the pressure of atmosphere, by the hour of occurrence of the critical phases, or, particularly, barometer or the temperature of boiling water. In the physical causes on which the observed differences dereducing the observations, it is necessary to assume a sea pend. This arises chiefly from the want of a sufficient level pressure if the place is at a considerable distance from number of ascertained facts; and to remedy this deficiency, any meteorological observatory. Previous to the publica observations have, in the preparation of this present article, tion of Buchan's Mean Pressure of the Atmosphere, it been collected and calculated from upwards of 250 places in appears that a mean sea-level pressure of 29.92 or 30.00 different parts of the globe, and the data set down on charts. inches was in such cases universally assumed.
The chief results of this inquiry are the following, attention pressure at Barnaul, Siberia, being 29:536 inches in July, being entirely confined to the chief oscillation, viz., that 30:293 inches in January, and 29.954 inches for the year, occurring from the A.M. maximum to the P.m. minimum. it follows that, by the former method of calculating the The A.M. Maximum.-In January this occurs from 9 to 10 heights, observations made in January to ascertain the in tropical and temperate regions as far as 50° N. lat.; in height of Lake Balkash would make the lake 350 feet too higher latitudes the time of occurrence varies from 8 A.M. high, and observations made in July would make it 330 to noon. In July it occurs from 9 to 10 everywhere only feet too low,—the difference of the two observations, each as far as about 40° N. lat.; the time at Tiflis (41° 42' N. set being supposed to be made under the most favourable lat.) being between 7 and 8 A.M. In higher latitudes the circumstances, and with the greatest accuracy, being 680 time varies from 8 to 11 A.M., the last hour being general feet. This illustration will serve to account for many of in north-western Europe. the discrepancies met with in books regarding the heights The P.M. Minimum.-In January this occurs from 3 to 4 of mountains and plateaus
P.M. nearly everywhere over the globe, a few exceptions Of the periodical variations of atmospheric pressure, the occurring in north-western Europe, the extremes being most marked is the daily variation, which in tropical and 2 P.M. at Utrecht and 6 P.M. at St Petersburg. It is quite sub-tropical regions is one of the most regular of recurring different in July, when the time from 3 to 4 P.M. is reguphenomena. In higher latitudes the diurnal oscillation is larly kept as far north as about 40° N. lat. In higher masked by the frequent fluctuations to which the pressure latitudes the hour is very generally 5, but at some places is subjected. If, however, hourly observations be regularly it is as early as 4 P.m., and at others as late as 6 P.M. made for some time, the hourly oscillation will become In the northern hemisphere, in summer, the afternoon apparent. The results show two maxima occurring from minimum falls to a greater extent below the mean of the 9 to 11 A.M. and 9 to 11 P.m., and two minima occurring day than the forenoon maximum rises above it, at 82 per from 3 to 6 A.M. and 3 to 6 P.M. The following are the ex. cent, of the stations; but in winter the percentage is only treme variations for January, April, July, and October 61. In the southern hemisphere the same relation is from the daily mean pressure at Calcutta, deduced from observed in the summer and winter months, thus showing the observations made during six years, viz., 1857-62:
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 January ..023 3 +:079 10 - .053 4 + :010 10 entered on charts of the globe, it is seen that the amplitude April +070 - 071 + 016 10
of this fluctuation is, speaking generally, greatest in the July
- 019 +040 10 - .051 +029 10 October
tropics, diminishing as we advance into higher latitudes; 026 - 047 4 + 018 10
greater over the land than over the sea, increasing greatly
on proceeding inland ; nearly always greater with a dry Similarly the maxima and minima at Vienna, with the than with a moist atmosphere ; and generally, but by hour of their occurrence, are as follows :
no means always, it is greatest in the month of highest
temperature and greatest dryness combined. The regions which is completely shut in by land, and the Atlantic, of largest amplitude include the East India Islands, Eastern which is bounded by two great continents, show a much Peninsula, India, Arabia, tropical Africa, and tropical South smaller oscillation than prevails over the land adjoining and Central America, where it either closely approaches them, and the lines of equal oscillation now attain their or exceeds 0.100 inch. At Silchar, in Assam, it is 0.133 annual maximum. On the other hand, in January, when inch. In the tropical parts of the ocean the oscillation is the sun's rays fall perpendicularly over the most uniform from 0-020 to 0·030 inch less than on land. The influence surface, or over the maximum extent of ocean, the lines are of the Mediterranean Sea in lessening the amount over all almost everywhere parallel with the parallels of latitude. regions bordering it is very strongly marked. The line Again, on advancing inland from the Atlantic, the showing an oscillation of 0.050 inch crosses North effects of comparatively local influences are very striking, America about lat. 44°, curves southward at some distance as the following mean July oscillations, from places from the east coast to lat. 23°, then north-eastward along the situated in lines running in different directions, show :coast of Africa, passes eastwards near the north coast of that Dublin, 0·012; Oxford, 0·022; Ostend, 0·009; Brussels, continent, thence strikes northwards, cutting the easternt 0.019; Vienna, 0.049; Odessa, 0.024 ; and Tiflis, 0.077: part of the Black Sea, and eastward across the Caspian to Limerick, 0·010; Helston, 0.007; Paris, 0·020; Geneva, a point to northward of Peking, and then bends southward 0.045 ; Turin, 0.052; Rome, 0.036; Palermo, 0.008; and to the Loo Choo Islands. The line of 0.020 inch cuts the Malta, 0.020. But the most remarkable illustration is the N.W. of Spain and N.W. of France, and runs northward following, the places being all situated between 38° and through Great Britain as far as the Tweed, thence to 42° N. lat. : San Francisco, 0.068; Fort Churchhill, 0.091; Christiania, then southwards to Copenhagen and to Cracow, Washington, 0·063; Angra do Heroisma, 0.006; Lisbon, the latitude of which it follows eastward through Asia. 0.030; Campo Maior, 0.054; Palermo, 0.008; Tiflis,
The more marked seasonal changes are these:-In India 0.077; and Peking, 0.060. the oscillations during the dry and wet seasons, or in It follows from what has been stated that much which January and July, respectively, are-Bombay, 0·120 and has been written regarding these fluctuations, and in ex0.067 inch ; Poonah, 0-133 and 6.059 inch ; and Calcutta, planation of them, does not rest on facts; and nearly 0.132 and 0.091 inch. At Madras, where the rain-bringing everything yet requires to be done in the way of collectcharacters of the monsoons are reversed, the numbers are ing data towards the representation and explanation of 0114 and 0·115 inch, and at Roorkee, where rain falls all the daily oscillations of atmospheric pressure which are, the year round, 0·088 and 0.079. Again, at Aden, in as regards two-thirds of the globe, perhaps, as already Arabia, where the weather of July is peculiarly hot and stated, the most regular of recurring phenomena, and dry, the oscillation in December is 0.106, but in July it an explanation of which cannot but throw much light on rises to 0:137 inch. The point to be insisted on here is, many of the more important and difficult problems of the that, whatever be the cause or causes to which the daily atmosphere. The data chiefly required are—barometric barometric oscillation is due, the absolute amount is largely data from which the amplitude of the four daily oscillations dependent on comparatively local influences.
can be represented in their distribution and times of While illustrations similar to the above may be adduced occurrence for each of the months; temperature data, comfrom many other parts of the globe, showing the influence parable inter se, from which the diurnal march of temperain the same direction of prevailing dry or wet, hot or cold ture for each month can be ascertained; hygrometric data seasons on the amplitude of the oscillation, the North for hourly values ; rain data also for the hours; wind Atlantic and regions adjoining present an apparent excep- observations conducted on a satisfactory and uniform plan; tion to the law which seems to be indicated by these together with magnetic and electrical observations.
It is results. The whole of the North Atlantic, particularly singularly unfortunate that the disposition of meteorologists north of lat. 20°, and the sea-boards which bound it, to of recent years has been to recommend as hours of observawhich the Mediterranean and its immediate sea-board may tions for places which observe only twice or thrice daily, be added, are strikingly characterised by a small summer hours which do not correspond with the times when the oscillation; and this diminution is most strongly marked great barometric and thermometric daily phases occur; along the eastern part of the ocean. Thus, in July, at Ponta hence these phases cannot be noted except at the great Delgada, in the Azores, the oscillation is only 0.06 inch; at observatories, which are too few and far apart to give Angra do Heroisma, also in the Azores, 0.010 inch ; at sufficient data for the proper discussion of many of those Funchal, Madeira, 0·011 inch ; at Oporto, 0.018; Lisbon, questions. 0-030; and Lagos, 0·021 ; at Naples and Palermo, 0.008; Since the two maxima of daily pressure occur when the and at Malta, 0.020 inch. Now, with reference to this temperature is about the mean of the day, and the two extensive region, it is to be noted that the rainfall of July minima when it is at its highest and lowest respectively, there is either zero or very small; and yet with this dry state is thus suggested a connection between the daily barometric of the atmosphere and high temperature (the annual maxi- oscillations and the daily march of temperature; and mum occurring at the time), this oscillation is extraor- similarly a connection with the daily march of the amount dinarily diminished, being exactly the reverse of what takes of vapour and humidity of the air. The view enterplace during the dry and wet seasons in India. The tained by many of the causes of the daily oscillations may diminution on the western half of the Atlantic, though be thus stated :—The forenoon maximum is conceived to be not so great, is also striking, the January and July oscilla- due to the rapidly increasing temperature, and the rapid tions being 0.056 and 0.036 inch in Barbadoes, 0·080 evaporation owing to the great dryness of the air at this and 0.056 at Jamaica, 0·082 and 0.054 at Havanna, time of the day, and to the increased elasticity of the 0-053 and 0.024 in the Bahamas, and 0.054 and 0·022 in lowermost stratum of air which results therefrom, until a Bermuda. Over the whole of the region here indicated the steady ascending current has set in. As the day advances, rainfall of July is largely in excess of that of January. the vapour becomes more equally diffused upwards through The apparently exceptional character of this region is pro- the air, an ascending current, more or less strong and bably due to the circumstance, that at this time of the year steady, is set in motion, a diminution of elasticity follows, the sun's rays fall perpendicularly over a more diversified and the pressure falls to the afternoon minimum. From surface of the earth, that is, on a greater extent of land, this point the temperature declines, a system of descending than at any other season. At this time the Mediterranean, currents set in, and the air of the lowermost stratum