WIND is a sensible agitation of the atmosphere, occasioned by a quantity of air flowing from one place to another.
As navigation depends in a great measure upon the direction and force of the winds, as the temperature of climates is greatly influenced by them, and as they are absolutely necessary to preserve the salubrity of the atmosphere, it is not surprising that they have very much engaged the attention of mankind. To be acquainted with the laws by which they are regulated, and to be able to calculate beforehand the consequences of these laws, has been in every age the eager wish of philosophers. But whether it has been owing to an improper method of studying this subject, or to its lying beyond the reach of the human faculties, philosophers have not made that progress in it which the fannigue imaginations of some individuals led them to expect. Many discoveries indeed have been made; and from the numbers and the genius of the philosophers at present engaged in this study, others equally important may be expected.
But, notwithstanding this, many of the phenomena remain unexplained, and a rational and satisfactory theory seems still beyond our reach. It will not be expected, that where philosophers in general have failed, we shall succeed. If we can collect the facts hitherto ascertained, and explain such of them as the late discoveries have enabled us to understand, we trust we shall obtain the indulgence of the Public, though we cannot boast of throwing much new light on this difficult subject.
History of the Winds.
As the winds of the torrid zone differ in several important particulars from those which blow without the tropics, we shall first describe them, and afterwards those of the temperate zones.
1. In those parts of the Atlantic and Pacific oceans which lie near the equator, there is a regular wind during the whole year called the trade-wind. On the north side of the equator it blows from the north-east, varying frequently a point or two towards the north or east; and on the south side of it, from the south-west; changing sometimes in the same manner towards the south or east. The space included between the second and fifth degree of north latitude is the internal limit of these two winds. There the winds can neither be said to blow from the north nor the south; calms are frequent, and violent storms. This space varies a little in latitude as the sun approaches either of the tropics.—In the Atlantic ocean the trade-winds extend farther north on the American than on the African coast; and as we advance westward, they become gradually more easterly, and decrease in strength. Their force diminishes likewise as we approach their utmost boundaries. It has been remarked, that as the sun approaches the tropic of Cancer, the fourth-east winds become gradually more southerly, and the north-east winds more easterly: exactly the contrary takes place when the sun is approaching the tropic of Capricorn. § Ibid.
The trade-wind blows constantly in the Indian ocean from the tenth degree of south latitude to near the thirty-fifth. But to the northward of this the winds change every six months, and blow directly opposite to their former course. These regular winds are called monsoons, from the Malay word monsoon, which signifies "a season." When they shift their direction, variable winds and violent storms succeed, which last for a month and frequently longer; and during that time it is dangerous for vessels to continue at sea.
The monsoons in the Indian ocean may be reduced to two; one on the north and another on the south side of the equator; which extend from Africa to the longitude of New Holland and the east coast of China, and which suffer partial changes in particular places from the situation and influence of the neighbouring countries.
1. Between the third and tenth degrees of south latitude the south-east trade-wind continues from April to October; but during the rest of the year the wind blows from the north-west. Between Sumatra and New Holland this monsoon blows from the south during our summer months, approaching gradually to the south-east as we advance towards the coast of New Holland; it changes about the end of September, and continues in the opposite direction till April. Between Africa and Madagascar its direction is influenced by the coast; for it blows from the north-east from October to April, and during the rest of the year from the south-west.
2. Over all the Indian ocean, to the northward of the Tract, vol. 3rd degree of south latitude, the north-east trade-wind blows from October to April, and a south-west wind from April to October. From Borneo, along the coast of Malacca, Dr H. and Ky., ibid. and as far as China, this monsoon in summer blows nearly from the south, and in winter from the north by east §.
Near the coast of Africa, between Mozambique and Cape Guardafui, the winds are irregular during the whole year, owing to the different monsoons which surround that particular place.—Monsoons are likewise regular in the Red Sea; between April and October they blow from the north-west, and during the other months from the south-east, keeping constantly parallel to the coast of Arabia §.
Monsoons are not altogether confined to the Indian Ocean; on the coast of Brazil, between Cape St Augustine and the island of St Catherine, the wind blows between September and April from the east or north-east, and between April and September from the south-west †.—The bay of Panama is the only place on the west side of a great continent where the wind shifts regularly at different seasons: there it is easterly between September and March; but between March and September it blows chiefly from the south and south-west.
Such in general is the direction of the winds in the torrid zone all over the Atlantic, Pacific, and Indian Oceans; but they are subject to particular exceptions, which we shall now endeavour to enumerate.—On the coast of Africa, from Cape Bojador to Cape Verde, the winds are generally north-west; from hence to the island of St Thomas near the equator they blow almost perpendicular to the shore, bending gradually, as we advance southwards, first to the west and then to the south-west ‡.—On the coast of New Spain likewise, from California to the Bay of Panama, the winds blow almost constantly from the west or south-west, except during May, June, and July, when land-winds prevail, called by the Spaniards Pecos ††. On the coast of Chili and Peru §, from 20° or 30° south latitude, to the equator, and on the parallel coast of Africa, the wind blows during the whole year from the south, varying according to the direction of the land towards which it inclines, and extending much farther out to sea on the American than the African coast. The trade-winds are also interrupted sometimes by westerly winds in the Bay of Campeachy and the Bay of Honduras.
As to the countries between the tropics, we are too little acquainted with them to be able to give a satisfactory history of their winds.
In all maritime countries between the tropics of any extent, the wind blows during a certain number of hours every day from the sea, and during a certain number towards the sea from the land; these winds are called the sea and land breezes. The sea breeze generally sets in about ten in the forenoon, and blows till five in the evening; at seven the land-breeze begins, and continues till eight in the morning, when it dies away †. During summer the sea-breeze is very perceptible on all the coasts of the Mediterranean Sea ‡, and even sometimes as far north as Norway §.
In the island of St Lewis on the coast of Africa, in 16° north latitude, and 16° west longitude, the wind during the rainy season, which lasts from the middle of July to the middle of October, is generally between the south and east; during the rest of the year it is for the most part east or north-east in the morning; but as the sun rises, the wind approaches gradually to the north, till about noon it gets to the west of north, and is called a sea-breeze. Sometimes it shifts to the east as the sun declines, and continues there during the whole night. In February, March, April, May, and June, it blows almost constantly between the north and west §. In the island of Balama, which lies likewise on the west coast of Africa, in the 11th degree of north latitude, the wind during nine months of the year blows from the south-west; but in November and December a very cold wind blows from the north-east †.
In the kingdom of Bornou, which lies between the 16th and 25th degree of north latitude, the warm season is introduced about the middle of April by sultry winds from the north-east, which bring along with them a deluge of rain ††. In Fezzan, which is situated about the 25th degree of north latitude, and the 3rd degree of east longitude, the Bornou wind from May to August blows from the east, south-east, or south-west, and is intensely hot ‡.
In Abyssinia the winds generally blow from the west, north-west, north, and north-east. During the months of June, July, August, September, and October, the north and northeast winds blow almost constantly, especially in Abyssinia, the morning and evening; and during the rest of the year they are much more frequent than any other winds ††.
At Calcutta, in the province of Bengal, the wind blows during January and February from the south-west and south; in March, April, and May, from the south; in June, July, August, and September, from the south and south-east; in October, November, and December, from the north-west ††. At Madras the most frequent winds are the north and north-east ††. At Tivoli in St Domingo, and at Isles de voil i. and Vaches, the wind blows oftentimes from the south and south-east ††. From these facts it appears, that in most tropical countries with which we are acquainted, the wind generally blows from the nearest ocean, except during the coldest months, when it blows towards it.
In the temperate zones the direction of the winds is by no means so regular as between the tropics. Even in the same degree of latitude, we find them often blowing in different directions at the same time; while their changes are frequently so sudden and so capricious, that to account for them has hitherto been found impossible. When winds are violent, and continue long, they generally extend over a large tract of country; and this is more certainly the case when they blow from the north or east than from any other points ††. By the multiplication and comparison of Meteorological Tables, for a regular connection between the changes of the atmosphere in different places may in time be obser¬ved, which will at last lead to a satisfactory theory of the winds. It is from such tables chiefly that the following facts have been collected.
In Virginia, the prevailing winds are between the south-west and north-west; the most frequent is the south-west, which blows more constantly in June, July, and August, than at any other season. The north-west winds blow most constantly in November, December, January, and February ††. At Ipswich in New England the prevailing winds are also between the north-west, north, and north-west ††; the most frequent is the north-west ††. But at Cam ii. act. 10, bridge, in the same province, the most frequent wind is the north-west ††. The predominant winds at New York are the north and north-west ††. And in Nova Scotia north-west winds blow for three-fourths of the year ††. The same wind blows most frequently at Montreal in Canada; but at Quebec the wind generally follows the direction of the river St Lawrence, blowing either from the north-west or north-west ††. At Hudson's Bay north-west winds blow for three-fourths of the year; the north-west wind occasions the greatest cold, but the north and north-west are the vehicles of snow ††.
It appears from these facts, that westerly winds are most frequent over the whole eastern coast of North America ††, that in the southern provinces south-west winds predominates; and that the north-west become gradually more frequent as we approach the frigid zone.
In Egypt, during part of May, and during June, July, and August, August, and September, the wind blows almost constantly from the north, varying sometimes in June to the west, and in July to the east and the south; during part of September, and in October and November, the winds are variable, but blow more regularly from the east than any other quarter; in December, January, and February, they blow from the north, north-west, and west; towards the end of February they change to the south, in which quarter they continue till near the end of March; during the last days of March and in April they blow from the south-east, south, and south-west, and at last from the east; and in this direction they continue during a part of May.
In the Mediterranean the wind blows nearly three-fourths of the year from the north; about the equinoxes there is always an easterly wind in that sea, which is generally more constant in spring than in autumn. These observations do not apply to the gulf of Gibraltar, where there are seldom any winds except the east and the west.—At Baflia, in the island of Corfuca, the prevailing wind is the south-west.
In Syria the north wind blows from the autumnal equinox to November; during December, January, and February, the winds blow from the west and south-west; in March they blow from the south, in May from the east, and in June from the north. From this month to the autumnal equinox the wind changes gradually as the sun approaches the equator; first to the east, then to the south, and lastly to the west.—At Bagdad the most frequent winds are the south-west and north-west; at Pekin, the north and the Arctic south; at Kamchatka, on the north-east coast of Asia, the prevailing winds blow from the west.
In Italy the prevailing winds differ considerably according to the situation of the places where the observations have been made: At Rome and Padua they are northerly, at Milan easterly.—All that we have been able to learn concerning Spain and Portugal is, that on the west coast of these countries the west is by far the most common wind, particularly in summer; and that at Madrid the wind is north-east for the greatest part of the summer, blowing almost constantly from the Pyrenean mountains.—At Berne in Switzerland the prevailing winds are the north and west; at St Gotthard, the north-east; at Lausanne, the north-west and south-west.
Father Cotte has given us the result of observations made at 86 different places of France; from which it appears, that along the whole south coast of that kingdom the wind blows most frequently from the north, north-west, and north-east; on the west coast, from the west, south-west, and north-west; and on the north coast, from the south-west. That in the interior parts of France the south-west wind blows most frequently in 18 places; the west wind in 14; the north in 13; the south in 6; the north-west in 4; the south-east in 2; the east and no west each of them in one.—On the west coast of the Netherlands, as far north as Rotterdam, the prevailing winds are probably the south-west, at least this is the case at Dunkirk and Rotterdam. It is probable also that along the rest of this coast, from the Hague to Hamburg, the prevailing winds are the north-west, at least these winds are most frequent at the Hague and at Franeker. The prevailing wind at Delft is the south-east; and at Breda the north and the east.
In Germany the east wind is most frequent at Gottingen, Munich, Weissenburg, Dusseldorf, Saganum, Erford, and at Euda in Hungary; the south-east at Prague and Wurtzburg; the north-west at Ratibone; and the west at Manheim and Berlin.
From an average of ten years of the register kept by order of the Royal Society, it appears, that at London the winds blow in the following order:
| Wind | Days | |------|------| | South-west | 112 | | North-east | 58 | | North-west | 50 | | West | 53 |
It appears, from the same register, that the south-west wind blows at an average more frequently than any other wind during every month of the year, and that it blows longest in July and August; that the north east blows most constantly during January, March, April, May, and June, and most seldom during February, July, September, and December; and that the north-west wind blows oftenest from November to March, and more seldom during September and October than any other months. The south-west winds are also most frequent at Bristol, and next to them are the north-east.
The following table of the winds at Lancaster has been drawn up from a register kept for seven years at that place:
| Wind | Days | |------|------| | South-west | 92 | | North-east | 67 | | South | 51 | | West | 41 |
The following table is an abstract of nine years observations made at Dumfries by Mr Copland:
| Wind | Days | |------|------| | South-west | 82 | | West | 69 | | East | 68 | | South-west | 50 |
The following table is an abstract of seven years observations made by Mr Meek at Cambuslang near Glasgow:
| Wind | Days | |------|------| | South-west | 174 | | North-west | 140 |
It appears, from the register from which this table was extracted, that the north-west wind blows much more frequently in April, May, and June, and the south-west in July, August, and September, than at any other period. We learn from the Statistical Account of Scotland, that the south-west is by far the most frequent wind all over that kingdom, especially on the west coast. At Saltcoats in Ayrshire, for instance, it blows three-fourths of the year; and along the whole coast of Murray, on the north-west side of Scotland, it blows for two-thirds of the year. East winds are common over all Great Britain during April and May; but their influence is felt most severely on the eastern coast.
The following table exhibits a view of the number of days during which the westerly and easterly winds blow in a year at different parts of the island. Under the term westerly are included the north-west, west, south-west, and south; the term easterly is taken in the same latitude.
| Years of Obser. | Places | Westerly | Easterly | |----------------|--------|----------|----------| | 10 | London | 233 | 132 | | | Lancaster | 216 | 149 | | 5 | Liverpool | 170 | 175 | | 9 | Dumfries | 227.5 | 137.5 | | 10 | Braxholm, 54 miles south-west of Berwick | 232 | 133 | | | Cambuslang | 214 | 151 | | 7 | Hawkhill, near Edinburgh | 229.5 | 135.5 | | | Mean | 217.4 | 144.7 |
In In Ireland the south-west and west are the grand trade-winds blowing most in summer, autumn, and winter, and least in spring. The north-east blows most in spring, and nearly double what it does in autumn and winter. The south-west and north-west are nearly equal, and are most frequent after the south-west and west.
At Copenhagen the prevailing winds are the east and south-east; at Stockholm, the west and north. In Russia, from an average or register of 16 years, the winds blow from November to April in the following order:
| West | N.W. | East | S.W. | South | N.E. | N.S.E. | |------|------|------|------|-------|------|--------| | Days | 45 | 26 | 23 | 22 | 20 | 19 |
And during the other six months,
| West | N.W. | East | S.W. | South | N.E. | N.S.E. | |------|------|------|------|-------|------|--------| | Days | 27 | 27 | 29 | 24 | 22 | 15 |
The west wind blows during the whole year 72 days; the north-west 53; the south-west and north 46 days each. During summer it is calm for 41 days, and during winter for 21. In Norway the most frequent winds are the climate south, the south-west, and south-east. The wind at Bergen, seldom directly west, but generally south-west or south-east; a north-west, and especially a north-east wind, are but little known there.
From the whole of these facts, it appears that the most frequent winds on the south coasts of Europe are the north, the north-east, and north-west; and on the western coast, the south-west; that in the interior parts which lie most contiguous to the Atlantic Ocean, south-west winds are also most frequent; but that westerly winds prevail in Germany. Westerly winds are also most frequent on the north-east coast of Asia.
It is probable that the winds are more constant in the south temperate zone, which is in a great measure covered with water, than in the north temperate zone, where their direction must be frequently interrupted and altered by mountains and other causes.
M. de la Baille, who was sent thither by the French king to make astronomical observations, informs us, that at the Cape of Good Hope the main winds are the south-east and north-west; that other winds seldom last longer than a few hours; and that the east and north-east winds blow very seldom. The south-west wind blows in most months of the year, but chiefly from October to April; the north-west prevails during the other six months, bringing along with it rain, tempests, and hurricanes. Between the Cape of Good Hope and New Holland the winds are commonly westerly, and blow in the following order: north-west, south-west, west, north.
In the great South Sea, from latitude 30° to 40° south, the south-east trade-wind blows most frequently, especially when the sun approaches the tropic of Capricorn; the wind next to it in frequency is the north-west, and next to that is the south-west. From south latitude 40° to 50° the prevailing wind is the north-west, and next the south-west. From 50° to 60° the most frequent wind is also the north-west, and next to it is the west.
Thus it appears that the trade-winds sometimes extend farther into the south temperate zone than their usual limits, particularly during summer; that beyond their influence the winds are commonly westerly, and that they blow in the following order: north-west, south-west, west.
Thus have we finished the history of the direction of the winds. In the torrid zone they blow constantly from the north-east on the north side of the equator, and from the south-east on the south side of it. In the north temperate zone they blow most frequently from the south-west; in the south temperate zone from the north-west, changing, however, frequently to all points of the compass, and in the north temperate zone blowing particularly during spring from the north-east.
As to the velocity of the wind, its variations are almost infinite; from the gentlest breeze to the hurricane, which tears up trees and blows down houses. It has been remarked, that our most violent winds take place when neither the heat nor the cold is greatest; that violent winds generally extend over a great tract of country; and that they are accompanied with sudden and great falls in the mercury of the barometer. The wind is sometimes very violent at a distance from the earth, while it is quite calm at its surface. On one occasion Lunardi went at the rate of 70 miles an hour in his balloon, though it was quite calm at Edinburgh when he ascended, and continued so during his whole voyage. See NEUMATICS.
For the instruments invented to measure the velocity of the wind, see ANEMOSCOPE and ANEMOMETER.
Theory of the Winds.
The atmosphere is a fluid surrounding the earth, and extending to an unknown height. Now all fluids tend invariably to a level; if a quantity of water be taken out of any part of a vessel, the surrounding water will immediately flow in to supply its place, and the surface will become level as before; or if an additional quantity of water be poured into any part of the vessel, it will not remain there, but diffuse itself equally over the whole. Such exactly would be the case with the atmosphere. Whatever therefore destroys the equilibrium of this fluid, either by increasing or diminishing its bulk in any particular place, must at the same time occasion a wind.
Air, besides its qualities in common with other fluids, is capable of also capable of being dilated and compressed. Suppose a vessel filled with air: if half the quantity which it contains be drawn out by means of an air-pump, the remainder will fill the vessel completely; or if twice or three times the original quantity be forced in by a condenser, the vessel will still be capable of holding it.
Rarefied air is lighter, and condensed air heavier, than common air. When fluids of unequal specific gravities are mixed together, the heavier always descend, and the lighter ascend. Were quicksilver, water, and oil, thrown into the same vessel together, the quicksilver would uniformly occupy the bottom, the water the middle, and the oil the top. Were water to be thrown into a vessel of oil, it would immediately descend, because it is heavier than oil. Exactly the same thing takes place in the atmosphere. Were a quantity of air, for instance, to be suddenly condensed at a distance from the surface of the earth, being now heavier than before, it would descend till it came to air of its own density; or, were a portion of the atmosphere at the surface of the earth to be suddenly rarefied, being now lighter than the surrounding air, it would immediately ascend.
If a bladder half filled with air be exposed to the heat of a fire, the air within will soon expand, and distend the bladder; if it be now removed to a cold place, it will soon become flaccid as before. This shows that heat rarefies and that cold condenses air. The surface of the torrid zone is much more heated by the rays of the sun than the frozen or temperate zones, because the rays fall upon it much more perpendicularly. This heat is communicated to the air near the surface of the torrid zone, which being thereby rarefied, ascends; and its place is supplied by colder air, which rushes in from the north and south.
The diurnal motion of the earth is greatest at the equator, and diminishes gradually as we approach the poles, where the sun... it ceases altogether. Every spot of the earth's surface at the equator moves at the rate of 15 geographical miles in a minute; at the 40° of latitude, it moves at about 11½ miles in a minute; and at the 30°, at nearly 13 miles. The atmosphere, by moving continually round along with the earth, has acquired the same degree of motion; so that those parts of it which are above the equator move faster than those which are at a distance. Were a portion of the atmosphere to be transported in an instant from latitude 30° to the equator, it would not immediately acquire the velocity of the equator; the eminences of the earth therefore would strike against it, and it would assume the appearance of an easterly wind. This is the case in a smaller degree with the air that flows towards the equator, to supply the place of the rarefied air which is continually ascending; and this, when combined with its real motion from the north and south, must cause it to assume the appearance of a north-easterly wind on this side the equator, and of a south-westerly beyond it.
The motion westwards occasioned by this difference in celebrity alone would not be great; but it is farther increased by another circumstance. Since the rarefaction of the air in the torrid zone is owing to the heat derived from the contiguous earth, and since this heat is owing to the perpendicular rays of the sun, those parts must be hottest where the sun is actually vertical, and consequently the air over them must be most rarefied; the contiguous parts of the atmosphere will therefore be drawn most forcibly to that particular spot. Now since the diurnal motion of the sun is from east to west, this hottest spot will be continually shifting westwards, and this will occasion a current of the atmosphere in that direction. That this cause really operates, appears from a circumstance already mentioned: when the sun approaches either of the tropics, the trade-wind on the same side of the equator assumes a more easterly direction, evidently from the cause here mentioned; while the opposite trade-wind, being deprived of this additional impulse, blows in a direction more perpendicular to the equator.
The westerly direction of the trade-winds is still farther increased by another cause. Since the attraction of the sun and moon produces so remarkable an effect upon the ocean, we cannot but suppose that an effect equally great at least is produced upon the atmosphere. Indeed as the atmosphere is nearer the moon than the sea is, the effect produced by attraction upon it ought to be greater. When we add to this the elasticity of the air, or that disposition which it has to dilate itself when freed from any of its pressure, we cannot but conclude that the tides in the atmosphere are considerable. Now since the apparent diurnal motion of the moon is from east to west, the tides must follow it in the same manner, and consequently produce a constant motion in the atmosphere from east to west. This reasoning is confirmed by the observations of several philosophers, particularly of M. Caffin, that in the torrid zone the barometer is always two-thirds of a line higher twice every 24 hours than during the rest of the day; and that the time of this rise always corresponds with the tides of the sea; a proof that it proceeds from the same cause.
All these different causes probably combine in the production of the trade-winds; and from their being sometimes united, and sometimes distinct or opposite, arise all those little irregularities which take place in the direction and force of the trade-winds.
Since the great cause of these winds is the rarefaction of the atmosphere by the heat of the sun, its ascension, and the consequent rushing in of colder air from the north and south, the internal boundary of the trade-winds must be that parallel of the torrid zone which is hottest, because there the ascension of the rarefied air must take place. Now since the sun does not remain stationary, but is constantly shifting from one tropic to the other, we ought naturally to expect that this boundary would vary together with its exciting cause; and therefore when the sun is perpendicular to the tropic of Cancer, the north-east trade-winds would extend no farther south than north latitude 23½°; that the south-east wind would extend as far north; and that when the sun was in the tropic of Capricorn, the very contrary would take place. We have seen, however, that though this boundary be subject to considerable changes from this very cause, it may in general be considered as fixed between the second and fifth degrees of north latitude.
Though the sun be perpendicular to each of the tropics during part of the year, he is for one half of it at a considerable distance; so that the heat which they acquire while present is more than lost during his absence. But the sun is perpendicular to the equator twice in a year, and never farther distant from it than 23½°; being therefore twice every year as much heated, and never so much cooled, as the tropics, its mean heat must be greater, and the atmosphere in consequence generally most rarefied at that place. Why then, it will be asked, is not the equator the boundary of the two trade-winds? To speak more accurately than we have hitherto done, the internal limit of these winds must be that parallel where the mean heat of the earth is greatest. This would be the equator, were it not for a reason which shall now be explained.
It has been shown by astronomers, that the orbit of the earth is an ellipse, and that the sun is placed in one of the foci. Were this orbit to be divided into two parts by a straight line perpendicular to the transverse axis, and passing through the centre of the sun, one of these parts would be less than the other; and the earth, during its passage through this smaller part of its orbit, would constantly be nearer the sun than while it moved through the other portion. The celerity of the earth's motion in any part of its orbit is always proportioned to its distance from the sun; the nearer it is to the sun, it moves the faster; the farther distant, the slower. The earth passes over the smaller portion of its orbit during our winter; which must therefore be shorter than our summer, both on account of this part of the orbit being smaller than the other, and on account of the increased celerity of the earth's motion. The difference, according to Caffin, is 7 days, 23 hours, and 53 minutes. While it is winter in the northern, it is summer in the southern hemisphere; wherefore the summer in the northern hemisphere must be just as much shorter than the winter as our winter is shorter than our summer. The difference therefore between the length of the summer in the two hemispheres is almost 16 days. The summer in the northern hemisphere consists of 190½ days, while in the southern it consists only of 174½. They are to one another nearly in the proportion of 14 to 12½; and the heat of the two hemispheres may probably have nearly the same proportion to one another.
The internal limit of the trade-winds ought to be that parallel where the mean heat of the globe is greatest: this would be the equator, if both hemispheres were equally hot; but since the northern hemisphere is the hottest, that parallel ought to be situated somewhere in it; and since the difference between the heat of two hemispheres is not great, the parallel ought not to be far distant from the equator (A).
(a) This parallel could be determined by calculation, provided the mean heat of both the segments into which it divides... The trade-wind would blow regularly round the whole globe if the torrid zone were all covered with water. If the Indian Ocean were not bounded by land on the north, it would blow there in the same manner as it does in the Atlantic and Pacific Oceans. The rays of light pass through a transparent body without communicating any, or at least but a small degree of heat. If a piece of wood be inclosed in a glass vessel, and the focus of a burning glass directed upon it, the wood will be burnt to ashes, while the glass through which all the rays passed is not even heated. When an opaque body is exposed to the sun's rays, it is heated in proportion to its opacity. If the bulb of a thermometer be exposed to the sun, the quicksilver will not rise so high as it would do if this bulb were painted black. Land is much more opaque than water; it becomes therefore much warmer when both are equally exposed to the influence of the sun.
For this reason, when the sun approaches the tropic of Cancer, India, China, and the adjacent countries, become much hotter than the ocean which washes their southern coasts. The air over them becomes rarefied, and ascends, while colder air rushes in from the Indian Ocean to supply its place. As this current of air moves from the equator northward, it must, for a reason already explained, assume the appearance of a south-west wind; and this tendency eastward is increased by the situation of the countries to which it flows. This is the cause of the south-west monsoon, which blows during summer in the northern parts of the Indian Ocean. Between Borneo and the coast of China its direction is almost due north, because the country to which the current is directed lies rather to the west of north; a circumstance which counteracts its greater velocity.
In winter, when the sun is on the fourth side of the equator, these countries become cool, and the north-east trade-wind resumes its course, which, had it not been for the interference of these countries, would have continued the whole year.
As the sun approaches the tropic of Capricorn, it becomes almost perpendicular to New Holland; that continent is heated in its turn, the air over it is rarefied, and colder air rushes in from the north and west to supply its place. This is the cause of the north-west monsoon, which blows from October to April, from the third to the tenth degree of south latitude. Near Sumatra, its direction is regulated by the coast; this is the case also between Africa and Madagascar.
The same cause which occasions the monsoons, gives rise to the winds which blow on the west coasts of Africa and America. The air above the land is hotter and rarer, and consequently lighter than the air above the sea; the sea air therefore flows in, and forces the lighter land atmosphere to ascend.
The same thing will account for the phenomena of the sea and land breezes. During the day, the cool air of the sea and sea, loaded with vapours, flows in upon the land, and takes the place of the rarefied land air. As the sun declines, the rarefaction of the land air is diminished; thus an equilibrium is restored. As the sea is not so much heated during the day as the land, neither is it so much cooled during the night; because it is constantly exposing a new surface to the atmosphere. As the night approaches, therefore, the cooler and denser air of the hills (for where there are no hills there are no sea and land breezes) falls down upon the plains, and pressing upon the now comparatively lighter air of the sea, causes the land-breeze.
The rarefied air which ascends between the second and fifth degrees of north latitude, has been shown to be the principal cause of the trade-winds. As this air ascends, it must become gradually colder, and consequently heavier; it would therefore descend again if it were not buoyed up by the constant ascent of new rarefied air. It must therefore spread itself to the north and south, and gradually mix in its passage with the lower air; and the greater part of it probably does not reach far beyond the 30°, which is the external limit of the trade-wind. Thus there is a constant circulation of the atmosphere in the torrid zone; it ascends near the equator, diffuses itself toward the north and south, descends gradually as it approaches the 30°, and returning again towards the equator, performs the same circuit. It has been the opinion of the greater part of those who have considered this subject, that the whole of the rarefied air which ascends near the equator, advances towards the poles and descends there. But if this were the case, a constant wind would blow from both poles towards the equator, the trade-winds would extend over the whole earth; for otherwise the ascent of air in the torrid zone would very soon cease. A little reflection must convince us that it cannot be true: rarefied air differs nothing from the common air except in containing a greater quantity of heat. As it ascends, it gradually loses this superfluous heat. What then should hinder it from descending, and mixing with the atmosphere below? That there is a constant current of superior air, however, towards the poles, cannot be doubted; but it consists principally of hydrogen gas. We shall immediately attempt to assign the reason why its accumulation at the pole is not always attended with a north wind.
If the attraction of the moon and the diurnal motion of the sun have any effect upon the atmosphere, and that they have some effect can hardly be disputed, there must be a real
\[ \frac{2n - 2s}{3n + 3s} = \frac{214}{804}. \]
To reduce this value of \( x \) to degrees, we must multiply it by 60, since a great circle was made = 6; it gives 1° 48' 27" as the internal limit of the trade wind. This is too small by 2° 11' 33". But the value which we have found is only that of the fine of the arc intercepted between the equator and the internal limit; the arc itself would be somewhat greater; besides, the proportion between the heat of the two segments is an assumed quantity, and may probably be greater than their difference in bulk; and one reason for this may be, the great proportion of land in the northern compared with the southern segment. See the Journal de Physique, Mai 1791. Wind. real motion of the air westwards within the limits of the trade-winds. When this body of air reaches America, its further passage westwards is stopped by the mountains which extend from one extremity of that continent to the other. From the momentum of this air, when it strikes against the sides of these mountains, and from its elasticity, it must acquire from them a considerable velocity, in a direction contrary to the first, and would therefore return eastwards again if this were not prevented by the trade-winds. It must therefore rush forwards in that direction where it meets with the least resistance; that is, towards the north and south. As air is nearly a perfectly elastic body, when it strikes against the sides of the American mountains its velocity will not be perceptibly diminished, though its direction be changed. Continuing, therefore, to move with the velocity of the equator, when it arrives at the temperate zones it will assume the appearance of a north-east or south-west wind. To this is to be ascribed the frequency of south-west winds over the Atlantic Ocean and western parts of Europe. Whether these winds are equally frequent in the Northern Pacific Ocean, we have not been able to ascertain; but it is probable that the mountains in Asia produce the same effect as those in America.
It is not impossible that another circumstance may also contribute to the production of these winds. In the article Weather, we endeavoured to prove that the annual evaporation exceeds considerably the quantity of rain which falls; and found reason to conclude, therefore, that part of the evaporated water was decomposed in the atmosphere. In that case, the oxygen, which is rather heavier than common air, would mix with the atmosphere; but the hydrogen (a cubic foot of which weighs only 41.41 grams, while a cubic foot of oxygen weighs 593.32 grams) would ascend to the higher regions of the atmosphere.
By what means this decomposition is accomplished (if it takes place at all) we cannot tell. There are probably a thousand causes in nature of which we are entirely ignorant. Whether heat and light, when long applied to vapours, may not be able to decompose them, by uniting with the hydrogen, which seems to have a greater attraction for heat than oxygen has; or whether the electrical fluid may not be capable of producing this effect—are questions which future observations and experiments must determine. Dr Franklin filled a glass tube with water, and passed an electric shock through it; the tube was broken in pieces, and the whole water disappeared. He repeated the experiment with ink instead of water, and placed the tube upon white paper; the same effects followed; and the ink, though it disappeared completely, left no stain on the paper. Whether the water in these cases was decomposed or not, it is impossible to say; but the supposition that it was, is not improbable. An experiment might easily be contrived to determine the point.
This decomposition would account for the frequency of south-west winds, particularly in summer; for thus new air is furnished to supply the place of that which is forced northwards by the causes already explained. Perhaps it may be a confirmation of this conjecture, that the south-west winds generally extend over a greater tract of country than most other winds which blow in the temperate zones.
What has been said of south-west winds, holds equally with regard to north-west winds in the south temperate zone.
After south-west winds have blown for some time, a great quantity of air will be accumulated at the pole, at least if mutating they extend over all the northern hemisphere; and it appears from comparing the tables kept by some of our late navigators in the Northern Pacific Ocean with similar tables kept in this island, that this is sometimes the case so far as relates to the Atlantic and Pacific Oceans. When this accumulation becomes great, it must, from the nature of fluids, and from the elasticity of air, press with a considerable and increasing force on the advancing air; so that in time it becomes stronger than the south-west wind. This will occur at first a calm, and afterwards a north wind; which north-east will become gradually easterly as it advances southwards, from its not assuming immediately the velocity of the earth. The mass of the atmosphere will be increased in all those places over which this north-east wind blows; this is confirmed by the almost constant rise of the barometer during a north-east wind.
Whatever tends to increase the bulk of the atmosphere near the pole, must tend also to increase the frequency of north-east winds; and if there be any season when this increase takes place more particularly, that season will be most liable to these winds. During winter the northern parts of Europe are covered with snow, which is melted in the beginning of summer, when the heat of the sun becomes more powerful. Great quantities of vapour are during that time raised, which will augment both the bulk and weight of the atmosphere; especially if the conjecture about the conversion of vapour into air has any foundation. Hence north-east winds are most prevalent during May and June (n).
But it will be said, if this hypothesis were true, the south-west and north-east winds ought to blow alternately, and continue each of them for a stated time; whereas the south-west wind blows sometimes longer and sometimes shorter, neither is it always followed by a north-east wind.
If the conjecture about the decomposition of vapour in the torrid zone be true, the hydrogen which formed a part of it will ascend from its lightness, and form a stratum above the atmospheric air, and gradually extend itself, as additional hydrogen rises, towards the north and south, till at last it reaches the poles. The lightness of hydrogen is owing to the great quantity of heat which it contains; as it approaches the poles it must lose a great part of this heat, and may in consequence become heavy enough to mix with the atmosphere below. Oxygen makes a part of the atmosphere; and its proportion near the poles may sometimes be greater than ordinary, on account of the additional quantity brought thither from the torrid zone. Mr Cavendish mixed oxygen and hydrogen together in a glass jar; and upon making an electrical spark pass through them, they immediately combined, and formed water.
That there is electric matter at the poles, cannot be doubted. The Abbé Chappe informs us, that he saw thunder and lightning much more frequently at Toboliski and other parts of Siberia than in any other part of the world. In the north of Europe the air, during very cold weather, is exceedingly electric: sparks can be drawn from a person's hands and face, by combing his hair, or even pow-
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(n) The frequency of north-east winds during these months is the greatest defect in the climate of Scotland, and is felt indeed severely over all Great Britain. In the United States of America, these winds keep pace with the clearing of the land. Some time ago, in Virginia, they did not reach farther than Williamsburgh; now they reach to Richmond, which is situated considerably farther west, and are even beginning to be felt still farther within the country. Might not it be possible then to prevent the frequency of these winds in this country, by planting trees along the whole coast? It is a pity that the experiment were not tried; were it to succeed, it would very materially improve the climate. May not the appearance of the aurora borealis be owing to the union of oxygen and hydrogen by the intervention of the electric fluid? That it is an electrical phenomenon at least, can hardly be doubted. Artificial electricity is much strengthened during an aurora, as Mr Volta and Mr Canton have observed; and the magnetic needle moves with the same irregularity during an aurora that has been observed in other electrical phenomena. This fact we learn from Bergman and De la Lande. Many philosophers have attempted to demonstrate, that aurora boreales are beyond the earth's atmosphere; but the very different results of their calculations evidently prove that they were not possessed of sufficient data.
If this conjecture be true, part of the atmosphere near the poles must at times be converted into water. This would account for the long continuance of south-west winds at particular times: when they do so, a decomposition of the atmosphere is going on at the pole. It would render this conjecture more probable, if the barometer fell always when a south-west wind continues long.
If this hypothesis be true, a south-west wind ought always to blow after aurora boreales; and we are informed by Mr Winn*, that this is actually the case. This he found never to fail in 23 instances. He observed also, that when the aurora was bright, the gale came on within 24 hours, but did not last long; but if it was faint and dull, the gale was longer in beginning, and less violent, but it continued longer. This looks like a confirmation of our conjecture. Bright aurora are probably nearer than those which are dull. Now, if the aurora borealis be attended with a decomposition of a quantity of air, that part of the atmosphere which is nearest must first rush in to supply the defect, and the motion will gradually extend itself to more distant parts. Just as if a hole were bored in the end of a long vessel filled with water, the water nearest the hole would flow out immediately, and it would be some time before the water at the other end of the vessel began to move. The nearer we are to the place of precipitation, the sooner will we feel the south-west wind. It ought therefore to begin sooner after a bright aurora, because it is nearer than a dull and faint one. Precipitations of the atmosphere at a distance from the pole cannot be so great as those which take place near it; because the cold will not be sufficient to condense so great a quantity of hydrogen; south-west winds, therefore, ought not to last so long after bright, as after dull aurora. Winds are more violent after bright aurora, because they are nearer the place of precipitation; just as the water near the hole in the vessel runs swifter than that which is at a considerable distance.
If these conjectures have any foundation in nature, there are two sources of south-west winds; the first has its origin in the trade-winds, the second in precipitations of the atmosphere near the pole (c). When they originate from the first cause, they will blow in countries farther south for some time before they are felt in those which are farther north; but the contrary will take place when they are owing to the second cause. In this last case, too, the barometer will sink considerably; and it actually does so constantly after aurora, as we are informed by Mr Madison†, who paid particular attention to this subject. By keeping accurate meteorological tables in different latitudes, it might easily be discovered whether these consequences be true, and consequently whether the above conjectures be well or ill grounded.
There are also two sources of north-east winds; the first another is an accumulation of air at the pole (b), the second a precipitation of the atmosphere in the torrid zone. For the discovery of this last cause we are indebted to Dr Franklin. In 1740 he was prevented from observing an eclipse of the moon at Philadelphia by a north-east storm, which came on about seven o'clock in the evening. He was surprised to find afterwards that it had not come on at Boston till near 11 o'clock; and upon comparing all the accounts which he received from the several colonies of the beginning of this and other storms of the same kind, he found it to be always an hour later the farther north-east, for every 100 miles.
"From hence (says he) I formed an idea of the course of the storm, which I will explain by a familiar instance. I suppose a long canal of water stopped at the end by a gate. The water is at rest till the gate is opened; then it begins to move out through the gate, and the water next the gate is first in motion, and moves on towards the gate; and so on successively, till the water at the head of the canal is in motion, which it is last of all. In this case all the water moves indeed towards the gate; but the successive times of beginning the motion are in the contrary way, viz. from the gate back to the head of the canal. Thus, to produce a north-east storm, I suppose some great rarefaction of the air in or near the gulf of Mexico; the air rising thence has its place supplied by the next more northern, cooler, and therefore denser, and heavier air; a successive current is formed, to which our coast and inland mountains give a north-east direction."
Currents of air from the poles naturally, as has been observed, assume a north-east direction as they advance southwards; because their diurnal motion becomes less than that of the earth. Various circumstances, however, may change this direction, and cause them to become north, or even south-west, winds. The south-west winds themselves may often prove sufficient for this; and violent rains, or great heat,
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(c) We are now rather doubtful whether the first cause here assigned be so general as we at first imagined. The almost constant sinking of the barometer when a south wind blows, seems to indicate, that it is generally occasioned by decompositions of the atmosphere. Nor are we certain that mountains are adequate to produce the effect assigned them.
(b) When the ice, which in Russia accumulates on the inside of the windows of the common people's houses, thaws, it lets loose a quantity of mephitis air, producing all the dangerous effects of charcoal (Dr Guthrie of the Climate of Russia, Edin. Trans. vol. ii. p. 220.). May not then a quantity of air be extricated from ice during its thawing? And may not this be another source of north-east winds? We are not ignorant of the experiment which Dr Garnier made to discover this (see Manchester Transactions, vol iv.); and that he found that ice in this country sets loose no air in the act of thawing. But Dr Guthrie has shown us, in the essay above referred to, that water, by being long exposed to intense cold, changes its nature, and acquires qualities which it had not before. Would it not be worth the while of the philosophers in Russia, and other cold countries, to investigate this a little farther? We would recommend it to the consideration of the ingenious Dr Guthrie himself; who, from his situation, has the best opportunities of investigating the matter completely. It is certainly of very great importance, and might lead to discoveries that would remove our present difficulties in meteorology, and enable us to give a satisfactory and useful theory of the weather. heat, by lessening or rarefying the atmosphere in any country, will produce the same effect in countries to the westward when north winds happen to be blowing.
In North America, the north-west winds become gradually more frequent as we advance northwards. The east coast of this continent, where the observations were made from which this conclusion was drawn, is alone cultivated; the rest of the country is covered with wood. Now cultivated countries are well known to be warmer than those which are uncultivated; the earth in the latter is shaded from the sun, and never heated by his rays. The air, therefore, in the interior parts of America, must be constantly colder than near the east coast. This difference will hardly be perceptible in the southern parts, because there the influence of the sun is very powerful; but it will become gradually greater as we advance northwards, because the influence of the sun diminishes, and the continent becomes broader. Hence north-west winds ought to become more frequent upon the east coast as we advance northwards; and they will probably cease to blow so often as soon as the whole continent of North America becomes cultivated.
Thus have we attempted to explain the causes which produce the more general winds that prevail in the torrid and temperate zones. The east and west winds, when they are not partial and confined to a very small portion of the atmosphere, seem to be nothing else but currents of air brought from the north or south by the causes already mentioned, and prevented from proceeding farther by contrary currents. If these currents have come from the north, they will assume the appearance of east winds; because their diurnal motion will be less than that of the more southern latitudes over which they are forced to remain stationary. The southern currents will become west winds, for a contrary reason. This will furnish us with a reason for the coldness of east winds, compared with west winds. If this account be true, there ought very frequently to be a west wind in a latitude to the south of those places where an east wind blows. This might easily be determined by keeping accurate registers of the winds in different latitudes, and as nearly as possible under the same meridian; and upon the result of these observations the truth or falsehood of the above conjecture must finally rest.
Besides these more general winds, there are others which extend only over a very small part of the earth. These originate from many different causes. The atmosphere is composed of three different kinds of air, oxygen, azote, and carbonic acid; to which may be added water. Great quantities of each of these ingredients are constantly changing their aerial form, and combining with various substances; or they are separating from other bodies, affuming the form of air, and mixing with the atmosphere. Partial voids, therefore, and partial accumulations, must be continually taking place in different parts of the atmosphere, which will occasion winds varying in direction, violence, and continuance, according to the suddenness and the quantity of air destroyed or produced. Besides these there are many other ingredients constantly mixing with the atmosphere, and many partial causes of condensation and rarefaction in particular places. To these, and other causes probably hitherto unknown, are to be ascribed all those winds which blow in any place besides the general ones already explained; and which, as they depend on causes hitherto at least reckoned contingent, will probably for ever prevent uniformity and regularity in the winds. All these causes, however, may, and probably will, be discovered; the circumstances in which they will take place, and the effects which they will produce, may be known; and whenever this is the case, the winds of any place may in some measure be reduced to calculation.
It is of importance, in the first place, to know the general winds, and the causes which produce them; they will blow oftenest in every country, continue longest, and in a great measure stamp the nature of the climate. To explain these has been the intention of this essay; and though we have probably failed of success, our attempt, we hope, will not be altogether useless. The facts which are here collected will at least facilitate the labours of the future inquirer. Were accurate observations made over the whole globe of the direction and velocity of the winds, and especially of the time when they begin and cease to blow, so much light would be thrown in a short time upon this important subject, that a theory of the winds might be formed, capable of explaining all the phenomena, and really useful to the human race.
**Hot Winds.** See SAMUEL.
**Wind-Flower.** See ANEMONY.
**Wind-Mill,** a kind of mill, the internal parts of which are much the same with those of a water-mill: from which, however, it differs, in being moved by the impulse of the wind upon its sails or vanes, which are to be considered as a wheel in axis. See MECHANICS, no. 62.
**Wind-Gage.** See Wind-GAGE.
**Wind-Galls,** in farriery. See there § xxxiii.
**Wind-Gun.** See Air-Gun.
**Instruments for measuring the strength, velocity, &c. of the Wind.** See Wind-Gage, Anemometer, and Anemoscope.
**Wind-Hatch,** in mining, a term used to express the place at which the ore is taken out of the mines.
**Wind-Shock,** a name given by our farmers to a distemper to which fruit-trees, and sometimes timber-trees, are subject. It is a sort of bruise and shiver throughout the whole substance of the tree; but the bark being often not affected by it, it is not seen on the outside, while the inside is twisted round, and greatly injured. It is by some supposed to be occasioned by high winds; but others attribute it to lightning. Those trees are most usually affected by it whose boughs grow more out on one side than on the other. The best way of preventing this in valuable trees, is to take care in the plantation that they are sheltered well, and to cut them frequently in a regular manner while young.
**Wind-Taught,** in sea-language, denotes the same as stiff in the wind. Too much rigging, high masts, or anything catching or holding wind aloft, is said to hold a ship wind-taught; by which they mean, that she flops too much in her falling in a stiff gale of wind. Again, when a ship rides in a main stress of wind and weather, they strike down her top masts, and bring her yards down, which else would hold too much wind, or be too much dintended and wind-taught.
**Wind-Sails,** a sort of wide tube or funnel of canvas, employed to convey a stream of fresh air downward into the lower apartments of a ship.
This machine is usually extended by large hoops situated in different parts of its height. It is let down perpendicularly through the hatches, being expanded at the lower end like the base of a cone; and having its upper side open on the side which is placed to windward, so as to receive the full current of wind; which entering the cavity, fills the tube, and rushes downwards into the lower regions of the ship. There are generally three or four of these in our capital ships of war, which, together with the ventilators, contribute greatly to preserve the health of the crew.
**Windage of a Gun,** is the difference between the diameter of the bore and the diameter of the ball.