WIND. As the air is a fluid, subjected to the same laws of gravitation as other fluids, it necessarily has a constant tendency to preserve an equilibrium in every part; so that if, by any means whatever, it is rendered lighter in any one place than another, the weightier air will rush in from every side towards this place, till
Wind. as much be there accumulated as makes it of an equal weight with the rest of the atmosphere: it is these currents of air which are called winds.
Many are the causes which may vary the weight of the atmosphere, and occasion particular tropical winds.
Although other causes may occasion winds in certain circumstances, yet their principal and most universal cause is the sun, which warmeth the air to a much greater degree in some places of the atmosphere than in others; and as the air is susceptible of a great degree of expansion by heat in those places where it is heated to any considerable degree, it is expanded so much as to become lighter than the air in those places where it is colder; so that the weightier cold air from all the circumjacent parts rushes towards this point to restore the equilibrium which had been destroyed. So that if there be any particular part upon the earth's surface where the sun acts constantly with greater force than on any other part, a current of air will constantly flow from these towards the warmer region: but the sun acts with greater force upon those parts of the earth which are nearest the equator, than those which approach towards either poles; so that we might naturally expect that a wind would constantly blow from the polar regions towards the equator; which is really found to be the case in the torrid zone, where the influence of the sun overcomes almost all the other lesser causes which produce the variable winds in our more northerly regions. However, even in the torrid zone, these north and south winds are varied in different ways.
Although the heat of the equatorial region is greater than any other, yet as the sun acts perpendicularly in his diurnal course upon one point of the equator only at one time, and immediately passes over it, and as the air retains the heat communicated to it by the sun but for a short time, cooling gradually as he retires, and continuing still to decrease till his influence again returns the following day; the degree of heat upon this great circle must be very different in different parts, and perpetually varying in every point; which must in some measure tend to disturb those winds coming from the polar regions, which we have already mentioned. To comprehend clearly what will be the effects of this rotation, let us consider what effect it would naturally produce upon the equator with regard to wind, supposing no other cause should interrupt it. And here we must observe, that as the point upon which the sun acts with the greatest power is constantly moving from east to west, the air to the east of that point over which the sun has more lately passed will be more rarefied than that to the west, and will naturally flow towards that point from east to west with greater velocity than from west to east, as the cool air to the west of that point will be interrupted in its motion towards it by the motion of the sun meeting it. Hence therefore it follows, that from the diurnal motion of the earth from west to east a constant wind would always be produced, were it not obstructed by other causes. But as there is a constant stream of air flowing from the polar towards the equatorial regions, a composition of these two currents of air acting at the same time will produce a north-east wind in all parts of the northern hemisphere, and a south-east wind in all parts of the southern one. These winds are known
by the name of the general trade-winds.
If there were no inequalities on the surface of our globe, and if it were composed of a substance perfectly homogeneous, this wind would invariably take place at all times on every part of the earth's surface: but as this is not the case, it is liable to several very considerable variations. In all those regions towards the poles, as the influence of the sun is there but weak, other lesser causes occasion particular winds, and disturb that regularity which at first view we might expect; so that the general trade wind does not invariably take place beyond the 28th or 30th degree of latitude; and the regions between that and the poles have nothing but variable winds. Even in the torrid zone, there are many causes which in particular places alter this direction of the wind; so that the genuine trade-winds do not take place except in the Atlantic and Pacific oceans on each side the equator to the distance of 28 or 30 degrees, and in the greatest part of the Indian ocean to the south of the equator, as appears more distinctly upon the Map, (Plate CCCXII.) where the course of the winds are marked by the direction of the strokes, the darts pointing in the same direction as the wind blows.
Having thus explained the nature and causes of the general trade-wind, we now proceed to take notice of the principal deviations which take place in the torrid zone. The general trade-wind, when thus altered at particular seasons, is known by the name of monsoons. There are other variations, which, although as general, are yet of smaller and more limited influence. These are known by the name of breezes; and as they blow periodically from the sea, they are denominated sea or land breezes, and take place more or less in every sea-coast within the tropics. As the causes of the monsoons will be more clearly comprehended after the nature of these breezes is explained, we shall first consider them.
The sea and land breezes of the torrid zone are gentle periodical winds regularly shifting twice every day, and blowing from the sea towards the land during the day time, and from the land towards the sea in the night. These breezes do not blow with an equal degree of force throughout the whole day and night, but are perpetually varying, being always strongest about mid-day and midnight, and becoming gradually weaker till the time of change in the evening and morning; about which time the air continues for a short space perfectly calm: but in a little the breeze begins to be felt on the side opposite to that from which it blew last, so faint at first as hardly to be perceived; but by degrees acquiring greater strength, it goes on increasing for five or six hours, after which it again as gradually sinks and dies away. They always blow directly off or towards the shore, and never extend their influence to a great distance from it, although this is varied by particular circumstances in different places; as they never extend so far from the points of capes and promontories, as in deep bays; nor upon the windward, as le-shores.
These breezes are produced by the same cause which gives rise to the trade-wind, viz. the heat of the sun. In these warm regions the days and nights are nearly of an equal length throughout the whole year; the sun rising high in the day time, and descending almost perpendicularly at night; which occasions a much greater
Wind. variation between the heat of the day and night than is experienced in the more temperate climates; and it is this great difference between the heat of the night and day which produces the breezes. For the rays of the sun are reverberated from the land during the day-time, much more powerfully than from the sea, whose surface is constantly evaporating; and the air above the land is rendered much warmer, and consequently more rarefied than above the sea; so that a current of air necessarily takes place at that time from the sea towards the land, increasing and diminishing in strength as the heat increases or declines. But when the sun descends below the horizon, the evaporation from the surface of the sea is stopped or greatly diminished, and the cold which it occasioned is of consequence removed: the reverberation of the sun's rays from the surface of the earth is likewise removed, and the air above the land quickly resumes its natural degree of cold, which is always greater than the sea when the influence of the sun is withdrawn: so that the air above the sea becomes warmer during the night than that above the land, and a current of air is of course established from the land to the sea, which forms the land-breeze, which acts as uniformly, although less powerfully, than the sea-breeze; blowing at first gently as the air begins to cool, and gradually gathering strength as the sun retires below the horizon; till his influence begins to be full again in the morning, when it gradually gives place to the more powerful influences of the sea-breeze. These breezes are not, however, entirely confined to the torrid zone. They are even felt in more northern regions; the sea-breeze in particular being almost as perceptible during the summer season along the coasts of the Mediterranean and the Levant, both on the African, European, and Asiatic shores, as within the tropics. Even in our own colder climate, the effects of this are often sensibly felt during the summer season; although, from the length of the day and shortness of the night, the difference between the heat of these is far less than in warmer climates. And although the shortness of our nights prevents us from feeling a nocturnal breeze, similar to the land-breezes of the torrid zone; yet in every serene evening, we have an opportunity of observing a phenomenon, proceeding from a similar cause with that which occasions them in warmer climates. For as the waters retain their heat longer than the earth after the sun withdraws, the moisture which was raised during the heat of the day to a small distance from the earth's surface is quickly condensed by the cold of the evening, and falls down in copious dews; whereas that which is above the surface of the water is more slowly condensed, by reason of the heat which that element retains longer, and hovers at a small distance above it in the form of a dense vapour, which slowly subsides as it loses its heat. This is the cause of those low mists which are so often seen hovering above the surface of rivers and other waters in the evenings towards the end of summer.
It was already observed, that in the Indian ocean the general trade-wind only took place in some parts to the south of the equator. To the north of the line, and in some places to the south of it in that ocean, the general trade-wind only blows regularly for six months; and during the other six months the wind blows in a direction entirely opposite. It is these winds, which
shift thus regularly, which are called monsoons, although they are also sometimes called trade-winds. Wind.
At the equator the days and nights are always of an equal length throughout the whole year; so that the heat being thus equally divided, it never arrives to such an intense degree as to be insupportable to the inhabitants. And as there is no vicissitude of seasons at the equator, so at the poles they never experience the more pleasing vicissitudes of day and night, the sun never setting during the summer season, nor rising above the horizon during the winter: and although the day decreases in length as we recede from the pole, from 6 months to 24 hours; yet in all high latitudes the sun descends for such a short space below the horizon, and in such an oblique direction, that the difference between the heat of the day and night is but very inconsiderable. From which it follows, that during this season, when the sun continues to act with such uninterrupted influence upon the surface of the earth, the air will then be rarefied more above the dry land than upon the surface of the water; so that a wind would naturally set in at that time from the sea towards the land, similar to the diurnal sea-breezes in the warmer climates; and on the contrary, during the winter season, the air in these northern regions being colder above the land than the water, the winds will naturally blow from the land towards the sea, similar to the land-breezes of the torrid zone. But as the influence of the sun, although of longer continuance, is in general more languid in climates of a high latitude than in those near the line, it is not to be expected that these effects will follow with the same regularity as in the torrid zone; being more apt to be interrupted by lesser causes which affect the atmosphere and produce winds in different directions. Yet these are not so totally interrupted but that we can easily trace their effects even in our own cold climate: for during the summer season, the large continent to the east of us, being more heated than the Atlantic ocean westward, produces a general tendency of the current of air towards the east, inasmuch that westerly winds are observed to prevail more than any other, not only here, but in all the frontier countries on the continent, during the summer season. And easterly winds become again more prevalent in the winter and spring. On the contrary, it is observed in North America, that the easterly winds prevail more in summer than at any other time; and the west winds always prevail during the cold months of winter. The same effects take place with a greater degree of constancy in other parts of Europe, particularly in Greece, and the countries in that neighbourhood; as the ancient Greeks have particularly remarked, that the winds blew from the south during the heat of summer, particularly about the dog-days, and from the north during the colder weather of winter.
Any attentive reader, who has accompanied us thus far, will readily see, that the monsoons which take place in the Indian ocean proceed from the same general cause. For when the sun, in his annual course, has crossed the line, and comes to act very strongly upon the extensive countries of Arabia, Persia, China, and the other parts of India, these become heated to a much higher degree than the ocean to the south of them; and the air above these extensive countries being so much rarefied, naturally draws the wind towards that place, which
Wind. which, by overcoming the general trade-wind, produces the southerly monsoons which take place in all those seas during the months of April, May, June, July, August, and September. But when the sun has again retreated towards the southern hemisphere, this great degree of heat in these countries subsides, and the genuine trade-wind again resumes its natural course; forming what they call the northerly monsoon, which blows in the months of October, November, December, January, February, and March: and as the continent of Asia now assumes a greater degree of cold than the Atlantic and Pacific oceans in the same latitude, it produces a brisker and more steady gale during the continuance of this monsoon, than is ever experienced in the general trade-wind.
Having thus explained the nature of the monsoons in general, we shall proceed to consider the particulars which influence the direction of these in those parts where they take place. In all that part of the Indian ocean which lies between the island of Madagascar and Cape Commorin, the wind blows constantly from the W. S. W. between the months of April and October; and in the opposite direction from the month of October till April, although with some variation in different places, as these winds are neither so strong nor constant in the bay of Bengal as in the Indian ocean. And it is likewise remarkable, that the S. W. winds in these seas are generally more southerly on the African side, and more westerly on the Indian, as appears distinctly in the map. But these variations are not repugnant to the general theory. For it is sufficiently known, that high lands in every part of the globe are much colder than low and flat countries; and as that part of Africa is very high and mountainous, the cold in these regions is much greater than in the more flat countries of Arabia and India; so that the wind naturally blows from these cold regions, in the summer season, towards the warmer continent of Asia; which occasions those inflections of the wind to the eastward which take place in these seas during the summer months; and is still farther assisted by the peninsula of India, the kingdom of Siam, and the islands of Sumatra and Java, on the eastern part of this ocean, lying so much farther south than the kingdoms of Arabia and Persia; so that these, being more heated than the ocean to the westward, naturally draw the wind towards them, and produce the easterly variation of the monsoon which takes place in this part of the ocean, while the warm and sandy deserts of Arabia draw the winds more directly northward near the African coast.
In the eastern parts of the Indian ocean, beyond the island of Sumatra, through the gulph of Siam and bay of Tonquin, and along the southern parts of China, and among the Philippine islands, &c. to the north of the equator, the monsoons observe a different direction, blowing nearly due south and north. Here the greatest part of the warm continent is to the west of this district, which makes the wind naturally assume this direction. A little to the east of this, among the Marianne islands, the general trade-wind takes place, there being no continent to the north of them to occasion monsoons.
The monsoons are as regular in the eastern part of the Indian ocean to the south of the equator, as they
are to the north of it; as here a northern monsoon sets in from the month of October till April, and a southern from April to October. And here, as to the north of the line, we find the direction of the monsoons varying according to particular circumstances in different places: for about the island of Sumatra, and towards the west end of Java, the monsoons set in nearly from the north and south; but towards Celebes and Timur, they begin to tend a little more to the east and west, gradually declining as they approach the coast of New Guinea; near to which the northerly monsoon from October till April blows from the N. W. and the opposite monsoon from the S. E. between October and April. The reader will easily perceive that these monsoons are occasioned by the continent of New Holland and Guinea; which being heated by the sun when in the southern signs, draws the wind towards that in the summer season, in the same manner as the continent of Asia produces the monsoons to the north of the line. And it is likewise sufficiently plain, that the inflection of these periodical winds about Celebes and Timur is occasioned by that part of the continent called New Guinea jutting out so near to the equator to the east of these, and drawing the wind toward that quarter.
These are the most general and extensive monsoons which take place in our globe. But there are other periodical winds, which occur in particular places in these warm regions, that deserves particular attention.
In the Red Sea, the monsoon shifts as regularly as in other places; but being influenced by the coasts, it tends a little more to the north and south than in the Indian ocean.
On the south coast of Africa, to the south of Cape Corientes, and about the southern parts of the island of Madagascar, the regular trade-wind from the S. E. takes place between October and April; but from April till October the wind blows from the W. or N. W. and is at that season exceedingly cold. This is evidently occasioned by a cause already taken notice of: for notwithstanding the high and cold nature of this continent, yet when the sun is to the south of the line, his powerful influence at that season so far abates their natural degree of cold, as not to interrupt the general trade-wind between the months of October and April: But when he returns to the northern hemisphere, the high mountains of Africa resume their native coldness, and repel the general trade-winds by their cold and more powerful blast, so as to produce the intemperate monsoon which here takes place between the months of April and October.
From Mozambique to Cape Guardafou, at the mouth of the Red Sea, the monsoons are a little more irregular than in the other parts of the Indian ocean. For it is observed, that between October and January the winds are variable, although chiefly from the north. In January the N. E. monsoon sets in, and continues regular till the month of May. From May till October the winds again become variable, but blow chiefly from the southern points; but in the months of June, July, and August, there are frequent calms, especially about the bay of Melinda, which sometimes continue for several weeks together, and extend only about 100 leagues from shore.
Before
Before we can explain the cause of this irregularity clearly, it will be necessary to attend to the direction of the wind on each side of this track at each particular season. In the months of October, November, and December, the winds are here variable, but chiefly from the north. Now during these three months, the wind to the south of this beyond Cape Corientes blows from the S. E. at the Red Sea, and all to the north of this the wind during this season of the year is from the N. E. And as the sun is then perpendicular to the bay of Melinda, these opposite winds here meeting and opposing one another, and being both of them stopped in their course westward by the cold regions of Africa near the Mountains of the Moon, will naturally produce the variable winds here observed, according as the one or the other of these three balancing powers shall predominate: although, as the coast here runs away towards the south-west, it is natural to expect that the northerly wind, which follows the same direction, should more frequently prevail than those which are opposed by it; especially when we consider, that the island of Madagascar, now beginning to be warmed by the influence of the sun, will concur in drawing the wind to the southward; and when the continent of Africa is more heated in the months of January and February, it does not oppose the easterly monsoon, so that the winds become then more fixed than before. But in the months of June, July, and August, the wind to the south of Cape Corientes is from the N. W.; and near the Red Sea, and throughout the northern part of the Indian ocean, the S. W. monsoon is then in its greatest vigour; so that on each end of this district the wind is blowing in an opposite direction; from which results these calms about Melinda, which we just now mentioned.
This much may suffice for the shifting winds on the African and Asiatic coasts. As to America, the only places where the wind shifts regularly are, the bays of Honduras and Campeachy on the east, and that of Panama and some parts on the coast of Mexico on the west, with a small track upon the coast of Brazil. In the south part of the bay of Honduras, between Cape Gratia de Dios and Cape la Bela, the common trade-wind between E. and N. E. blows between March and November; from October till March there are westerly winds, not constant or violent, but blowing moderately sometimes two or three days, or a week; and then the easterly breeze may again prevail for an equal length of time. The reason of the peculiarity here observed is this: During the summer season, the high land on the isthmus of Darien is so much warmed as not to interrupt the course of the general trade-winds; but when he retires to the southern hemisphere, the cold upon the isthmus at that season becomes so great, as to condense the air to such a degree as to repel the trade-wind for some time: but not being cooled to such an intense degree as in some of the larger continents, the trade-wind at times overcomes and repels these land-breezes in its turn, and produces the phenomena above described. And that this is really the case, appears evident from this circumstance, that the land breezes are most prevalent and of longest duration in the coldest months of December and January; before and after which two months, the trade-wind being generally checked only a day or two about the
full or change of the moon. As these western breezes on this coast take their rise from the same cause as the diurnal land-breeze in warm climates, they may be considered as land-breezes of two or three days continuance, and forming an intermediate step between the land-breezes and monsoons. Although the influence of these breezes is felt farther off at sea than the common diurnal breeze, yet they do not extend a great way, being seldom felt above 20, 30, or 40 leagues from shore; and about Cape La Vela, which is much exposed to the east wind, these breezes seldom extend above eight or ten leagues from shore. Land-breezes of the same nature, and proceeding from similar causes, are also experienced in the winter season in the bay of Campeachy, which are there known by the name of Sumaferas winds. Beyond Cape la Vela these western breezes are not felt, which is undoubtedly occasioned by the whole of that coast as far as Cape St Augustine being so much exposed to the general trade-wind, which here sweeps along the coast with so much violence, as almost totally to repress the weaker influence of the breezes. But between Cape St Augustine and St Catharine's island, or a little farther, we again meet with a variation of the wind at different seasons, as it is here observed to blow in an E. or N. E. direction from September till April, and from April till September from the S. W. This variable wind, or monsoon, like the others on this coast, extends but for a very short way from shore, and is evidently occasioned by the same causes as the other periodical winds. For in the summer months (which in this climate is between September and April), the land of the continent being heated by the sun, draws the trade-wind from its common course of S. E. a little to the westward; and as the coast here tends towards the S. W. the wind in some measure (as it always does) follows the same direction, and produces this E. N. E. monsoon. But in the winter, when this region becomes more cool, the east wind is repelled by the dense cold air from the mountains; by which means it is bent to the northward, and is forced along the coast to Cape St Augustine; where, meeting with no further hindrance, it again falls in with the general trade-wind, and is carried along with it in its proper direction.
We have purposely omitted mentioning the winds on the west coasts of Africa and America till the others were explained, as the causes of the peculiarities here observed will be now more easily comprehended. On the coasts of Chili and Peru, in America, from 25° or 30° of south latitude to the line; and on the parallel coast of Angola, &c. in Africa, the wind blows all the year from the south, varying in its direction a little in different places according to the direction of the coast, towards which it always inclines a little. But whatever is the direction at any one place, it continues the same throughout the whole year without any variation, and always blows from some southerly point. But there is this difference between this wind upon the coasts of Chili and Angola, that it extends much farther out to sea upon the former than upon the latter.
In order to explain the cause of this singular phenomenon, it is necessary to recollect, that the general trade-wind is produced by the concurrence of two separate causes. One is the great heat of the equatorial region,
Wind. region, by which alone would be produced a constant north or south wind. The other is the diurnal revolution of the earth, which would cause a perpetual tendency of the air in these warm regions from east to west. From the concurrence of these two causes result the general trade-wind, which would constantly blow from the S. E. or N. E. as we have already demonstrated. But if any one of these two causes, in any particular place, is prevented from producing its full effect, while the other continues to exert its influence, the general direction of the wind will be varied, and it will assume another. Thus, if the east wind was prevented from acting in any particular place, while nothing interrupted the south or north wind, it is evident that the air would rush towards the equator in that direction which was nearest and easiest, whether that should be pointing eastward or westward. Now as the high mountains in the internal parts of Africa and America interrupt the course of the east wind near the surface of the earth, while these coasts of which we now treat are entirely open to the south, the wind naturally rushes along the coasts of Chili and Angola from north to south; and as the low lands near the shore, in these warm regions, is generally warmer than the sea, the wind will naturally point in towards the shore, as is generally observed to happen.
This, then, is the obvious cause of the south wind which always prevails upon the coasts of Chili and Peru, as well as along the shores of Angola, Loango, &c. But it is only near the shore that this can take place; nor can it extend to a great height above these low and fertile regions. For as the internal parts of these countries are exceedingly high, but more especially the Andes of America, which experience a perpetual degree of cold more intense than some polar regions ever experience; the air must here be condensed to a very great degree, and sent forth from these high regions a perpetual wind to every side, which occasions almost all the peculiarities that have been remarked in these climates: for by opposing the general current of the trade-wind upon the eastern part of these continents, they produce these deluges of rain which supply the immense rivers of the Amazons, La Plata, &c. these do not, like the Nile and Gambia, swell only at a particular season, and then shrink into a diminutive size again; but continue throughout the whole year, with less variation of size, to pour their immense floods of water into the ocean. These cold winds likewise stretching to the westward, at a considerable distance above the warmer regions of the sea-coast, at length descend as low as the ocean, and form the general trade-wind, and occasion that unusual degree of cold which mariners have so often complained of even under the line to the westward of America. To the same cause also must we attribute the thick fogs so common upon the southern parts of Chili and along the coasts of Peru, with the other peculiarities of that singular climate about Lima and the kingdom of Valles in South America; for the vapours which are exhaled in such great abundance in the warm regions on the sea-shore, are, at a little height above the earth, condensed by the cold winds which come from the mountains, and form these thick mists which are so often observed in this climate. The same effects are felt in some degree on the similar coast of Africa. But
as the mountains of Africa are not so high as the Andes of America, nor approach so near the western coast, the effects are less sensible here than in America. The great height of the Andes above the mountains of the similarly situated country of Africa, is the only reason why the effects on that coast are not felt to an equal degree, although similar in kind.
Wind. A more singular deviation from the general trade-wind is observed to take place on the African and American coasts to the north of the line, than those we have taken notice of to the south of it. For it is observed, that from California to the bay of Panama, all along the coasts of New Spain, the winds blow almost constantly from the west or S. W. nearly directly opposite to the trade-wind; and on the coast of Africa from Cape Bayador to Cape Verde, they blow chiefly from the N. W. standing in upon the shore; from thence the wind bends gradually more and more from the north to the west, and so round to the S. W. all along the coast of Guinea, as will be distinctly seen by the map. After what we have said of the winds on the southern parts of these regions, it will be unnecessary to spend much time in explaining the cause of these peculiarities, as it will evidently appear that they are nearly the same, the variations here observed being occasioned by the particular direction of the coast.— Thus, along the coast of New Spain, the wind blows nearly the same direction in every place, as there are no remarkable bendings on the coast; being uniformly drawn towards the shore by the great heat of the low part of the continent near the sea; which in these regions is always more heated than the water of the ocean, and occasions that inflection. But as the coast of Africa is more irregular, the winds also are found to be more different in their direction. To the north of Cape Verde, as the coast stretches nearly south and north, the wind, being drawn towards it a little, blows from the N. W. But beyond that, the coast bends more eastward to Cape Palmas; from which it runs E. or N. E. all along the coast of Guinea, the wind shifting gradually more and more to the west, still pointing in upon the coast. And as there is nothing to oppose the current of air, which comes from the south along the coast of Angola, it stretches forward till it comes within the influence of the coast of Guinea, and is there drawn in towards the shore in a S. W. direction. But as it is only the lower regions of the coast of Guinea which are so much warmed, the high mountains within continuing cold; the northerly wind coming from these meeting and opposing the southerly winds in the higher regions of the air, by their mutual conflicts occasion those incessant rains and tremendous thunder so remarkable along the whole of this uncomfortable coast.
It has been often observed by mariners, that there is a track of sea to the west of Guinea from five to ten degrees of north latitude, in which the trade-wind blows with less steadiness than in any other part of that ocean, being almost constantly troubled with calms and tornadoes; the cause of which the reader will perceive by inspecting the map, as he will easily see that the winds are drawn from this quarter almost in every direction: so that there can be here no constant wind; but being exhausted of its air, it must become lighter than the circumjacent parts, and must then be
Wind. supplied from either side, as chance or occasional circumstances may direct, which occasions those sudden surges and tornadoes here observed.
Before we take leave of this subject, it is necessary to observe, that in the bay of Panama the winds between September and March are easterly; but from March till September they blow chiefly from the S. S. W. that is, during the winter months, while the sun is far from them, the winds are off shore; and during the summer months, the land being heated to a considerable degree, they are drawn towards the shore as usual. It is remarkable, however, that this is the only part on the west of a large continent where the wind shifts regularly at different seasons; which seems to be occasioned by the great height of the isthmus of Darien, and the Terra Firma to the east of it, and the nearness of these to the sea, in comparison of the mountains near Benin on the similarly situated coast of Africa; which is greatly assisted by the deepness of the bay, which, by bending so much to the eastward from Cape Lorenzo, is in a great measure screened from the force of the south winds, which allows the winter breeze to extend itself upon the bay with more facility. We ought here also to remark, that along the coast of Mexico, between Cape Pelanco and Guatemala, there are land-winds which blow in the months of May, June, and July, called by the Spaniards Popogais. They greatly resemble the Summefenta winds in the bay of Campeachy, as they blow both night and day a moderate breeze without intermission, sometimes three or four days or a week together. But as these blow from the land in summer only, whereas the Summefenta's blow only in winter, they must be occasioned by a different cause, which seems to be this: As the continent which divide the South Sea from the bay of Mexico and gulph of Honduras, is but of very small breadth, and in many places very high ground, the heat which it receives from the sun in summer is not so great as on the similar coast of Africa: and as the trade-wind coming from the great Atlantic ocean sweeps along the eastern part of the American coast from Cape St. Augustine to the bay of Honduras with very great violence at that season, the small heat of this narrow continent is not sufficient to stop it entirely during that season; so that at some times it blows for a short time quite across it, and occasions those winds called Popogais.
Besides these more general winds, there are likewise some particular winds which are only felt in particular places at certain times, whose effects are so singular as to merit attention; some of which we shall here take notice of. In the gulph of Persia, particularly at Ormuz, during the months of June and July, there sometimes blows from the west, for a day or two together, a hot suffocating fiery wind, which scorches up and destroys any animal that may be exposed to it; for which reason, almost every body leave their habitations at Ormuz during these two months, and retire to the mountains near Shiraz in Persia, where they enjoy a more comfortable climate. To explain the cause of which, it is necessary to observe, that along all the coasts of Asia, to the north of the Indian ocean, the diurnal sea and land breezes take place, as in every part of the torrid zone; by means of which the monsoons are not felt close in upon the shores. But as the monsoon
continues to blow regularly at a small distance from shore, so in all probability it continues its course without interruption at a small distance above the surface of the earth. Now when the monsoon is in its greatest vigour, its influence will sometimes descend even as low as the surface of the earth, and, interrupting the course of the breezes, hurry along with it these warm vapours, which ought to have ascended upwards, and produced the salutary sea breeze; and as the earth is thus deprived of the refreshing influence and moisture of the sea-breeze, the air, by the strong reverberation of the sun-beams from such dry and sandy countries as Arabia, must soon be heated to an amazing degree, and produce these hot and suffocating winds. It is also remarkable, that these hot winds are more often experienced near headlands, where the sea-breezes are weakest, which seems to confirm this hypothesis.—Winds similar to these in kind, though not in degree, are felt upon the coast of Coromandel during the months of June, July, and August, while the west monsoon reigns; and on the Malabar coast they are likewise felt in the months of December and January, while the east monsoon reigns; but these are much less powerful than either of the others. As these hot winds always come from the land, they are known upon these coasts by the name of Terreno's.
It has likewise been observed, that on the coast of Africa to the north of Cape Verde, during the months of December, January, and February, there sometimes blows, for a day or two together, an easterly wind, so very intensely cold as to be almost as destructive as the warm winds at Ormuz. We have already in some measure explained the cause of this phenomenon.—During these months, when the sun is far from them, his influence is less felt than at other seasons, and the northerly wind upon the coast is of course weakened, insomuch that the cold produced by the mountains in the heart of the country being now in its greatest degree of force, bursts its usual confinement for a time, spreading to the west with great violence, and producing those uncommon effects already mentioned. Those who sail on these coasts, distinguish this particular wind by the name of Harmattan.
These are the principal winds, whether constant or periodical, that take place within the tropics; and thus simple are their causes.
The succession of sea and land breezes renders the torrid zone not only habitable but comfortable. Besides, as these currents of cold air, rushing from each side of the globe, and carrying along with them vast quantities of aqueous vapours which they collect from the surface of the earth in their course, meet and oppose one another at that part of the atmosphere where the influence of the sun is greatest at the time, the water is there forced from the clouds in such prodigious quantities, as to produce a diversity of seasons in the torrid zone, something similar to what is experienced in more temperate climates; with this difference, however, that whereas, in temperate climates, the warmest and most comfortable season is when the sun approaches nearest perpendicular to them, in these warmer climates the heavy rains which fall upon them at that season moderates the heat, and prevents the sun from having such an effect as at other times; so that their coldest and most inconstant weather, which they
Wind. they call rointer, is at that season, when, without this cause, they would be exposed to the sun's most powerful influence.
We shall only take notice of one other instance of the happy effects produced on our globe by the laws of nature with respect to winds. We have seen, that in the great Atlantic and Pacific oceans, the trade-wind blows constantly from the easterly points throughout the whole year, so that ships sail from east to west within the tropics with the utmost facility; but it is absolutely impossible in these seas to sail from west to east, as the wind would be constantly against them; so that ships bound for any port to the eastward in these regions, must stand to the north or south till they are beyond the limits of the trade-winds, where they meet with variable breezes, by the help of which they sail to the eastward. But if the same constant trade-wind had taken place in the northern part of the Indian ocean, it would have been impossible to have sailed to the eastward at all; because the continent of Asia would have prevented the ships from sailing far enough north to find the variable winds. But here, as in almost every case in which the operations of nature are concerned, we find, that what produceth the disease, at the same time furnisheth a remedy; for that very continent which would have stood in our way going northward, draws the wind towards itself at one season, which makes that course of navigation unnecessary, the shifting of the monsoons supplying a nearer and more commodious course. Thus we see, that wherever the sea is open to the south or north, near the tropics, so as that ships are at freedom to reach the variable winds, the trade-wind constantly blows in one direction; but, wherever there is any extent of continent within the verge of the torrid zone, so as that they could not be at liberty to reach the variable winds, there the course of the trade-wind is altered, being drawn towards it in summer, and from it in winter, forming that shifting wind called monsoons. From which we may naturally infer, that as there are no monsoons in the Pacific or Atlantic, or in the western part of the Indian ocean, to the south of the line, there are no extensive continents near the tropics in either of these places.
Wind-Flower. See ANEMONE.
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 axes.
Wind-Gage. See Wind-GAGE.
Wind-Gall, in farrery. See there, § xxxii.
Instruments for measuring the strength, velocity, &c. of the WIND. See Wind-GAGE, ANEMOMETER, and ANEMOSCOPE.
The following ingenious method of measuring the velocity of the wind is extracted from a letter of the Rev. Mr. Bryce of Kirknewton to the president of the Royal Society.
"The velocity of the wind near the earth is very unequal, upon account of the frequent interruptions it meets with from hills, trees, and houses, and even in open plains; the surface of the earth, though much smoother than it commonly is, must reflect and interrupt such a fluid as the air, and occasion great irregu-
larity in the velocity of its current: this is the reason when a feather is let fly with the wind, why it seldom, if ever, describes a straight line, but moves sometimes in a kind of spiral, now high, and then low, sometimes to the right, and then again to the left; and why two feathers let fly at once seldom, if ever, keep together, or describe similar lines.
"But at some considerable distance from the earth, the velocity of the wind seems to be regular and steady: nothing can be more uniform than the velocity of a cloud in the sky appears to be even in the greatest storm: it is like a ship carried away insensibly by a smooth and gentle current, passing over equal spaces in equal times. This suggested the thought, that the motion of a cloud, or its shadow over the surface of the earth, would be a much more proper measure of the velocity of the wind.
"In the end of March 1763, I had as favourable an opportunity of putting this method into practice as I could have wished for: the storm was exceeding high, and moved with vast velocity; the sun was bright, the sky clear, except where it was spotted with light floating clouds. I took my station in the north window of my dining room, near the clock, from which I had a free prospect of the fields: the sun was in the meridian, the wind due west intersecting his rays at right angles. I waited until the fore-part of the shadow of a cloud, that was distinct, and well defined, just touched a south and north line, which I had marked upon the ground: at that instant I began my reckoning, and followed the shadow with my eye in its progress, counting seconds all the while by the clock, until I had reckoned up 15 seconds; then I observed exactly where the fore-faid edge of the shadow was.
"This experiment I repeated ten times in half an hour, and seldom found the difference of a second in the time which different clouds took to move over the same space. On the 5th of May current, I repeated the trial four different times, the sun being also near the meridian, the wind in the west, with light clouds floating in a clear sky as formerly; and found that the shadows of different clouds took some of them 44, and others 45 seconds, to pass over the same space which they had moved over in 15 seconds in the former trials.
Feet
This space measures exactly 1384 = space passed over in 15"
which multiplied by 4
gives 5536 = space passed over in 1 m.
which multiplied by 60
gives 332,160 = space passed over in 1 h.
"Which space is = 62.9 English miles per hour, the velocity of the wind in March 1763.
"One third of this (or 21 miles nearly) shows the velocity of the wind on May the 6th, when it blew a fresh gale.
"This day, May 12, there was a small westerly breeze, the velocity of which I measured upon the same line, the sun being 10 minutes past the meridian, and found that the shadow took 95 seconds to pass over the above space; which gives the velocity of the wind at the rate of 9.9 English miles per hour.
"Thus, by having several lines in different directions of a known length marked upon the ground, one may easily (and with great accuracy, I imagine) measure the velocity of the wind. If a person was provided
Wind ded with an instrument for measuring the force of the
Window. wind, it would perhaps be worth while to observe whether, when the velocities of different winds were the same (or nearly so) the force of these winds did not vary with the seasons of the year, the points of the compass from which the wind blows, and also with the different state of the barometer and thermometer; since the momentum of the wind depends not only upon its velocity, but also upon its density."