will flow through the contiguous air, not acted on by the rarefying force. A wind, therefore, will blow out of a place in which the air is suddenly rarefied; and on this principle, in all probability, it is, that, 7. Since the sun's power in rarefying the air is notorious, it must necessarily have a great influence on the generation of winds. 8. Most caves are found to emit wind, either more or less. M. Muschenbroeck has enume rated a variety of causes that produce winds, existing in the bowels of the earth, on its surface, in the atmosphere, and above it. See Intr. ad Phil. Nat. vol. ii. p. 1116, &c. as 57 to 2; i. e. if water move two feet in a second, the wind will fly 57 feet. 2 2. Add, that s = V ; and therefore the space any fluid, impelled by any impression, moves in any time, is determined by finding a fourth proportional to the two numbers that express the ratio of the specific gravities of the two fluids, and the square of the space the wind that fourth proportional is the space required, moves in, in the given time. The square root of M. Mariotte, e. gr. found, by various experiment, that a pretty strong wind moves 24 feet in a second of time; which is at the rate of 1440 in a 205. The rising and changing of the wind are determined experimentally, by means of weather-minute; i. e. at the rate of somewhat more than 16 miles in an hour: wheref re, if the space cocks, placed on the tops of houses, &c. But which the water, acted on by the same force as these only indicate what passes about their own the air, will describe in the same time, be reheight, or near the surface of the earth: Wolfius quired; then will c1, 24, b = 800; assures us, from observations of several years, that the higher winds, which drive the clouds, and we shall find s = 576 are different from the lower ones, which move the weather-cocks.. And Dr. Derham observes something not unlike this. Phys. Theol. lib. i. cap. 2. 206. The author last mentioned relates, upon comparing several series of observations made of the winds in divers countries, viz. England, Ireland, Switzerland, Italy, France, New England, &c., that the winds in those several places seldom agree; but when they do it is commonly when they are strong, and of long continuance in the same quarter; and more, he thinks, in the northerly and easterly than in other points. Also, that a strong wind in one place is oftentimes a - weak one in another, or moderate, according as the places are nearer, or more remote. Phil. Trans. No. 267 and 321. 207. Wind being only air in motion, and air being a fluid subject to the laws of other fluids, its force may be regularly brought to a precise computation: thus, The ratio of the specific gravity of any other fluid to that of air, together with the space that fluid, impelled by the pressure of the air, moves in any given time, being given; we can determine the space through which the air itself, acted on by the same force, will move in the same time.' By this rule : 1. As the specific gravity of air is to that of any other fluid; so, reciprocally, is the square of the space which that fluid, impelled by any force, moves in any given time, to the square of the space which the air, by the same impulse, will move in the same time. Supposing, therefore, the ratio of the specific gravity of that other fluid b с to that of air, to be == ; the space described by the fluid to be called s; and that which the air will describe by the same impulse r. The 800 nearly. Der hani estimated the velocity of the wind in very great storms at 66 feet per second; and de la Condamine at 90 feet per second. 3. The velocity of wind being given, to determine the pressure required to produce that velocity;' we have this rule. The space th wind moves in one second of time is to the height a fluid is to be raised in an empty tube, in order to have a pressure capable of producing that velocity, in a ratio compounded of the specific gravity of the fluid to that of the air, and of quadruple the altitude a body descends in the first second of time, to the aforesaid space of the air. C 1 = 1 ; 208. Suppose, e. gr., the space through which the air moves in a second, a = 24 feet, or 288 inches; call the altitude of the fluid r; and the b 800 X 14 11200 ratio of mercury to air and the altitude through which a body descends in the first second of time, 16 feet 1 inch; then, by the theorem, we shall have 288: :: 11200 x 762: 288, and consequently r =; 288 X 288 11200 x 762 01, &c., of an inch. Hence we see why a small but sudden change in the barometer is followed with violent winds. See an account of the principle upon which these calculations are founded under the article WATER. 209. When the direction of the wind is not perpendicular, but oblique to the surface of the solid, then the force of the former upon the latter will not be so great as when the impulse is direct, and that for reasons which are easily derived from the theory of the resolution and composition of forces, and from the theory of direct and oblique impulses. In short, the general proposition for compound impulses is, that the effective impulse is as the surface, as the square of the air's velocity, as the square of the sine of the angle of incidence, and as the sine of the obliquity of the solid's motion to the direction of the impulse, jointly; for the alteration of every one of these quantities will alter the effect in the same proportion. But these general rules, as we have already more than once observed, 2 N subject to great variations; so that their results seldom coincide with those of actual experi ments. 210. Philosophers have used various methods for determining the velocity of the wind, which is very different at different times. The method used by Dr. Derham was that of letting light downy feathers fly in the wind, and accurately observing the distance to which they were carried in any number of half seconds. This method he preferred to that of Dr. Hooke's mola alata, or pneumatica. (See Phil. Trans. No. 24, and Birch's Hist. Roy. Soc. vol. iv. p. 225.) He tells us that he thus measured the velocity of the wind in the great storm of August, 1705, and by many experiments found that it moved at the rate of thirty-three feet per half second, or of forty-five miles per hour: whence he concludes that the most vehement wind (as that of November, 1703) does not fly at the rate of above fifty or sixty miles per hour. Phil. Trans. No. 313, or Abr. vol. iv. p. 411. 211. Mr. Brice observes that experiments with feathers are subject to uncertainty: as they seldom or ever describe a straight line, but describe a sort of spirals, moving to the right and left, and rising to very different altitudes in their progress. He, therefore, considers the motion of a cloud, or its shadow, over the surface of the earth, as a much more accurate measure of the velocity of the wind. In this way he found that the wind, in a considerable storm, moved at the rate of 62.9 miles per hour; and that, when it blew a fresh gale, it moved in the same time about twenty-one miles; and that in a small breeze the wind moved at the rate of 9.9 miles per hour. Phil. Trans. vol. lvi. p. 226. 212. But it has been observed by Cavallo and others, that this method is very fallacious, partly because it is not known whether the clouds do or do not move exactly with the air in which they float; and partly because the velocity of the air in the region where the clouds float is by no means the same with that of the air which is nearer to the surface of the earth, and is sometimes quite contrary to it, as indicated by the motion of the clouds themselves. Others have estimated the velocity of the wind by the changes effected by it upon the motion of sound, which must of course be very inacurate. A very simple method of determining the velocity of the wind is that which M. Coulomb (Mem. de l' Acad. Roy. 1781, p. 70) employed in his experiments on wind-mills, because it requires neither the aid of instruments nor the trouble of calculation. Two persons were placed on a small elevation, at the distance of 150 feet from one another, in the direction of the wind; and, while the one observed, the other measured the time which a small and light feather employed in removing through this space. The distance between the two persons, divided by the number of seconds, gave the velocity of the wind per second. The best method, says Cavallo, of measuring the velocity of the wind, is by observing the velocity of the smoke of a low chimney, or by estimating the effect it produces upon certain bodies, and thus may be determined its force as well as its velocity. We shall here observe that 214. The force of the wind is as the square of its velocity, as Mr. Ferguson has shown by experiments on the whirling-table; and in moderate velocities this will hold very nearly. Upon this principle the numbers in the third column are calculated. The proposition upon which this column has been formed seems to be, that the impulse of a current of air, striking perpendicularly upon a given surface, with a certain velocity, is equal to the weight of a column of air which has that surface for its base, and for its height the space through which a body must fall, in order to acquire that velocity of the air. 215. It is observed, with regard to this table, that the evidence for those numbers, where the velocity of the wind exceeds fifty miles an hour, does not seem of equal authority with that of those of fifty miles, or under. Phil. Trans. vol. li. p. 165. 216. 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. 217. I. In those parts of the Atlantic and Pacific oceans which lie nearest 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 southeast, changing sometimes in the same manner towards the south or east. The space included between 2° and 5° of N. lat. 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, also, that, as the sun approaches the tropic of Cancer, the south-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. 218. The trade wind blows constantly in the Indian Ocean from 10° of S. lat. to near 30°: 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 moossin, 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. 219. 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 inflection of the neighbouring countries. 1. Between 3° and 10° of S. lat. 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 3° S. lat., 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 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 Guardafeu, the winds are irregular during the whole year, owing to the different monsoons which surround the particular place.-Monsoons are likewise regular in the Red Sea; between April and October they blow from the northwest, and during the other months from the south-east, keeping constantly parallel to the coast of Arabia. 220. Monsoons are not altogether confined to the Indian Ocean; on the coast of Brasil, 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 southwest.-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. 221. 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 Bayador 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 popogayos. On the coast of Chili and Peru, from 20° to 30° S. lat. 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. 222. 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 six 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. 223. In the island of St. Lewis on the coast of Africa, in 16° N. lat., and 16° W. long., 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 descends, 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 11° N. lat., 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 aorth-east. 224. In the kingdom of Bornou, which lies between 16° and 20° N. lat., the warm season is introduced about the middle of April by sultry winds from the south-east, which bring along with them a deluge of rain. In Fezzan, which is situated about 25° N. lat., and 35° E. long., the wind from May to August blows from the east, south-east, or south-west, and is intensely hot. 225. In Abyssinia the winds generally blow from the west, north-west, north, and north-east. During the months of June, July, August, Septemper, and October, the north and north-east winds blow almost constantly, especially in the morning and evening; and during the rest of the year they are much more frequent than any other winds. east; the most frequent is the north-west. But at Cambridge, in the same province, the most frequent wind is the south-east.-The predominant winds at New York are the north and 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-east or south-west.-At Hudson's Bay westerly winds blow for three-fourths of the year; the north-west wind occasions the greatest cold, but the north and north-east are the vehicles of snow. 229. It appears, from these facts, that west winds are most frequent over the whole eastern coast of North America; that in the southern provinces south-west winds predominate; and that the north-west become gradually more frequent as we approach the frigid zone. 230. In Egypt, during part of May, and during June, July, August, and September, the wind blows almost constantly from the north, varying sometimes in June to the west, and in July to the west and the east: 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 during the last days of March, and in April, they blow from the south-east, south, and southwest, and at last from the east; and in this direction they continue during a part of May. 226. 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-quarter they continue till near the end of March; 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 the Isles de Vaches, the wind blows oftenest 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. 227. II. 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, some regular connexion between the changes of the atmosphere in different places may in time be observed, 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. 228. In Virginia, the prevailing winds are be tween the south-west, west, north, and northwest; 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 be tween the south-west, west, north, and north 231. In the Mediterranean the wind blows nearly three-fourths of the year from the north; about the equinoxes there is always an east wind in that sea, which is generally more constant in spring than in autumn. These observation do not apply to the gut of Gibraltar, where there are seldom any winds except the east and west.—At Bastia, in the island of Corsica, the prevailing wind is the south-west. 233. 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 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 south; at Kamtschatka, on the north-east coast of Asia, the prevailing winds blow from the west. 233. 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. Gothard the north-east; at Lausanne the north-west and south-west. 234. Father Cotte has given us the result of observations made at eighty-six 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 eighteen places; the west wind in fourteen the north in thirteen; the south in six; the north-east in four; the south-east in two; the east and north-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 Hamburgh, the prevailing winds are the northwest, 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. 235. In Germany the east wind is most frequent at Gottingen, Munich, Weissemburg, Dusseldorf, Saganum, Erford, and at Buda in Hungary; the south-east at Prague and Wirtzberg; the north-east at Ratisbon; and the west at Manheim and Berlin. 236. From an average of ten years of the register kept by order of the Royal Society, it appears, that at London the wind blows in the following order : Winds. South-west North-east North-west West 237. 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 and June, and January, March, April, May, constantly during most seldom during February, July, September, and December; and that the north-west wind blows often from November to this table was extracted, that the north-east wind 241. It appears, from the register from which blows much more frequently in April, May, and June, and the south-west in July, August, and from the Statistical Account of Scotland that September, than at any other period. We learn the south-west is by far the most frequent wind all over that kingdom, especially on the west coast. At Saltcoats in Airshire, for instance, it blows three-fourths of the year; and along the whole coast of Murray, on the north-east 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. 242. 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: |