have the interval almost imperceptible: when this is the case, and the echo is so very near, the voice is said to be increased and not echoed; however, in fact, the increase is only made by the swiftly pursuing repetition. Our theatres and concert-rooms are best fitted for music or speaking, when they enlarge the sound to the greatest pitch in the smallest interval for a repetition which does not begin the word till the speaker has finished it, throws all the sounds into confusion. Thus the theatre at the Hay-market, in London, enlarges the sound very much; but then at a long interval after the singer or speaker. The old theatre at Drury-lane, before it was altered, enlarged the sound but in a small degree; but then the repetition was extremely quick in its pursuit, and the sounds, when heard, were therefore heard distinctly. Dergolise, the great musical composer, used to say, that an echo was the best school-mistress; for let a man's own music be ever so good, by playing to an echo she would teach him to improve it.

Reflected sounds may be magnified by much the same contrivances as are used in optics respecting light: hence it follows, that sounds uttered at one focus of an elliptical cavity, are heard much magnified in the other focus. The whispering gallery at St. Paul's Cathedral, in London, is of this description; a whisper uttered at one side of the dome, is reflected to the other, and may be very distinctly heard. The speaking and ear trumpets are constructed on this principle. The best form for these instruments is a hollow parabolic conoid, with a small orifice at the top or apex, to which the mouth is applied when the sound is to be magnified, or the ear when the hearing is to be facilitated.

CHAP. XX.

OF WINDS, IRREGULAR AND REGULAR.

pable of receiving or affording it, we are not to be surprised at those streams of air that are continually fleeting round the globe.—Minerals, vegetables, and animals, contribute to increase the current; and are sending off their constant supplies. These, as they are differently affected by cold or heat, by mixture or putrefaction, all yield different quantities of air at different times; and the loudest tempests, and most rapid whirlwinds, are formed from their united contributions.

The moon, the other

The sun is the principal instrument in rarefying the juices of plants, so as to give an escape to their imprisoned air; it is also equally operative in promoting the putrefaction of animals. Mineral exhalations are more frequently raised by subterranean heat. planets, the seasons, are all combined in producing these effects in a smaller degree. Mountains give a direction to the courses of the air. Fires carry a current of air along their body. Night and day alternately chill and warm the earth, and produce an alternate current of its vapours. These, and many other causes, may be assigned for the variety and the activity of the winds, their continual change, and uncertain duration.

With us on land, as the wind proceeds from so many causes, and meets such a variety of obstacles, there can be but little hopes of ever bringing its motions to conform to theory; or of foretelling how it may blow a minute to come. The great Bacon, indeed, was of opinion, that by a close and regular history of the winds, continued for a number of ages together, and the particulars of each observation reduced to general maxims, we might at last come to understand the variations of this capricious element; and that we could foretell the certainty of a wind with as much ease as we now foretell the return of an eclipse. Indeed, his own beginnings in this arduous undertaking seem to speak the possibility of its success; but, unhappily for mankind, this investigation is the work of ages, and we want a Bacon to direct the process.

WIND is a current of air. Experimental philosophers produce an artificial wind by an instrument called an colipile. This is nothing more than a hollow copper ball, with a long pipe; a tea-kettle might be readily made into one, if it were entirely closed at the lid, and the spout left open through this spout it is to be filled with water, and then set upon the fire, by which means it produces a violent blast, like wind, To be able, therefore, with any plausibility, to which continues while there is any water remain- account for the variations of the wind upon land, ing in the instrument. In this manner water is is not to be at present expected; and to underconverted into a rushing air; which, if caught stand any thing of their nature, we must have as it goes out, and left to cool, is again quickly recourse to those places where they are more converted into its former element. Besides this, permanent and steady. This uniformity and as was mentioned in the former chapter, almost steadiness we are chiefly to expect upon the every substance contains some portions of air. ocean. There, where there is no variety of subVegetables, or the bodies of animals left to pu- stances to furnish the air with various and intrefy, produce it in a very copious manner. But constant supplies, where there are no mountains it is not only seen thus escaping from bodies, but to direct the course of its current, but where all may be very easily made to enter into them. is extensively uniform and even; in such a place, A quantity of air may be compressed into water, the wind arising from a simple cause, must have so as to be intimately blended with it. It finds but one simple motion. In fact, we find it so. a much easier admission into wine, or any fer- There are many parts of the world where the mented liquor and an easier still into spirits of winds, that with us are so uncertain, pay their wine. Some salts suck up the air in such quan- stated visits. In some places they are found to tities, that they are made sensibly heavier there-blow one way by day, and another by night; in by, and often are melted by its moisture. In others, for one-half of the year they go in a directhis manner, most bodies being found either ca- tion contrary to their former course: but, what

it

is more extraordinary still, there are some places
where the winds never change, but for ever blow
the same way.
This is particularly found to
obtain between the tropics in the Atlantic and
Æthiopic oceans; as well as in the great Pacific

sea.

west. In this manner the air is put into motion by day; and by night the parts continue to impel each other till the next return of the sun, that gives a new force to the circulation.

To conceive his opinion clearly, let us for a Few things can appear more extraordinary moment suppose the whole surface of the earth to a person who has been out of our variable to be an ocean, and the air encompassing it on latitudes, than this steady wind, that for ever every side, without motion. Now it is evident, sits in the sail, sending the vessel forward; and that that part of the air which lies directly under as effectually preventing its return. He who the beams of the sun, will be rarefied; and if the has been taught to consider that nothing in the sun remained for ever in the same place, there world is so variable as the winds, must certainly would be a great vacuity in the air, if I may so be surprised to find a place where there is nothing express it, beneath the place where the sun stood. more uniform. With us their inconstancy has The sun moving forward from east to west, this become a proverb; with the natives of those dis- vacuity will follow too, and still be made under tant climates they may talk of a friend or a mis- it. But while it goes on to make new vacuities, tress as fixed and unchangeable as the winds, and the air will rush in to fill up those the sun has mean a compliment by the comparison. When already made; in other words, as it is still traour ships are once arrived into the proper lati- velling forward, the air will continually be rushtudes of the great Pacific ocean, the marinering in behind, and pursue its motions from east to forgets the helm, and his skill becomes almost useless neither storms nor tempests are known to deform the glassy bosom of that immense sheet of waters; a gentle breeze, that for ever blows in the same direction, rests upon the canvass, and speeds the navigator. In the space of six weeks, ships are thus known to cross an immense ocean, that takes more than so many months to return. Upon returning, the trade-wind, which has been propitious, is then avoided; the mariner is generally obliged to steer into the northern latitudes, and to take the advantage of every casual wind that offers, to assist him into port. This wind, which blows with such constancy one way, is known to prevail, not only in the Pacific ocean, but also in the Atlantic, between the coasts of Guinea and Brazil; and, likewise, in the Ethiopic | This seems to be the great universal wind, blowing from the east to the west, that prevails in all the extensive oceans, where the land does not frequently break the general current. Were the whole surface of the globe an ocean, there would probably be but this one wind, for ever blowing from the east, and pursuing the motions of the sun westward. All the other winds seem subordinate to this; and many of them are made from the deviations of its current. To form, therefore, any conception relative to the variations of the wind in general, it is proper to begin with that which never varies.

ocean.

ness.

There have been many theories to explain this invariable motion of the winds; among the rest we cannot omit that of Dr. Lyster, for its strange"The sea," says he, "in those latitudes, is generally covered over with green weeds, for a great extent; and the air produced from the vegetable perspiration of these, produces the trade-wind." The theory of Cartesius was not quite so absurd. He alleged that the earth went round faster than its atmosphere at the equator; so that its motion, from west to east, gave the atmosphere an imaginary one from east to west; and thus an east

In this manner is explained the constant east wind that is found blowing round the globe, near the equator. But it is also known, that as we recede from the equator on either side, we come into a trade-wind, that continually blows from the poles, from the north on one side, or the south on the other, both directing towards the equator. This also proceeds from a similar cause with the former; for the air being more rarefied in those places over which the sun more directly darts its rays, the currents will come both from the north and the south, to fill up the intermediate vacuity.

These two motions, namely, the general one from east to west, and the more particular one from both the poles, will account for all the phenomena of trade-winds; which, if the whole surface of the globe were sea, would undoubtedly be constant, and for ever continue to blow in one direction. But there are a thousand circumstances to break these air-currents into smaller ones; to drive them back against their general course; to raise or depress them; to condense them into storms, or to whirl them into eddies. In consequence of this, regard must be often had to the nature of the soil, the position of the high mountains, the course of the rivers, and even to the luxuriance of vegetation.

If a country, lying directly under the sun, be very flat and sandy, and if the land be low and extensive, the heat occasioned by the reflection of the sunbeams produces a very great rarefaction of the air. The deserts of Africa, which are conformable to this description, are scarcely ever fanned by a breath of wind by day; but the burning sun is continually seen blazing in intolerable splendour above them. For this reason, all along the coasts of Guinea, the wind is always

3

| that blow for certain months of the year one way, and the rest of the year the contrary way; these are called the Monsoons, from a famous pilot of that name, who first used them in navigation with success. In all that part of the ocean that lies between Africa and India, the east winds begin at the month of January, and continue till about the commencement of June. In the month of August or September, the contrary direction takes place: and the west winds prevail for three or four months. The interval be

of June to the beginning of August, there is no fixed wind; but the sea is usually tossed by violent tempests, proceeding from the north. These winds are always subject to their greatest variations, as they approach the land; so that on one side of the great peninsula of India, the coasts are, for near half the year, harassed by violent hurricanes and northern tempests: while, on the opposite side, and all along the coasts of Coromandel, these dreadful tempests are wholly un

to reign in the month of September; but at fifteen degrees of south latitude, this wind is found to be lost, and the great general trade-wind from the east is perceived to prevail. On the contrary, at Cochin, in China, the west wind begins

perceived blowing in upon the land, in order to fill up the vacuity caused by the sun's operation. In those shores, therefore, the wind blows in a contrary direction to that of its general current; and is constantly found setting in from the west. From the same cause it happens, that those constant calms, attended with deluges of rain, are found in the same part of the ocean. For this tract being placed in the middle, between the westerly winds blowing on the coast of Guinea, and the easterly trade-winds that move at some distance from shore, in a contrary direc-tween these winds, that is to say, from the end tion, the tendency of that part of the air that lies between these two opposite currents is indifferent to either, and so rests between both in torpid serenity; and the weight of the incumbent atmosphere, being diminished by the continual contrary winds blowing from hence, it is unable to keep the vapours suspended that are copiously borne thither; so that they fall in continual rains. But it is not to be supposed, that any theory can account for all the phenomena of even those winds that are known to be most regular. In-known. At Java and Ceylon, a west wind begins stead of a complete system of the trade-winds, we must rather be content with an imperfect history. These,' as was said, being the result of a combination of effects, assume as great a variety as the causes producing them are various. Besides the great general wind above mention-in March; so that these monsoons prevail, at ed, in those parts of the Atlantic that lie under the temperate zone, a north wind prevails constantly during the months of October, November, December, and January. These, therefore, are the most favourable months for embarking for the East Indies, in order to take the benefit of these winds, for crossing the Line; and it has been often found by experience, that those who had set sail five months before, were not in the least farther advanced in their voyage, than those who waited for the favourable wind. During the winter, off Nova Zembla, and the other arctic countries, a north wind reigns almost continually. In the Cape de Verde islands, a south wind prevails during the month of July. At the Cape of Good Hope, a north-west wind blows during the month of September. There are also regular winds, produced by various causes, upon land. The ancient Greeks were the first who observed a constant breeze, produced by the melting of the snows, in some high neighbouring countries. This was perceived in Greece, Thrace, Macedonia, and the Ægean sea. The same kind of winds are now remarked in the kingdom of Congo, and the most southern parts of Africa. The flux and reflux of the sea also produces some regular winds, that serve the purposes of trade; and, in general, it may be observed, that wherever there is a strong current of water, there is a current of air that seems to attend it."

Besides these winds that are found to blow in one direction, there are, as was said before, others 1 Buffon, vol. ii. p. 230.

2 See Supplementary Note A, p. 166.

different seasons, throughout the Indies. So that the mariner takes one part of the year to go from Java to the Moluccas; another from Cochin to Molucca; another from Molucca to China; and still another to direct him from China to Japan.*

There are winds also that may be considered as peculiar to certain coasts; for example, the south wind is almost constant upon the coasts of Chili and Peru; western winds almost constantly prevail on the coast of Terra Magellanica, and in the environs of the Straits le Maire. On the coasts of Malabar, north and north-west winds prevail continually; along the coast of Guinea, the north-west wind is also very frequent; and, at a distance from the coasts, the north-east is always found prevailing. From the beginning of November to the end of December, a west wind prevails on the coasts of Japan; and, during the whole winter, no ships can leave the port of Cochin, on account of the impetuosity of the winds that set upon the coast. These blow with such vehemence, that the ports are entirely choked up with sand, and even boats are not able to enter. However, the east winds that prevail for the other half of the year, clear the mouths of their harbours from the accumulations of the preceding winter, and set the confined ships at liberty. At the straits of Babelmandel, there is a south wind that periodically returns, and which is always followed by a north-east.

Besides winds thus peculiar to certain coasts, there are others found to prevail on all the coasts, 3 Varenii Geographia Generalis, cap. 20. 4 See Supplementary Note B, p. 169.

L

that being the time of high tides, the sea may Such is the nature and the inconstancy of the communicate a part of its motions to the winds. irregular winds, with which we are best acquaintThe sun and moon, also, which then have a great-ed. But their effects are much more formidable er effect upon the waters, may also have some in those climates near the tropics, where they influence upon the winds: for there being a great are often found to break in upon the steady body of air surrounding the globe, which, if con- course of the trade-winds, and to mark their pasdensed into water, would cover it to the depth of sage with destruction. With us the tempest is thirty-two feet, it is evident that the sun and but rarely known, and its ravages are registered moon will, to a proportionable degree, affect the as an uncommon calamity; but in the countries atmosphere, and make a tide of air. This tide that lie between the tropics, and for a good space will be scarcely perceivable indeed; but, without beyond them, its visits are frequent, and its doubt, it actually exists; and may contribute to effects are anticipated. In these regions the increase the vernal and autumnal storms, which winds vary their terrors; sometimes involving are then known to prevail. all things in a suffocating heat; sometimes mixing all the elements of fire, air, earth, and water, together; sometimes, with a momentary swiftness, passing over the face of the country, and destroying all things in their passage; and sometimes raising whole sandy deserts in one country, to deposite them upon some other. We have little reason, therefore, to envy these climates the luxuriance of their soil, or the brightness of their skies. Our own muddy atmosphere, that wraps us round in obscurity, though it fails to gild our prospects with sunshine, or our groves with fruitage, nevertheless answers the call of industry. They may boast of a plentiful, but precarious, harvest; while with us, the labourer toils in a certain expectation of a moderate, but a happy, return.

Upon narrowing the passage through which the air is driven, both the density and the swiftness of the wind is increased. For, as currents of water flow with greater force and rapidity by narrowing their channels; so also will a current of air driven through a contracted space, grow more violent and irresistible. Hence we find those dreadful storms that prevail in the defiles of mountains, where the wind, pushing from behind through a narrow channel, at once increases in speed and density, levelling or tearing up every obstacle that rises to obstruct its passage.

Winds reflected from the sides of mountains and towers, are often found to be more forceful than those in direct progression. This we frequently perceive near lofty buildings, such as churches or steeples, where winds are generally known to prevail, and that much more powerfully than at some distance. The air in this case, by striking against the side of the building, acquires additional density, and, therefore, blows with more force.

These different degrees of density, which the air is found to possess, sufficiently show that the force of the winds does not depend upon their velocity alone: so that those instruments called anemometers, which are made to measure the velocity of the wind, will by no means give us certain information of the force of the storm. In order to estimate this with exactness, we ought to know its density; which also these are not calculated to discover. For this reason we often see storms, with very powerful effects, that do not seem to show any great speed; and, on the contrary, we see these wind-measurers go round with great swiftness, when scarcely any damage has followed from the storm.7

In Egypt, a kingdom so noted for its fertility, and the brightness of its atmosphere, during summer the south winds are so hot that they almost stop respiration; besides which, they are charged with such quantities of sand, that they sometimes darken the air as with a thick cloud.9

50 miles per hour

60

80 100

A storm or tempest.
A great storm.
A hurricane.

A hurricane that tears up trees,
and carries buildings before it.

The force of the wind is nearly as the square of ties. But the force is much more than in the simple the velocity, or but little above it, in these velociratio of the surfaces, with the same velocity, and this increase of the ratio is the more, as the velocity is the more. By accurate experiments with two planes, the one of 174 square inches, the other of 32, which are nearly in the ratio of 5 to 9, Dr. Hutton found their resistances, with a velocity of 20 feet per second, to be the one 1,196 ounces, and the other, 2,542 ounces: which are in the ratio of 8 to 17, being an increase of between one-fifth and one-sixth parts more than the ratio of the surfaces.-ED. 8 Buffon, vol. ii. p. 258.

9 The most destructive wind of Egypt is what is called the Kamsin, which generally prevails in March, April, and May. Denon thus describes it: "The Kamsin is equally terrible by the frightful spectacle it exhibits when present, and by the consequences which follow its ravages. We had already passed with security one-half of the season in which it usually appears: when, in the evening of the 18th of May, I felt myself entirely overcome by a suffocating heat; it seemed as if the fluctuation of the air was suddenly suspended. I was struck on my arrival with my companions at the bank of the Nile, with a new appearance of nature all around me; this was a