fies season.

These winds blow with the greatest regu

larity between Hindustan and the eastern coast of Africa. When the sun is south of the equator, from October to April, a north-east monsoon prevails; but when the sun is north of the equator, from April to October, a south-western current is established. When the sun passes the equator, and the monsoons are changing their direction, variable winds or tempests generally occur, a disturbance which is called by seamen the "breaking up of the monsoons."

The theory of the monsoons will be readily understood after the foregoing account of the trades. When the sun has great northern declination, the peninsula of Hindustan, the north of India and China being strongly heated, the quick moving air of the equator rushes to the northward, to fill up the slower moving rarefied space, and this supply of air, having not only a rapid velocity towards the east, but also a motion from the south, produces the south-west monsoon in the Indian Ocean, in the Bay of Bengal, and in the China Sea. When the sun, on the other hand, has great southern declination, the same seas are occupied by air, which, coming from regions beyond the northern tropic, hast less easterly velocity than the space it is drawn into. This gives the air an easterly direction, which, combined with its proper motion towards the equator from the north, produces the north-east monsoon.

42. In addition to the grand effects of the trades and the monsoons, the heating action of the sun produces a diurnal interchange of air in some parts of the world, in what are called land and sea breezes. The ocean is but little affected by sudden changes of temperature, such as are produced by the succession of day and night, and it participates only to a certain extent in the alternating temperatures of the seasons. Such changes

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become equalized over large tracts of salt water, and serve to temper the rigours of climate on coasts and in islands. The land, on the contrary, becomes rapidly heated by the sun, and as rapidly cooled in his absence. Thus, the great continents act as heaters and coolers of the atmosphere, and produce those changes in the aerial currents already noticed; and not only continents, but islands of moderate size, produce the daily alternation of land and sea breezes, so refreshing upon the coasts of hot climates. During the day the land becomes much more heated by the sun than does the adjacent water, and consequently the air resting upon the land is much more heated and rarefied than that upon the water. The cooler and denser air, therefore, flows from the water towards the land, constituting the sea breeze, and, displacing the warmer and lighter air over the land, forces it into a higher region, along which it flows in an upper current to seaward. At night a contrary effect takes place. After sunset the land cools much more rapidly than the water, and the air over the shore becoming cooler, and consequently heavier than that over the sea, flows towards it, and forms the land breeze.

The phenomena of land and sea-breezes may be well illustrated by a simple experiment. Fill a large dish with cold water, and place in the middle of it a saucer full of warm water; let the dish represent the ocean, and the saucer an island heated by the sun, and rarefying the air above it; blow out a wax taper, and if the air of the room be still, on applying it successively to every side of the saucer, the smoke will be seen moving towards it and rising over it, thus indicating the course of the air from sea to land. On reversing the experiment, by filling the saucer with cold water, and the dish with warm, the land breeze will be shown by holding

the smoking wick over the edge of the saucer; the smoke will then be wafted to the warmer air over the dish.

43. In most mountain districts alternating currents may be observed in the atmosphere, which partake of much of the nature and regularity of land and sea breezes; they have been distinguished by the name of hill and valley breezes, and, as the subject is almost new to science, it may be interesting to give an example of these breezes *.

At Nyons, in the department of La Drôme, a wind has been known from time immemorial, under the name of Pontias. It is a cold wind, and rises every evening about 9 or 10 o'clock in summer, and at 6 in winter. Its first approach is from a narrow, deep, winding gorge, about two leagues in length, and extending on one side into the plains of the Rhone, near Nyons, and on the other side into a large valley, inclosed by the mountains of La Drôme. The breeze increases progressively during the night, until sunrise, when it begins to decline in intensity, and finally dies away when the ground has been warmed by the sun. This breeze is much more cold and violent in winter than in summer, and it often produces such a depression of temperature as to congeal the moisture of the atmosphere. In summer it has the effect of a fresh morning breeze. Its regularity is very remarkable, although subject to certain disturbances. Thus, during the greatest heats of summer when the short nights are not sufficient to cool the surface of the parched ground, the Pontias is scarcely perceptible. A similar effect is produced by a rainy or a cloudy night. Snow, on the contrary, appears greatly to favour this breeze. The effect of these variations is to limit its progress. In winter, or imme

*The subject has been raised into the importance which it seems to merit by M. Fournet, in the Annales de Chimie et de Physique for 1840.

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diately before or after rains, it sometimes descends as far as the Rhone, a distance of seven leagues; but in summer, or during serene weather, it extends to a much shorter distance.

This breeze does not exist in the upper regions of the atmosphere, nor even above the hills in the neighbourhood of Nyons, but seems to be entirely confined to the defile, at the entrance to which stands the town. It does not blow in an equal current, but is subject to certain swells at intervals of a few minutes, and these are most distinct when an east wind opposes its exit; it then escapes by irregular puffs. On ascending the defile, which leads into the upper gorge, the force of the wind diminishes in proportion to the ascent, and at length is entirely lost at a certain elevation.

In connection with this breeze is an upper compensating current, called la Vesine, or the bad wind, which, ascending the river of Eygues, passes over a defile, and is lost in a larger valley. This wind increases in violence with the heat of the day or of the season.

Thus we observe two breezes periodically opposed to each other; the one a nocturnal breeze, and the other a day breeze, blowing in opposite directions, according to the hour of the day, and developed entirely by the physical features of the locality. M. Fournet has also established the existence of hill and valley breezes in other mountain districts of France and Switzerland, caused chiefly by the asperities of the soil, producing daily an atmospheric flux and reflux, which manifests itself in ascending and descending breezes known in different places by local names.

44. There is, however, a distinction to be attended to in comparing the daily alternations of wind produced by mountains with those produced by coasts. The sea breeze is much more general and decided than the

nocturnal or land breeze, whereas, in mountain breezes, the nocturnal or hill breeze seems to be the most regular; because, although the mountain summits receive by day more and stronger sunshine than the lowlands, yet they also reflect and radiate more heat, besides being more cooled by the colder air in contact with them, so that on the whole it is doubtful whether they would generally become warmer than the plains, so as to receive a diurnal breeze from them, except in particular localities. But by night everything conspires to reduce the temperature of the summits, which, from their isolated position, radiate their heat in every direction through the clear rarefied air into boundless space, while the lowlands can only radiate their heat upwards and through a denser and less transparent air; and even that which they do radiate is often reflected back by a canopy of clouds, so that the nocturnal breeze from the hills must be far more general than the diurnal one towards them. In the case of sea and land breezes, on the contrary, although the land must always be more heated by day than the sea, thus producing the sea breeze, there is no apparent cause why the land, which is a worse radiator of heat than water, should ever become cooler than the water by night, so as to send forth a land breeze, properly so called. The winds so designated appear, therefore, to be truly not land, but hill breezes. We are not aware that they are ever felt on flat shores, such as Demerara, although this is one of the hottest coasts in the world, nor on the flat island of Barbadoes; but they are felt off the neighbouring island of Grenada, which is smaller, but mountainous.

45. Our space will not allow us to notice the interesting phenomena of the winds at greater length*; • The reader interested in the subject will find the principal phenomena