returns anew toward the extremities of the terrestrial axis. On this principle, we ought to find a north wind in the northern hemisphere, and a south wind in the southern; but these two directions combine with the motion of the earth from west to east, and there results a N.E. wind in one hemisphere, and a S.E. wind in the other. Indeed, as the diameter of the parallel circles continues diminishing in proportion as we recede from the equator, and as all the points situated in the same meridian turn round the axis of the earth in twenty-four hours, it follows that they move with a velocity much greater, as they are nearer to the equinoctial line. But the masses of air which flow from the north toward the equator have an acquired velocity less than that of the region toward which they are directed. They, therefore, turn more slowly than do the points situated near the equator, and they oppose to the elevated

of floating. Thus it is that, according to circumstances, smoke, and especially vapour of water, condense during the day around lofty peaks (valleys of Aoste, Maurienne, Ossela, Anzasca, Sesia, vale of Illiers, Col du Géant); whence it follows that the air dries during the night, and becomes moist during the day, on these heights; while the inverse effect takes place at night in hollows (Geneva, Col du Géant, St. Paul). It is easy to see from this that these tides must play an important part in the developement of parasitic clouds, and in the phenomena of the distribution of rains and storms.

9th. The hot air of the plains, ascending during the day, tends to warm the valleys and summits; but this effect is partly counterbalanced by the evaporation which it occasions, so that it may become dry and cold (Marienne) on the other hand, the night breeze tends to cool the valleys, by bringing down the cold of the upper regions; hence the explanation of the sudden coolness occasioned by the aloup du vent, the congelations of watery vapour occasioned by the pontias, the spring frosts, which, at an equal radiation, affect more particularly the vegetation of the valleys. We might even find in this effect the explanation of some of the anomalies of temperature, which travellers have recognised at different heights on the side of mountains.

10th. The general upper winds might, under certain circumstances, alter the aerial wave or ebb (Maurienne, Aoste, Ossola, Martigny, Mont Cenis), or even complicate them (Cogne); but their effect is not always sufficiently powerful to destroy them entirely (Mont Tharbor, vale of Sesia): sometimes they produce a dead calm (Tarentaise). It follows from this that the prognostics of fine weather, deduced from the regularity in the behaviour of breezes, are often contradicted by experience (valley of Brévenne, Chessy, Bex). However, we may say that the destruction of the currents is generally followed by rain (Maurienne).

11th. Finally, circumstances of local temperature may also nullify mountain breezes; thus it is that the pontias ceases to blow when, during the short interval of summer nights, the earth, heated by a burning sun, has not time to become sufficiently cool.

M. FOURNET explains these alternations of ascending diurnal currents, and descending nocturnal currents, by the heating of summits by the rising sun, which determines an ascending current; whilst the heating of the plain, which is greater during the day than that of the mountain, determines a descending current towards evening. I am the more induced to admit this explanation, as it follows, from the experiment made during the summer of 1842, at the summit of the Faulhorn, by M. BRAVAIS, that the mean heating of the surface of the soil during the day is sensibly equal to that of the maximum of the air (Vide Annales de Chimie et de Physique, t. lxxiv. p. 337; 1840).-M.

parts of the surface of the globe a resistance analogous to that of a well-defined N.E. wind. For the same reason the trade-wind of the southern hemisphere blows from the S.E.

On approaching the equator from the parallel of 30°, few changes are observed in the direction of the winds; they vary from N.N.E. to N.E. or E.N.E.; and in the neighbourhood of the equator they are E. It is, in fact, at the equator that the motion by the earth's rotation is most rapid; and there it is that the masses of air remain most behindhand, and oppose the greatest resistance: it is on this line, also, that the trade-winds from both hemispheres meet, and as one comes from the N.E., and the other from the S.E., an east wind is the result; as when one billiard-ball is met by another, it takes a direction intermediate between that of the two balls. The trade-winds of our hemisphere also affect all the directions comprised between E. and N.N.E.

In the upper regions of the atmosphere, there also exist constant currents; in the northern hemisphere the heated air is determined toward the north; and, in proportion as it advances toward the pole, it gets more and more in advance of the earth in its rotatory motion. The combination of this motion from the west toward the east, with the primordial direction from south to north, gives rise to a S.W. wind. For the same reason a N.W. wind is observed in the upper currents of the southern hemisphere.

TRADE-WINDS OF THE GREAT OCEAN.-Bounded on one side by the western coast of America, and on the other by the eastern coast of New Holland, sprinkled merely with small groups of islands, this sea presents us with the greatest masses of water on our globe. The N.E. trade-wind blows very regularly at some distance from the earth, between the equator and the northern tropical circle. To the credit of this wind it is that the Spanish galleons always went direct from Acapalco to Manilla, without wandering from their route; and thus they did not discover a multitude of islands which have since been seen. The northern limit of this trade-wind advances, during the summer of our hemisphere, toward the north pole, and retreats during our winter; according as our hemisphere is hotter or colder than the opposite hemisphere. The N.E. trade-wind may be said to prevail between the 2d and the 25th degree of N. latitude; the S.E. trade-wind also blows regularly on the south of the equator; its limits are not so well known, but we

shall not be far from the truth in saying that it extends from the 10th to the 21st degree of south latitude.

These winds reign over the whole extent of this sea, as far as the Philippines and New Holland; but they are only found at a certain distance from the American coast. In the belt, from 2° N. to 2° S., which separates the two trade-winds, the air is most highly heated; and it rises with such force, that it neutralises the horizontal motion. So that, in this belt, the dead calm is only disturbed by storms, which the Spaniards and Portuguese term tornados or travados. We will call this belt the region of calms; and we shall see that torrents of rain, and almost daily storms, joined to the causes mentioned, are opposed to the establishment there of regular winds.

TRADE-WINDS OF THE ATLANTIC OCEAN. —

Navigators who traverse these seas in all directions have fixed their limits with great precision. In the north, the N.E. trade-wind does not prevail beyond the 28th or 30th degree. Its southern limit is at a mean 8° N.; then comes the region of calms, extending to 3° N., whence commences the S.E. trade-wind, which extends onward to 28° of south latitude. The extent of the region of calms also depends on the season; in August it extends from 3° 15' N. to 13° N.; in February, from 1° 15′ N. to 6° N.

In this sea the S.E. trade-wind always extends to the north of the equator. M. Prevost thought of explaining this anomaly by remarking that the southern hemisphere is colder than ours; the region of calms being, in his opinion, limited by two bands, the mean temperature of which ought to be the same, the S.E. trade-wind must pass beyond the equator. This explanation, which was very favourably received at the time, is subject, however, to real difficulties. First, in the Great Ocean the equator forms the limit of the trade-winds; then, if M. Prevost's explanation were correct, the following absurd consequence would follow that the winter of the northern hemisphere must be hotter than the summer of the southern hemisphere, because in this season the S.E. trade-wind always blows to the north of the equator.

The solution of this problem must be sought where M. de Humboldt seeks it, in the configuration of the basin of the Atlantic Ocean. The portion of South America, situated to the north of the equator, presents to us the lofty mountains of Columbia, which separate the sea from the Antilles of the Great Ocean. When the sun is at the south

of the equator, and, consequently, during the winter of the northern hemisphere, these seas are already hotter than the continent; but the current which flows into the sea of the Antilles, and which is, so to speak, the origin of the Gulf Stream, elevates their temperature still higher. This circumstance would of itself be sufficient to determine a current of air from the south to the north, which, by combining with the east wind, would produce a S.E. wind, that would neutralise the N.E. wind before the latter had reached the equator. Add to this, that the general direction of the coast is from S.E. to N.W. which singularly favours the extension of the south-east wind.

WEST WIND OF THE HIGHER REGIONS.

We have already shewn that this wind must constantly prevail in the higher regions of the atmosphere within the tropics. The following proofs are still more conclusive: The inhabitants of Barbadoes, an island situated to the north of the chain of the Antilles, one day to their great astonishment saw volcanic ashes fall from the sky. They came from the volcano of St. Vincent, which is situated to the west of their isle. These ashes, having been launched into the air as high as to the region of the upper current, had been transported by it in the direction from west to east. At the summit of the peak of Teneriffe almost all travellers have found west winds, even while the tradewind was prevailing at the level of the sea. Paludan, a navigator who is well acquainted with these localities, relates that little clouds often float in a reverse direction to the trade-wind; and Bruce made the same observations in Abyssinia. A recent fact confirms all the preceding on the 25th of February, 1835, the ashes emitted from the volcano of Cosiguina, in the state of Guatimala, obscured the light of the sun for five days; they mounted into the region of the upper trade-wind, and fell a short time afterwards in the streets of Kingston in Jamaica, which is situated to the N.E. of Guatimala.

WINDS IN THE INDIAN OCEAN.-The winds which prevail in these latitudes, being influenced by the configuration of the lands, present phenomena that are very difficult of analysis. The sea is bounded on the west by Africa, which extends from the S.S.W. to the N.N.E. Every document, after having been subjected to severe criticism by Ch. Ritter, tends to shew that this continent is an elevated table-land, of which the first steps only have been explored. On the north are situated Persia and Arabia, which form two slightly elevated table-lands, void of rivers,

and covered with nothing but a miserable vegetation. On the south of these deserts of sand, the Indian peninsula advances into the sea in a southerly direction, whilst it is bounded on the north by the Ilimalaya Mountains, covered with perpetual snow, and by the table-land of Thibet. The coast of Malabar rises abruptly from the bosom of the ocean, whilst that of Coromandel rises gradually above the level. The height of the Indian table-land is every where almost the same; it is lower, however, after passing the north point of the island of Ceylon. At the east of the Bay of Bengal is the Birman Empire, the land extends toward the N.E.; the Chinese Sea is bounded on the north by a chain of mountains, and on the east by the Archipelago of the Philippines. On the south and on the east of the peninsula of Malacca are islands studded with lofty mountains, such as Sumatra, Java, Borneo, and the Celebes. To the south of this group is New Holland, the interior of which is unknown; but the absence of rivers makes it fair to suppose that it presents neither table-land nor elevated chains. The dryness of the winds that blow from the interior enables us to presume, also, that it is not furnished with large lakes or numerous ponds.

This predominance of land, and the differences of temperature constantly existing between it and the sea, disturb the regularity of the trade-winds. Regular winds prevail during the winter and the summer, but their direction is not the same; they are known by the name of monsoons, a word derived from the Malay, moussin, signifying season.

In January, the temperature of Southern Africa is at its maximum, that of Asia at its minimum. The northern part of the Indian Ocean is hotter than the continent, but not so hot as the southern part of the same ocean at a similar latitude. In both hemispheres, therefore, we find east winds directed toward the hotter parts. From October to April, the S.E. trade-wind prevails in the southern hemisphere; the N.E. trade-wind blows in the opposite hemisphere, and it obtains the name of N.E. monsoon; between the two is the region of calms. When the sun advances toward the north, the temperature of the continent and that of the sea tend to an equilibrium; so that, about the vernal equinox, there cease to be any prevailing winds in the northern hemisphere, but variable winds alternate with dead calms and hurricanes, whilst the S.E. monsoon prevails throughout the year in the southern hemisphere. In proportion as the northern declination of the sun increases, the temperature of Asia increases more than