of the northern hemisphere are from the south-west, and those in the southern hemisphere from the north-west; and though many causes-unequal distribution of sea and land, position of continents, irregularity of land-surface, &c.-tend to disturb this continuity, still, on the whole, there is a marked predominance of westerly winds. In our own islands, the fact must be sufficiently obvious to every observer; in the North Atlantic, the average packet-voyage from New York to Liverpool is 23 days, while the return voyage requires 40 days; and in the vicinity of Cape Horn, westerly winds are three times more frequent than those from an easterly direction.

217. Among the constant currents of the atmosphere must also be enumerated the north and south polar winds, which, as mentioned in the theory of the trade-winds, are continually flowing north and south from either pole. And further, as an undercurrent almost invariably implies the existence of an upper current in a contrary direction, we may notice also the upper west wind of the tropics, which, high above the trades, seems constantly flowing in an opposite direction. Proofs of this upper tropical wind are found in the circumstance that dust ejected from the volcanoes of the West Indies has fallen on ship-deck several hundred miles to the eastward; and also in the often-observed fact, that a similar wind prevails near the summit of Teneriffe (12,000 ft.), while the regular trade-wind is blowing from an opposite direction below.

["The existence of the upper trade-wind, coming from the west, or of the return trade-wind, which has often been doubted, seems to be proved in the Atlantic," says M. Guyot, "by two facts often cited and very conclusive. The volcano of the island of St Vincent, belonging to the Lesser Antilles, in one of its eruptions hurled a column of volcanic cinders to a great height in the atmosphere; the inhabitants of Barbadoes, situated east of St Vincent, saw with astonishment the cinders falling upon their island. The 25th of February 1835, the volcano of Cosiguina, in Guatemala, threw into the air such a quantity of cinders that the light of the sun was darkened during five days; a few days after, they were seen to cover the streets of Kingston, in Jamaica, situated north-east of Guatemala. these two cases it is evident that the cinders had reached the region of the upper trade-wind, and had been carried by it from west to east, in the opposite direction to the lower trade-wind. At the summit of the peak of Teneriffe, most travellers have found a west wind, even when the northeast trade-wind prevailed on the seaboard."]

In

218. Of the periodical winds the most important are the MONSOONS (from the Malay word moussin, signifying seasons), which in certain countries within and near the tropics blow from a certain quarter for one-half the year, and from an opposite point during the other half-the period of change being marked by calms, tempests, and variables. In other words, the monsoons are but the trade-winds interrupted in their regular action by the

geographical peculiarities of the regions in which they occur. From April to October, the south-west monsoon prevails north of the equator, and the south-east in the southern hemisphere; but from October to April the north-west monsoon blows south of the equator, and the north-east in the northern hemisphere. Of course, the farther from the equator, the later in the season will the southwest and north-west monsoons occur; and thus it happens that in India, at Anjengo, on the Malabar coast, 8° 30′ N. lat., the southwest monsoon commences as early as the 8th of April; at Bombay, 10° N. lat., about the 15th of May; in Arabia it commences a month earlier than on the coast of Africa; and in the northern part of Ceylon, fifteen or twenty days earlier than on the coast of Coromandel.

219. The cause of the monsoons is to be sought in the effect produced by the sun during his apparent annual progress from one tropic to the other, and has been thus explained:-"In the Indian Ocean, for example, as the sun advances towards the north, the zone of greatest rarefaction recedes from the equator, and the north-east monsoon (which is nothing more than the trade-wind) then subsides, and is succeeded by calms and variable winds; but as the summer approaches, and the sun arrives at the northern tropic, the southern portions of the Asiatic continent become hotter than the ocean, and the humid air from the equatorial seas flows northward to the land; south-west winds will therefore arise, which prevail from the peninsula of India to the Arabian Gulf, until the time of the autumnal equinox. During the same period the south-east monsoon, in the southern hemisphere, tempers the heat of Lower Guinea, and brings rain to the shores of Brazil. The motions of the atmosphere, however, are reversed as the sun crosses the equator and approaches the southern tropic. Pouring his fervid rays upon Southern Africa, the vast tract of New Holland, and the splendid clime of Brazil, the air flows in from the north and north-west towards these highly heated regions, and winds from these quarters prevail for several months-the monsoon extending along the coast of Brazil from Cape St Augustine to the Isle of St Catherine. But now the influence of the sun is partially withdrawn from Southern Asia; it glows no longer beneath its vertical rays, and over the cooled earth the north-east monsoon resumes its wonted course."

220. Equally remarkable for their persistency, though local in their areas and limited in their times, are the land and sea breezes, which occur on almost every seaboard, but most notably, of course, within the tropics. As formerly explained, these breezes arise from the unequal heating of the land and water surfaces, and be

come most decided where this inequality is greatest. During day the land-surface, from its low conducting power, acquires a more elevated temperature than the adjacent waters, and the air above it, partaking of this heat, becomes rarefied and ascends, while the cooler and denser air from the ocean sets in as a sea-breeze to restore the equilibrium. This sea-breeze, especially in tropical latitudes, commences about nine in the morning, gradually increases in force till the middle of the day, and falls away as the sun declines in the afternoon. As evening approaches, the air over the surface of the land becomes more rapidly cooled by radiation than that over the water, and then a cool land-breeze sets out towards the ocean, blows freshly during the night, and dies away towards morning, when the sea-breeze again commences. The extent of these breezes is exceedingly variable. In some localities they blow only for a few miles out and in of the shoreline, while in others their bracing and refreshing influence is experienced for many leagues in either direction.

221. Owing to the mobility of the atmosphere, and the many causes that may temporarily affect its temperature and density, the variable winds are exceedingly numerous and capricious-so capricious, that "fickle as the wind" has become an established simile in every extra-tropical country. We say extra-tropical, for within the tropics the trades and monsoons are the prevalent winds, and these, as we have seen, maintain a remarkable degree of permanency and periodicity. But in extra-tropical regions, where the force of the permanent winds becomes feeble, a thousand causes (as unequal distribution of land and water, nature of soil, irregularity of surface, &c.) occur to disturb these currents, and thus almost every district has its temporary winds—varying in direction, force, and duration. Fickle and uncertain as they may appear, they are, nevertheless, the results of law and lawdirected forces; but these forces being so complicated, comparatively little has been determined in this department of meteorology. Still, we know that in the middle latitudes of the northern hemisphere there are, properly speaking, only two normal winds-that of the north-east, and that of the south-west -the former being the cold polar hurrying towards the equator, and the latter the warm equatorial trending towards the poles. "The winds blowing in other directions are local winds, or transition winds, from one of the general currents to the other; and Professor Dove has shown that in Europe, at least, these winds succeed each other in an order always the same, which he has called the law of rotation of the winds. This will be easily under

stood if we remember that in advancing along their course the south-west tends always to become more west, and the north-east more and more east; we shall see that the result of this disposition ought to be, whenever they meet and change places, a rotation from west by north to east, and from east by south to west. In the place of the conflict of the two currents, the wind will then blow successively from three different regions, and in this order, until it is established in the direction of that one of the currents which has overpowered the other. But no one of these transition winds blows for any great length of time. In the southern hemisphere the order of succession is the reverse."

[Referring to the winds of the northern hemisphere, the eloquent author of Earth and Man' has the following suggestive remarks:-"This conflict of polar and equatorial winds, opposite in character and direction, gives to our climate one of its most characteristic features, that changeableness, that extreme variety of temperature, of dryness, and of moisture, of fair weather and of foul, that uncertainty of the seasons, which always keep the merchant and the farmer in anxious suspense, between the hope of a good harvest and the fear of a dearth. Not only are the variations in the same year considerable, but they are still more so from one year to another. The system of these currents oscillates from east to west, and changes place. The polar winds will prevail in a country, and will endanger the crops by the prolonged dryness of their atmosphere; while further east or west the trade-wind will spread fertility by its beneficent rains. Or the opposite: the south winds acquire such a preponderance, that the harvests perish by the moisture; while, at a somewhat greater distance, on the limit of the same wind, Nature lavishes all her treasures upon the labourer. It has been remarked that a mild winter in Europe corresponds frequently to a severe winter in America and Asia; while the mildness of the winter in America affords a presumption of a colder winter on the other side of the Atlantic. The years 1816 and 1817 were marked, as is known, in the history of Europe, by a general famine and distress. The wet was such that the harvests failed entirely. But the south-west wind, which blew without cessation over the western part of the continent, and which drenched it in its vapours, did not extend beyond Poland; and it was the south of Russia whose corn supported famished Europe for many long months. Then was revealed the commercial importance of those countries, hitherto unknown, and constantly increasing since. Who does not still remember the immense impulse given to the commerce between Europe and America by the drought of 1846, which damaged the corn-crop in Europe while America had an abundant harvest? These examples alone tell us the important part played in the life of nations by those variations of the atmospheric currents belonging to our temperate countries."]

222. Generally speaking, in the higher latitudes of the northern hemisphere, a south wind is warm and moist, because it comes from warmer regions, and passes over a greater extent of ocean; while, on the contrary, and for opposite reasons, a north wind is cold and dry. It is for this reason that the westerly and southwesterly winds of Europe are humid and genial, and the north and north-easterly hard and ungenial. These major currents are of course much modified in their passage over certain localities, and