adjacent islands. It is also for this reason that the northern and southern hemispheres, having unequal distributions of sea and land, are differently affected in their zones of climate-the greater extent of land in the northern imparting to it, as it were, a continental climate; while the greater expanse of ocean in the southern confers on its similar latitudes more of an insular climate. It is, further, to this unequal reception of heat by land and water that we owe the phenomena of sea and land breezes. During day the atmosphere immediately above the land is more rapidly heated by radiation than that above the ocean, hence the cooler sea-breeze sets in towards the land to restore the balance; while during the absence of the sun the more rapidly cooled land-atmosphere sets out as a land-breeze towards the more slowly cooling surface of the ocean. In fine, it is mainly owing to the unequal distribution of sea and land, and to the different modes in which the two elements are affected by the sun's heat, that we owe the great essentials of climatic diversity—the air and ocean-currents, the vapours and rains, of one region, varying from the winds, currents, and moisture of another.

208. Whatever the temperature of the air, it is incessantly receiving moisture from the surface of the land and water. This vapour arises and is diffused through its mass in an invisible form, and becomes visible only when condensed into mists, fogs, rains, and snows. It arises alike from land and water, but most copiously and persistently, of course, from the surface of the water. The superficial moisture of the land, unless where clothed with vegetation, is rapidly converted into vapour; but the supply from this source is irregular and limited compared with that derived from the water. Surface for surface, salt water is less vaporisable than fresh; hence a slight check to evaporation from the vast and extended surface of the ocean. It arises from rivers, lakes, and seas in every latitude, but most abundantly, of course, within the tropics, where the sun's heat is the greatest. So great is the evaporation within that zone, that it has been estimated as annually sufficient to reduce the surface of the sea to a depth of 8 or 10 feet—a waste, of course, that is incessantly counterbalanced by influx from other latitudes. The warmer the air the greater its capacity for moisture; and this capacity for holding moisture in suspension increases up to the point of saturation, when any reduction in temperature produces condensation, and rain ensues.

209. As in the case of other meteorological phenomena, the circumstances connected with the production and retention of atmospheric vapour are extremely complicated: but in general terms

it may be stated that heat is the grand promoter of evaporation; that a gentle current of air, by removing the pressure of the vapour as it is formed, is more favourable than a stagnant atmosphere; and that it is retarded by moist and cloudy conditions of the air, even though the sensation of heat be considerable. As previously mentioned, the humidity of the atmosphere is confined chiefly to its lower strata; and so rapidly does the quantity diminish as we ascend, that at the height of five or six miles it becomes all but inappreciable. The quantity of moisture evaporated from the surface of the globe may differ from day to day, and from year to year; but on the whole, and for any given number of years, it is returned again in the state of dew, rain, hail, or snow. Understanding the general conditions of the atmosphere as regards its heat and moisture, we may now proceed to its special effects in the production of winds, rains, and other kindred pheno.

mena.

Winds-Constant, Periodical, and Variable.

210. As already explained, any current of the atmosphere produced by inequality of temperature or density is termed a wind -the denser and colder air rushing in to supply the place of the lighter and warmer. In other words, wind is air in motion; and the most frequent cause of such motion is disparity of temperature between adjacent districts. As inequality of temperature is ever arising alike from general and from local causes, the occurrence of winds is incessant and universal, either along the earth's surface, or in the higher strata of the atmosphere. While the colder current is flowing below from the colder to the warmer region, a warm current is floating above from the warmer to the colder. As the homogeneity of the ocean is maintained by its currents, so the equilibrium and uniformity of the atmosphere is preserved by its winds. These winds, having different times, directions, and characters, are necessarily arranged into different classes, and distinguished by various designations.

211. As regards times, those blowing constantly in one direction between, and within a few degrees beyond, the tropics (the Trades), are spoken of as permanent winds; those blowing at certain periods (the Monsoons, &c.), as periodical; and those obeying no fixed period, as variable. As concerns direction, they are distinguished by the points of the compass, as North, South, East, West, South-west, South-south-west, &c., according to the quarter from which they blow. And as regards inherent or cli

N

matic character, they are cold or hot, dry or moist, gentle or boisterous, healthful or unhealthful, according to locality and other accompanying conditions. One of the most obvious characteristics of winds is their velocity, which may vary from a few miles to upwards of a hundred miles an hour; and between these extreme rates we have every variety, from the gentlest zephyr to the most violent hurricane. According to meteorological authorities, a velocity of 7 miles per hour is regarded as a gentle air; of 14, as a light breeze; of 21, a good sailing breeze; of 41, a gale ; of 61, a great storm; of 82, a tempest; and of 92, a hurricane, producing universal devastation-tearing up trees and sweeping away buildings. Or more minutely, according to their velocity and force, they have been arranged as follows::

212. In whatever character they occur, winds are important agents in the production and modification of climate. By their agency the moist and heated atmosphere of one region is transferred to another; and on their agency also depend, in a great measure, those currents of the ocean which are ever producing interchanges between the colder and warmer surface-waters of different latitudes. Besides these great climatological functions, they are intimately concerned in the production of rains and other aqueous phenomena; while their incessant commotions tend to preserve the atmosphere in ever-healthful equilibrium. They are the great bond, as has been well observed, between the land and water surfaces-transferring the moisture of the one to the thirsty uplands of the other, and the dry cold air of the former to disperse and rarefy the humid and depressing atmosphere of the latter. Geologically, winds have considerable effect in removing, piling

up, and re-assorting all loose superficial matters, as the sanddunes of the sea-shore and the sand-drift of the desert; while through the agency of the ocean-waves, which are created by their power, important changes are produced along every shore of the world. In their gentler manifestations they assist in the fertilisation of plants and in the dispersion of their seeds; while in their fiercer demonstrations their track is marked by ruin and devastation. By their impulse the commerce of distant nations is wafted from shore to shore; and fickle and fitful as they may appear, man not unfrequently avails himself of their power to turn the wheels and shafts of his machinery.

213. The most remarkable of the permanent, constant, or perennial air-currents are the TRADE WINDS (so called from their influence on the trade or commerce of the world), which, within the torrid zone and a few degrees beyond it on either side, are ever sweeping round the globe in a westerly direction. This is occasioned by the fact that the region near the equator is most intensely heated the other zones becoming colder and colder towards either pole. Under these circumstances, the air of the torrid zone becomes rarefied and ascends, while the colder and denser air sets in from either side to supply the deficiency. There are thus generated two great sets of currents-colder under-currents setting in towards the equator respectively from the north and south, and warm upper-currents flowing off from the equator towards either pole. If, then, the atmosphere were subject to no other influence than temperature, a north wind would always prevail along the earth's surface in the northern hemisphere, and a south wind in the southern hemisphere. But as the polar currents proceed towards the equator, they gradually come into zones having a greater velocity of rotation, and thus they are deflected in a westerly direction (as exhibited in the accompanying SketchMap), so as to become a north-east wind in the northern hemisphere, and a south-east wind in the southern.

214. In the Pacific, the north-east trade-wind may be said to prevail between the 25th and 2d degree of N. latitude; while the south-east trade ranges more widely between the 10th and 21st of S. latitude. In the Atlantic, on the other hand, the former is comprised between the 30th and 8th degree of N. latitude, and the latter, within the 3d of N. and the 28th of S. latitude. These limits, however, are not altogether stationary, but depend on the seasons-advancing towards the north during the summer of the northern hemisphere, and receding to the south as the sun withdraws to the southern tropic. Within these somewhat fluctuating zones, the trade-winds are steady and perennial, travelling

at the rate of from ten to twenty miles an hour, and wafting onwards in safety the merchandise of the globe. As they approach the continents, their courses become interrupted in consequence of the unequal heating of the land and water surfaces; hence within these coast-areas they assume the character of periodical, rather than of constant currents, being deflected at certain seasons from their normal directions.

215. As the north-east and south-east trades approach the equator of temperature, their currents begin to fail, and this effect, augmented by the upward tendency of the highly heated air of that region, produces a zone or belt of calms, which fluctuates a few degrees north and south, according to the seasons. If the approaching trades be equally reduced in any locality, a dead calm will be the result; but as this is seldom the case, the zone is also characterised by short gusty winds, which vary in force and direction according as either main current prevails. This zone of calms and variables, as it is termed, is further characterised by the frequency and intensity of its thunder-storms-the crossing and collision of currents of unequal densities being favourable to the manifestation of electrical phenomena. In the Atlantic, the region of calms (see Sketch-Map) ranges in August between 3° and 12° N. lat.; but in February it extends from 1° 15′ to 6° N. lat. A portion of this zone near the Cape Verde Isles is known to sailors as the "rainy sea ;" and is described as a region doomed to continual calms, broken up only by terrific storms of thunder and lightning, accompanied by torrents of rain. A suffocating heat prevails, and the torpid atmosphere is disturbed at intervals by short and sudden gusts, of little extent and power, which blow from every quarter of the heavens in the space of an hour-each dying away ere it is succeeded by another. In these latitudes vessels are sometimes detained for weeks." In the Pacific, the region of calms is comprised within the 2d degree of north and south latitude, near Cape Francis and the Galapagos Islands—a narrow belt separating the two trades, and characterised by the same phenomena as the calm zone of the Atlantic.

216. Besides the trade-winds, which are the most persistent of all aërial currents, there are certain winds in the higher latitudes that also blow with considerable continuity. The heated air which ascends from the tropics, and flows off towards either pole, gradually descends as it proceeds; but as it passes from latitudes having a high to others having a less rotatory velocity, it is deflected from its original course, and assumes a south-easterly direction in the northern hemisphere, and a north-easterly in the southern. In this way the prevailing winds in the higher latitudes