south-west wind from April to October.* From Berneo, along the coast of Malacca, and as far as China, this monsoon in our summer blows nearly from the south, and in winter from the north by east. Near the coast of Africa, between Mozambique and Cape Guardafui, the winds are irregular during the whole year, owing to the different monsoons which surround that particular place. Monsoons are likewise regular in the Red Sea; between April and October they blow from the north-west, and during the other months from the south-east, keeping constantly parallel to the Arabian coast.† On the coast of Brazil, between Cape St. Augustine and the island of St. Catharine, from September to April the wind blows from the east or north-east; and from April to September it blows from the south-west; so that monsoons are not altogether confined to the Indian Ocean. On the coast of Africa, from Cape Bajador, opposite the Canary Islands, to Cape Verd, the winds are generally north-west; and from hence to the island of St. Thomas, near the equator, they blow almost perpendicular to the shore. In all maritime countries of considerable extent, any between the tropics, the wind blows during a certain number of hours from the sea, and during a certain number from the land; these winds are called sea and land breezes. During the day, the air above the land is hotter and more rare than that above the sea; the sea air, therefore, flows in upon the land and supplies the place of the rarefied air, which is made to float higher in the atmosphere; as the sun descends, the rarefaction of the land air is diminished, and an equilibrium is restored. As the night approaches, the denser air of the hills and mountains (for where there are no hills, there are no sea and land breezes) falls down * The student will find these winds represented on Adams' globes and some others, by arrows, having the barbed points fying in the direction of the wind, as if shot from a bow; and, where the winds are variable, these arrows seem to be flying in all directions. † Bruce's Travels, vol. i. chap 4. upon the plains, and pressing upon the air of the sea, which has now become comparatively lighter than the land air, causes the land breeze. The Cape of Good Hope is famous for its tempests, and the singular cloud which produces them: this cloud appears at first only like a small round spot in the sky, called by the sailors the Ox's Eye, and which probably appears so minute from its exceedingly great height. In Natolia, a small cloud is often seen, resembling that at the Cape of Good Hope, and from this cloud a terrible wind issues, which produces similar effects. In the sea between Africa and America, especially at the equator and in the neighbouring parts, tempests of this kind very often arise, and are generally announced by small black clouds. The first blast which proceeds from these clouds is furious, and would sink ships in the open sea, if the sailors did not take the precaution to furl their sails. These tempests seem to arise from a sudden rarefaction of the air, which produces a kind of vacuum, and the cold dense air rushing in to supply the place. Hurricanes, which arise from similar causes, have a whirling motion which nothing can resist. A calm generally precedes these horrible tempests, and the sea then appears like a piece of glass; but, in an instant, the fury of the winds raises the waves to an enormous height. When from a sudden rarefaction, or any other cause, contrary currents of air meet in the same point, a whirlwind is produced. The force of the wind upon a square foot of surface is nearly as the square of the velocity; that is, if on a square board of one foot in surface, exposed to a wind, there be a pressure of one pound, another wind, with double the velocity, will press the board with a force of four pounds, &c. The following table, extracted from the Philosophical Transactions, shews the velocity and pressure of the winds, according to their different appellations. * This wind seems to be described by St. Paul, in the 27th chap. ter of the Acts, by the name of Euroclydon. of Vapours, Fogs and Mists, Clouds, Dew, and HoarFrost, Rain, Snow and Hail, Thunder and Lightning, Falling Stars, Ignis Fatuus, Aurora Borealis, and the Rainbow. 1. VAPOURS are composed of aqueous or watery par ticles, separated from the surface of the water, or moist earth, by the action of the sun's heat; whereby they are so rarefied, attenuated, and separated from each other, as to become specifically lighter than the air; and, consequently, they rise and float in the atmosphere. 2. FOGS AND MISTS. Fogs are a collection of vapours which chiefly rise from fenny, moist places, and become more visible as the light of the day decreases. If these vapours be not dispersed, but unite with those that rise from water, as rivers, lakes, &c. so as to fill the air in general, they are called mists. 3. CLOUDS are generally supposed to consist of vapours exhaled from the sea and land.* These vapours ascend till they are of the same specific gravity as the surrounding air; there they coalesce; and, by their union, become more dense and weighty. The more thin and rare the clouds are, the higher they soar; but their height seldom, if ever, exceeds two miles. The generality of clouds are suspended at the height of about a mile; sometimes, when the clouds are highly electrified, their height is not above seven or eight hundred yards. The wonderful variety in the colours of the clouds, is owing to their particular situation to the sun, and the different reflections of his light. The various figures of the clouds probably proceed from their loose and voluble texture, revolving in any form according to the different force of the winds, or from the electricity contained in them. 4. DEW. When the earth has been heated in the daytime by the sun, it will retain that heat for some time after the sun has set. The air being a less dense, or less compact substance, will retain the heat for a less time so that in the evening the surface of the earth will be warmer than the air about it; and, consequently, the vapours will continue to rise from the earth; but, as these vapours come immediately into a cool air, they will only rise to a small height; as the rarefied air in : * Dr. Thomson, in vol iii. of his Chemistry, page 321, &c. says, it is remarkable that, though the greatest quantity of vapour exists in the lower strata of the atmosphere, clouds never begin to form there, but always at some considerable height. The heat of the clouds is sometimes greater than that of the surrounding air. The formation of clouds and rain is neither owing to the saturation of the atmosphere, to the diminution of heat, nor to the mixture of airs of different temperatures. Evaporation often goes on for a month to gether in hot weather, especially in the torrid zone, without any rain. The water can neither remain in the atmosphere, nor pass through it, in a state of vapour: What then becomes of the vapour after it enters the atmosphere? What makes it lay aside the new form which it must have assumed, and return again to its state of vapour, and fall down in rain? Till these questions are experimentally answered, Dr. Thomson concludes that the formation of clouds and rain cannot be accurately accounted for. which they began to rise becomes condensed, the small particles of vapours will be brought nearer together. When many of these particles are united, they form dew; and, if this dew freeze, it will produce hoar-frost. 5. RAIN. When the weight of the air is diminished, its density will likewise be diminished, and consequently the vapours that float in it will be less resisted, and begin to fall; and, as they strike one upon another in falling, they will unite and form small drops. But when the small drops of which a cloud consisted, are united into such large drops that no part of the atmosphere is sufficiently dense to produce a resistance able to support them, they will then fall to the earth, and constitute what we call rain. If these drops be formed in the higher regions of the atmosphere, many of them will be united before they come to the ground, and the drops of rain will be very large. The drops of rain increase so much both in bulk and motion, during the descent, that a bowl placed on the ground would receive, in a shower of rain, almost twice the quantity of water that a similar bowl would receive on a neighbouring high steeple. The mean annual quantity of rain is greatest at the equator, and decreases gradually as we approach the poles. Thus, at On the contrary, the number of rainy days is smallest at the equator, and increases in proportion to the distance from it. The number of rainy days is often greater in winter than in summer; but the quantity of * Dr. Rutherford's Natural Philosophy, vol. ii. chap. 10 Signior Beccaria, whose observations on the general state of electricity in the atmosphere have been very accurate and extensive, ascribes the cause of rain, hail, snow, &c. &c. to the effect of a moderate electricity in the atmosphere. Mr. Adam Walker's Familiar Philosophy, Lect. v. page 215. |