If you light a fire in a room, and afterward stop up every chink by which air can gain access to the fire, except the chimney, the fire will go out in a short time. Again, if a lamp is burning on the table, and you stop up the chimney at the top, the lamp will go out at once. The reason of this is that the flame, in each case, attracts the air, and if either the supply of air is cut off below, or its escape above is checked, the flame cannot go on burning. This explanation, however, does not bear to be pushed too far. The reason that the fire goes out if the supply of air is cut off is, that the flame, so to speak, feeds on air; while the sun cannot be said, in any sense, to be dependent on the earth's atmosphere for the fuel for his fire. We have chosen the illustration of the flame, because the facts are so well known. If, instead of a lamp in the middle of a room, we were to hang up a large mass of iron, heated, we should find that currents of air set in from all sides, rose up above it, and spread out when they reached the ceiling, descending again along the walls. The existence of these currents may be easily proved by sprinkling a handful of fine chaff about in the room. What is the reason of the circulation thus produced? The iron, unless it be extremely hot, as it is when melted by Mr. Bessemer's process, does not require the air in order to keep up its heat; and, in fact, the constant supply of fresh air cools it, as the metal gives away its own heat to the air as fast as the particles of the latter come in contact with it. Why, then, do the currents arise? Because the air, when heated, expands or gets lighter, and rises, leaving an empty space, or vacuum, where it was before. Then the surrounding cold air, being elastic, forces itself into the open space, and gets heated in its turn.
From this we can see that there will be a constant tendency in the air to flow toward that point on the earth's surface where the temperature is highest—or, all other things being equal, to that point where the sun may be at that moment in the zenith. Accordingly, if the earth's surface were either [{209}] entirely dry land, or entirely water, and the sun were continually in the plane of the equator, we should expect to find the direction of the great wind-currents permanent and unchanged throughout the year. The true state of the case is, however, that these conditions are very far from being fulfilled. Every one knows that the sun is not always immediately over the equator, but that he is at the tropic of Cancer in June, and at the tropic of Capricorn in December, passing the equator twice every year at the equinoxes. Here, then, we have one cause which disturbs the regular flow of the wind-currents. The effect of this is materially increased by the extremely arbitrary way in which the dry land has been distributed over the globe. The northern hemisphere contains the whole of Europe, Asia, and North America, the greater part of Africa, and a portion of South America; while in the southern hemisphere we only find the remaining portions of the two last-named continents, with Australia and some of the large islands in its vicinity. Accordingly, during our summer there is a much greater area of dry land exposed to the nearly vertical rays of the sun than is the case during our winter.
Let us see for a moment how this cause acts in modifying the direction of the wind-currents. We shall find it easier to make this intelligible if we take an illustration from observed facts. It takes about five times as much heat to raise a ton weight of water through a certain range of temperature, as it does to produce the same effect in the case of a ton of rock. Again, the tendency of a surface of dry land to give out heat, and consequently to warm the air above it, and cause it to rise, is very much greater than that of a surface of water of equal area. Hence we can at once see the cause of the local winds which are felt every day in calm weather in islands situated in hot climates. During the day the island becomes very hot, and thus what the French call a courant ascendant is set in operation. The air above the land gets hot and rises, while the colder air which is on the sea all round it flows in to fill its place, and is felt as a cool sea-breeze. During the night these conditions are exactly reversed: the land can no longer get any heat from the sun, as he has set, while it is still nearly as liberal in parting with its acquired heat as it was before. Accordingly, it soon becomes cooler than the sea in its neighborhood; and the air, instead of rising up over it, sinks down upon it, and flows out to sea, producing a land-wind.
These conditions are, apparently, nearly exactly fulfilled in the region of the monsoons, with the exception that the change of wind takes place at intervals of six months, and not every twelve hours. In this district—which extends over the southern portion of Asia and the Indian ocean—the wind for half the year blows from one point, and for the other half from that which is directly opposite. The winds are north-east and south-west in Hindostan; and in Java, at the other side of the equator, they are south-east and north-west. The cause of the winds—monsoons they are called, from an Arabic word, mausim, meaning season—is not quite so easily explained as that of the ordinary land and sea breezes to which we have just referred. Their origin is to be sought for in the temperate zone, and not between the tropics. The reason of this is that the districts toward which the air is sucked in are not those which are absolutely hottest, but those where the rarefaction of the air is greatest. When the air becomes lighter, it is said to be rarefied, and this rarefaction ought apparently to be greatest where the temperature is highest. This would be the case if the air were the only constituent of our atmosphere. There is, however, a very important disturbing agent to be taken into consideration, viz., aqueous vapor. There is always, when it is not actually raining, a quantity of water rising from the surface of [{210}] the sea and from every exposed water-surface, and mingling with the air. This water is perfectly invisible: as it is in the form of vapor, it is true steam, and its presence only becomes visible when it is condensed so as to form a cloud. The hotter the air is, the more of this aqueous vapor is it able to hold in the invisible condition.
We shall naturally expect to find a greater amount of this steam in the air at places situated near the coast, than at those in the interior of continents, and this is actually the case. The amount of rarefaction which the dry air on the sea-coast of Hindostan undergoes in summer, is partially compensated for by the increased tension of the aqueous vapor, whose presence in the air is due to the action of the sun's heat on the surface of the Indian ocean. In the interior of Asia there is no great body of water to be found, and the winds from the south lose most of the moisture which they contain in passing over the Himalayas. Accordingly the air is extremely dry, and a compensation, similar to that which is observed in Hindostan, cannot take place. It is toward this district that the wind is sucked in, and the attraction is sufficient to draw a portion of the south-east trade-wind across the line into the northern hemisphere. In our winter the region where the rarefaction is greatest is the continent of Australia; and accordingly, in its turn, it sucks the north-east trade-wind of the northern hemisphere across the equator. Thus we see that in the region which extends from the coast of Australia to the centre of Asia we have monsoons, or winds which change regularly every six months. As to the directions of the different monsoons, we shall discuss them when we have disposed of the trade-winds—which ought by rights, as Professor Dove observes, rather to be considered as an imperfectly developed monsoon, than the latter to be held as a modification of the former.
The origin of the trade-winds is to be sought for, as before, in the heating power of the sun, and their direction is a result of the figure of the earth, and of its motion on its axis. When the air at the equator rises, that in higher latitudes on either side flows in, and would be felt as a north wind or as a south wind respectively, if the earth's motion on its axis did not affect it. The figure of the earth is pretty nearly that of a sphere, and, as it revolves round its axis, it is evident that those points on its surface which are situated at the greatest distance from the axis, will have to travel over a greater distance in the same time than those which are near it. Thus, for instance, London, which is nearly under the parallel of 50, has only to travel about three-fifths of the distance which a place like Quito, situated under the equator, has to travel in the same time. A person situated in London is carried, imperceptibly to himself, by the motion of the earth, through 15,000 miles toward the eastward in the twenty-four hours; while another at Quito is carried through 25,000 miles in the same time. Accordingly, if the Londoner, preserving his own rate of motion, were suddenly transferred to Quito, he would be left 10,000 miles behind the other in the course of the twenty-four hours, or would appear to be moving in the opposite direction, from east to west, at the rate of about 400 miles an hour. The case would be just as if a person were to be thrown into a railway carriage which was moving at full speed; he would appear to his fellow-passengers to be moving in the opposite direction to them, while in reality the motion of progression was in the train, not in the person who was thrown into it. The air is transferred from high to low latitudes, but this change is gradual, and the earth, accordingly, by means of the force of friction, is able to retard its relative velocity before it reaches the tropics so that its actual velocity, though still considerable, is far below 400 miles an hour.
This wind comes from high latitudes and becomes more and more easterly [{211}] reaching us as a nearly true north-east wind; and as it gets into lower latitudes becoming more and more nearly east, and forming a belt of north-east wind all round the earth on the northern side of the equator. In the southern hemisphere, there is a similar belt of permanent winds, which are, of course, south-easterly instead of north-easterly. These belts are not always at equal distances at each side of the equator, as their position is dependent on the situation of the zone of maximum temperature for the time being. When we reach the actual district where the air rises, we find the easterly direction of the wind no longer so remarkable, as has been noticed by Basil Hall and others. The reason is, that by the time that the air reaches the district where it rises, it has obtained by means of its friction with the earth's surface a rate of motion round the earth's axis nearly equal to that of the earth's surface itself.
The trade-wind zones, called, by the Spaniards, the "Ladies' Sea"—El Golfo de las Damas—because navigation on a sea where the wind never changed was so easy, shift their position according to the apparent motion of the sun in the ecliptic. In the Atlantic the north-east trade begins in summer in the latitude of the Azores; in winter it commences to the south of the Canaries.
In the actual trade-wind zones rain very seldom falls, any more than it does in these countries when the east wind has well set in. The reason of this is, that the air on its passage from high to low latitudes is continually becoming warmer and warmer. According as its temperature rises, its power of dissolving (so to speak) water increases also, and so it is constantly increasing its burden of water until it reaches the end of its journey, where it rises into the higher regions of the atmosphere, and there is suddenly cooled. The chilling process condenses, to a great extent, the aqueous vapor contained in the trade-wind air, and causes it to fall in constant discharges of heavy rain. Throughout the tropics the rainy season coincides with that period at which the sun is in the zenith, and in this region the heaviest rain-fall on the globe is observed. The wettest place in the world, Cherrapoonjee, is situated in the Cossya hills, about 250 miles northeast of Calcutta, just outside the torrid zone. There the ram-fall is upward of 600 inches in the year, or twenty times as much as it is on the west coasts of Scotland and Ireland. However, in such extreme cases as this, there are other circumstances to be taken into consideration, such as the position of the locality as regards mountain chains, which may cause the clouds to drift over one particular spot.
To return to the wind: When the air rises at the equatorial edge of the trade-wind zone, it flows away above the lower trade-wind current. The existence of an upper current in the tropics is well known. Volcanic ashes, which have fallen in several of the West Indian islands on several occasions, have been traced to volcanoes which lay to the westward of the locality where the ashes fell, at a time when there was no west wind blowing at the sea-level. To take a recent instance: ashes fell at Kingston, Jamaica, in the year 1835, and it is satisfactorily proved that they had been ejected from the volcano of Coseguina, on the Pacific shore of Central America, and must consequently have been borne to the eastward by an upward current counter to the direction of the easterly winds which were blowing at the time at the sea-level.