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tic, 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 I towards 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 entirely dry land, or entirely water, and the sun were contin- i nally in the plane of the equator, we should expect to find the directon 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 por- i tion 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. Accord-! ingly, 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 sur-' face of dryland 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, near-: ly 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 Hindustan; 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 «n Arabic word, mausitn, 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. v The reason of this is . that the districts towards 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 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 j 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 Hindustan 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 ah' here is extremely dry, and a compensation, similar to that which is observed in Hindustan, can not take placa It is towards 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 rarefac-! tion 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 center 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 •onsidered 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 towards 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 bo 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, 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, Southeasterly 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 tho 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 Bo 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, bj- 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 tp 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 jt 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 j zenith, and in this region the heaviest rainfall on the globe is observed. The wettest place in the world, Cherrapoonjee, is situated in the Cossya Hills, about , 250 miles North-east of Calcutta, just. outside the ton-id zone. There the rain- \ fall is upwards of £00 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 drill over one particular spot
To return to the wind: When the 1 air rises at the equatorial edge of the trade-wind zone, it flows away above the i lower trade-wind current The existj ence of an upper current in the tropics ! is well known. Volcanic ashes, which have fallen in several of the West India 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 Kings'ton, 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 bome to the Eastward by an upper current counter to the direction of the easterly winds which were blowing at the time at the sea-level.
Captain Hanry supposes that when the air rises, at either side of the equator, it crosses over into the opposite hemisphere, so that there is a constant interchange of air going on between the Northern and Southern hemispheres. This he has hardly sufficiently proved, and his views are not generally accepted. One of the arguments on which he lays great stress in support of his theoiy is, that on certain occasions dust has fallen in various parts of Western Europe, and that in it there have been discovered microscopical animals similar to those which are found in South America. This appears to be scarcely an incontrovertible proof; as Admiral Fitzroy observes: "Certainly such insects tnay be found in Brazil, but does it follow that they are not also in Africa under nearly the same parallels V
This counter-current, or "anti-trade," as Sir J. Herschel has called it, is at a high level in the atmosphere between the tropics, far above the top of the highest mountains; but at the exterior edge of the trade-wind zone, it descends to the surface of the ground. The Canary Islands are situated close to this edge, and accordingly we find that there is always a westerly wind at the summit of the Peak of Teneriffe, while the wind at tho sea-level, in the same island, is easterly throughout the summer months. Professor Piazzi Smith, who lived for some time on the top of that mountain making astronomical observations, has recorded some very interesting details of the con-; flicts between the two currents, which he was able to observe accurately from his elevated position. In winter the tradewiiid zone is situated to the south of ite' summer position in latitude, and at this; season the South-west wind is felt at the sea-level in the Canary Islands. Similar facts to these have been observed in other j localities where there are high mountains I situated on the edge of the trade-wind zone, as, for instance, Mouna Loa in the i Sandwich Islands. • There can therefore j be no doubt that the warm moist West wind which is felt Bo generally in the temperate zones, is really the air returning j to the Poles from the equator, which has Bow assumed a South-west direction on ite return journey, owing to conditions the reverse of those which imparted to it i a North-east motion on its way towards the equator. This, then, is our Southwest wind, which is so prevalent in the North Atlantic Ocean that the voyage from Europe to America is not unl'requently called the up-hill trip, in contradistinction to the down-hill passage home. These are the "brave West winds" of Maury, whose refreshing action on the soil he never tires of recapitulating.
The South-west monsoons of Hindus- | tan, which blow from May to October, and the North-west monsoons of the Java seas, which are felt between November and April, owe their westerly motion to a cause similar to that of the anti-trades which we have just described. To take the case of the monsoons of Hindustan:! we have seen above how the rarefaction of the air in Central Asia attracts the South-east trade-wind of the Southern hemisphere across the equator. This air, when it moves from the equator. into higher latitudes, brings with it the rate of motion, to the eastward, of the equatorial regions which it has lately left, and is felt as a South-west wind. Accordingly, the directions of the monsoons are thus accounted for. In the winter months the true North-east trade-wind is felt in Hindustan; while in the summer months its place is taken by the South-east trade [ of the Southern hemisphere, making its appearance as the South-west monsoon. In Java, conditions exactly converse to these are in operation, and the winds arc
South-east from April to November, and North-west during the rest of the year.
The change of one monsoon to the other is always accompanied by rough weather, called in some places the "breaking-out" of the moonsoon; just as with us the equinox, or change of the season from summer to winter, and vice vei-mi, is marked by "Windy weather," or "Equinoctial gales."
The question may, however, well be asked, why there are no moonsoons in the Atlantic Ocean 1
In the first place, the amount of rarefaction which the air in Africa and in Brazil undergoes, in the respective hot seasons of those regions, is far less considerable than that which is observed in Asia and Australia at the corresponding seasons.
Secondly, in the case of the Atlantic Ocean, the two districts towards which the air is attracted are situated within the torrid zone, while in the Indian Ocean they are quite outside the tropics, and in the temperate zones. Accordingly, even if the suction of the air across the equator did take place to the same extent in the former case as in the latter, the extreme contrast in direction between the two monsoons would not be perceptible to the same extent, owing to the fact that the same amount of westing could not be imparted to the wind, because it had not to travel into such high latitudes on either side of the equator. A tendency to the production of the phenomenon of the monsoons is observable along the coast of Guinea, where winds from the South and South-west are very generally felt. These winds are not really the South-east trade-wind, which has been attracted across the line into the Northern hemisphere. They ought rather to be considered as of the same nature as the land and sea breezes before referred to, since we find it to be very generally the case, that in warm climates the ordinary windcurrents undergo a deflection to a greater or less extent along a coast-line such as that of Guinea, Brazil, or the North of Australia.
Our readers may perhaps ask why it is, that when we allege that the whole of the winds of the globe owe their origin to a regular circulation of the air from the Polar regions to the equator, and back again, we do not find more definite traces of such a circulation in the winds of our own latitudes? The answer to this is, that the traces of this circulation arc easily discoverable if we only know how to look for them. In the Mediterranean Sea, situated near the.Northern edge of the trade-wind zone, the contrast between the equatorial and Polar currents of air is very decidedly marked. The two conflicting winds are known under various names indifferent parts of the district. The polar current, on its way to join the trade-wind, is termed the "Tramontane," in other parts the "Bora," the "Maestnil," <fcc.; while the return trade-wind bringing rain is well known under the name of the "Sirocco." In Switzerland the same wind is called the "Fiihn," and is a warm wind, which causes the ice and snow to inell rapidly, and constantly brings with it heavy rain.
lu these latitudes, the contrast is not so very striking, but even here every one knows that the only winds which last for more than a day or two at a time are the North-east and the South-west winds, the former of which is dry and cold, the lat-ter moist and warm. The difference between these winds is much more noticeable in winter than in summer, inasmuch as in the latter season Russia and the Northern part of Asia enjoy, relatively to the British islands, a much higher temperature than is the case in winter; so that the air which moves from those regions during the summer mouths does not come to us from a climate which is colder than our own, but from one which is warmer.
So far, then, we have attempted to trace the ordinary wind-currents, but as yet there are very many questions connected therewith which are not quite sufficiently explained. To mention one of these, we hear from many observers on the late Arctic expeditions, that the most marked characteristic of the winds in the neighborhood of Baffin's Bay, is the great predominance of North-westerly winds. It is not as yet decided, nor can it ever be satisfactorily decided, how far to the northward and westward this phenomenon is noticeable. The question then is, Whence doea this North-west wind come?
As to the causes of the sadden changes
I of wind, and of storms, they are as yet : shrouded in mystery, and we can not ! have much expectation that in our lifetime at least much will be done to unravel the web. Meteorology is a very young science—if it deserves the title of science at all—and until observations for ! a long series of years shall have been ; made at many stations, we shall not be 'in the possession of trustworthy facts on which to ground our reasoning. It is ! merely shoving the difficulty a step farther off to assign these irregular variations to atmospheric waves. It will be : time enough to reason accurately about the weather and its changes, when we ascertain what these atmospheric waves are, and what causes them. Until the "astro-meteorologists" will tell us the the principles on which their calculations are based, we must decline to receive their predictions as worthy of any credence whatever.
From tho Eclectic.
The work before us, which lias given to us the intention to review some of the chief features in the genius and work of the third great Italian, is every way worthy of its subject. It supplies a want we have long felt for such a life, and it is in itself written with considerable fervor and eloquence, witha delightful interest,sustained from the commencement to the close. It is an important chapter in the history of art, from the pen and matured judgment of a very able and sympathetic critic, and while it may scarce the less be called a history of Michael Angelo and his times, yet no reader will desire the work to be smaller than it is; for the great artist, and great pontiff, who appear in episode, are so closely related to the development of art in the age, and bring out so much more distinctly the chief characteristics of the great central man, that no more has been attempted or done than the proper province of the judicious biographer seemed to render necessary. In our own country, the name of Michael Angelo is not so
* Life of MicJtael Anyelo. By Herman Grimm. Translated, with the author's sanction, by Fanny Elizabeth Bunnett. With photographic portrait. •2 vols. Smith, Elder, and Co.