sure, as doubtless our readers well know, and, as in all fluids, whether gaseous or liquid, the heated mass, becoming lighter by the expansion consequent on heating, immediately ascends. This method of acquiring heat throughout is called convection, since in it the heated particles are conveyed from below upwards, and belongs to all fluid bodies; for they being bad conductors of heat, would require a great time to become heated throughout, as solid bodies do, by conduction, in which the particles of matter do not change their relative position. Suppose, now, that the sun shines on a large tract of land-for instance, the The lower portion of the atmosphere is Desert of Sahara; a vast volume of air much the denser, owing to the pressure being thus heated, begins immediately to of the upper portion; so much so, indeed, ascend; a vacuum is consequently left; that the half of the entire atmosphere is and air from all sides immediately rushes found below the hight of three and a half in to fill this vacuum, thus producing wind. miles, and at this point the barometer This is beautifully exemplified in the case stands at fifteen inches instead of thirty. of islands in the midst of the sea, as, for In air of such rarity-and even greater instance, the West-Indian Islands, where elevations have been reached, as by M. we find the wind every day blowing from Gay-Lussac in a balloon-respiration be-all sides towards the island, thus constitutcomes exceedingly laborious, and blood often starts from the mouth and eyes; birds are unable to fly, and if let go, they either return to the balloon, or, after ineffectual fluttering, descend with great rapidity in spiral lines like birds of prey; or from still greater elevations, where of course, their wings are still more useless, they drop directly downwards for a great distance. This elevation of three and a half miles may be called the line of repose of the atmosphere, being a calm space intervening between two regions liable to storms, and having currents of air flowing in different directions on each side of it, so that if air move from north to south below, its place is filled again by a counter-current moving from south to north above this line, so that a perpetual interchange is kept up. The great agent in causing currents of air, or winds, on the surface of the globe, is the heat of the sun. The air is not heated directly by the sun, or at least only in a very slight degree, for no transparent body will become hot by rays simply passing through it. The glass in a window remains quite cool though transmitting the fiercest rays of the sun; but if blackened on the inside, so that the rays can not pass through, it immediately becomes hot; similarly, the air becomes heated principally by radiation from the ground, ing what is known as the sea-breeze of the day. The cause of the wind in such cases being directed towards the island, is, that the heating, and consequent ascent of the air, takes place much more over land than over water, and this for two reasons-first, because a given amount of heat falling on a certain extent of surface of land, raises the temperature of that surface through no less than five times the number of degrees that it would raise a similar extent of surface of water; and secondly, because a large amount of the heat which the water would gain, is immediately rendered latent by the conversion of a portion of the water into vapor, and thus does not contribute to raise the temperature of the water. But if a surface of land, receiving a certain amount of heat, is raised in temperature through five times as many degrees as a surface of water of the same extent would be, it follows, as a matter of course, that in losing a certain amount of heat, it falls in temperature through five times as many degrees as water does; there is therefore, after sunset, a much greater condensation of the air over the land than there is over the water; and the former, increasing in density very much in consequence, presses outwards towards the sea with considerable force; the column of dense air over the land being lowered by thus flowing out over the sea, has its hight repaired in the upper regions by air flow- | the sun is at his extreme northern limit, ing in from the upper atmosphere over the sea to fill the vacuum; this is condensed, sinks down, and flows out to sea in its turn; and thus we have the wind blowing continuously in a direction exactly the reverse of its direction during the day, and forming the evening land-breeze, so well known in the West-Indian and other tropical islands. Let us now consider the trade-winds, which blow constantly from north-east and south-east towards the equator throughout the year. They are caused by the heat of the sun rarefying the air within the tropics; and on the ascent of this rarefied air to the higher regions, air, of course, flows towards the equator from north and south, to fill up the vacuum. Now, the speed with which any point on the earth's surface is carried along, in the daily rotation of the earth, varies according to the distance of the point from the equator, becoming less and less as it approaches the poles, the air, of course, over such a point, has no greater rotatory speed than the point itself, and does not readily acquire the greater speed at which the parts of the earth nearer the equator are traveling; therefore, as it passes over these parts, it is actually left behind by them; and since the earth rotates on its axis from west to east, these winds acquire an apparent motion from east to west-the motion being referred by the observer to them, instead of to himself, just as in a steam-boat passing along a river's bank. It is evident that when these two motions -namely, one towards the equator from the poles, and the other from east to west -when they, we say, are combined, the result will be that the direction of these winds will be from north-east in the northern hemisphere, and from south-east in the southern. As might be supposed, from the above account, these winds die away as they approach the equator, for there the upward motion takes the place of the horizontal, and accordingly, for some degrees at each side of the equator, there is a belt of calm air, which it is very difficult for ships to cross under canvas alone; they often require several days, or even weeks, to reach the region of winds on the other side; and as this belt must be twice crossed on the route round the Cape to India, we see where the principal advantage of steamers over sailing vessels is gained on that voyage. Sometimes, when in the month of June, this belt is so far north that the equator bounds its southern edge, and the south-east trade wind has even been found on the north of the equator; and similarly, during the month of December, the north-east trade-wind has been known to cross the line to the southern hemisphere. The monsoons of India and China next fall under our notice. These, as our readers must be aware, are strong winds blowing, in these countries from northeast during the winter months of the year, and from south-west during the summer. Of these, the south-west monsoon is by far the most violent, and also by far the most important, since it is this wind which brings to the parched and thirsty soil the copious rains which so materially enhance the climate of India, and the productiveness of its soil, although, perhaps, they prove a little disagreeable, in some respects, to European residents. It will easily be seen that the north-east monsoon is nothing but the north-east trade-wind, which has been prevented from blowing, during six months of the year, by the causes which produce the south-west monsoon. These causes are the same in principle as those which produce the trade-winds, but difference of circumstances alter the result. During the summer months, the sun shines in full strength on the continent of India, and since, as we explained above, land radiates heat much faster, and to a greater degree, than water, a great rarefaction, and consequent ascent of air, takes place over the whole continent; air rushes violently in from the Indian Ocean, which lies to the southwards, to supply the deficiency thus caused; but as the rotatory motion of the earth must be slower in all parts of India than at the equator, the earth is left behind by the wind, which arrives there traveling at the higher rate of speed which it has acquired at the equator. Combining these two motions as before, we get a south-west direction for the summer monsoon. The evaporation of water in the Indian ocean, is, of course, very great, and therefore the south-west monsoon arrives on the coast of India absolutely saturated with moisture. As the wind thus saturated rises to the higher regions, which it does on being rarefied after arriving over the land, it of course becomes much cooled, and therefore unable to retain its former amount of water in solution; the excess is, of course, precipitated; and hence the violent rains which accompany the south-west monsoon, and cool the country, so as to render it habitable during the summer months. As we said above, however, the under-currents of the atmosphere do not ascend higher than about three and a half miles; and, consequently, the south-west monsoon must be checked on reaching the Himalaya in the north-east, since their hight varies from four to five and a half miles; and, accordingly, it turns its course to a direction parallel to this great mountain-chain, and rushes up the country as a south-easterly wind, thus assuming along the north-east boundary of India the appearance of the southern trade-wind in latitude 25° to 30° north. After depositing a vast quantity of water on its way up the country, in consequence of the great fall in temperature caused by these mountains, its course is again checked at the north-west boundary of the Punjab by another bend in the chain; and turning along the mountains so as to become a north-east wind, it is neutralized by meeting with the ordinary south-west monsoon blowing from the Arabian Sea up the course of the Indus. It will be seen that a peculiar combination of circumstances is necessary in order to produce a south-west monsoon in any part of the world. In the first place, there must be land, not on the equator, but to the northwards of it, but yet sufficiently near the tropics to produce great rarefaction of the air on the sun's arriving at or near the zenith during the summer months; secondly, there must be water between this land and the equator, since otherwise there would be more rarefaction of air there, and no current could possibly set to the northwards, but the north-east trade-wind would continue blowing as before, though it would cease further from the equator than during the winter months. There is, therefore, no south-west monsoon, nor interruption to the north-east trade wind in the Atlantic, since here there is a uniform surface of water; nor in Africa, since here there is a uniform surface of land; but along the north sea board of the Mediterranean the same conditions are found; and accordingly, here, again, we find winds blowing correspond ing to the monsoons, though much weaker, for two reasons first, because the heat of the sun is much less, since it shines on land much further from the equator than the land along the north seaboard of the Indian Ocean; and secondly, because the surface of land which this heat has to act on here is much smaller; still, however, the winds are quite recognizable, and well known to mariners in those parts. We find them mentioned in the writings of antiquity; for instance, Demosthenes cautions the Athenians against delaying to send succors northwards to Olynthus, lest the Etesians should set in, and effectually prevent them. These Etesians correspond to the north-east monsoon of India, or the ordinary trade-wind of other parts, and in Greece, as in India, only blow during the six months that the sun is in the southern hemisphere, because checked during the other six months by the wind from the south-west. A south-west wind ought also to be found along the extreme south coast of the United States, in Louisiana, Alabama, Georgia, and Florida; and during our winter months we might expect to find a north-west monsoon blowing inland on the north coast of Australia. From constant and periodic winds, such as we have been considering, we must pass to those which, though not falling under either of these heads, are yet the prevailing winds in certain districts. Such are the south-west winds of Western Europe and the western parts of NorthAmerica on the seaboard of the Pacific, which become more westerly as we approach the northern parts of those continents; and again, such are the north-west winds of the Pacific coast of South-America, which become more westerly as we go further south. These are the most important winds of temperate zones; and it is to their influence that the western shores of great continents owe their decided superiority over the eastern, as regards mildness and equability of climate. Let us contrast, for instance, the climate of these kingdoms with that of Labrador and Newfoundland in about the same latitude on the east coast of America, and the latter, again, with British-Columbia and Vancouver's Island, on the west coast of the same continent. In both cases, we find that the average temperature of the whole year is 18° Fahrenheit higher on the western than on the eastern side; and again, we must.recollect that not only is lose their motion from the poles towards the equator, and not their motion from east to west. the average in favor of the western side | time to go round two thirds of the globe by so large a difference, but that the by the Cape Horn route, rather than to climate is also much more equable on the sail for one third in the teeth of the wind western side, the range from summer to on the direct route. Corresponding with winter temperature being much less; for these westerly winds in the lower atmothe heat of the Labrador summer is very sphere in high latitudes, there is a conmuch greater than that of ours, as is the stant current from east to west in the cold of winter more severe. We have an upper atmosphere within the tropics; for advantage of one or two degrees over the it is evident that by ascending in consewestern coast of America in the same lati-quence of rarefaction, the trade-winds only tude as regards average temperature, and this is probably owing to the influence of the Gulf Stream. But yet the climate of British-Columbia is a magnificent one; and when combined with its advantages of commercial situation, and roadsteads for shipping, will render it a colony of the very highest importance, both mercantile and political. It is to these prevailing south-west winds that these climatal advantages of the western sides of great continents are owing. We mentioned above that a current the reverse of the trade-wind must set from the tropics towards the poles in the upper regions of the atmosphere, in order to refill the vacuum caused by the flow of the tradewinds towards the equator. This upper current consists of the heated air which has been drawn up within the tropics, and rising from the ocean as it does over a great part of the globe, is perfectly saturated with moisture. It does not readily lose the whole of the more rapid motion of rotation which it had acquired within the tropics, and hence, by overtaking the earth in higher latitudes it impinges against the west sides of the continent as a south-west wind. It must be remembered, that partly in consequence of condensation, and partly to supply the vacu um caused by the current towards the equator, the upper current will in higher latitudes descend and become terrestrial; and it may also be seen that the further it goes from the equator the more will its west-to-cast motion predominate; so that in high latitudes it becomes almost a due west wind. This is more easily observed in the southern hemisphere than in the northern, since in the former the globe is circumnavigable, and there it is well known that it is almost impossible to round Cape Horn to the westward, and on the other hand, the voyage home from Australia is always made by Cape Horn; not only so, indeed, but if any vessel has to sail from Australia to the Cape of Good Hope, it is found to be a saving of Let us consider the effect of the southwest winds we have spoken of on the climate of the west sides of continents, and we may take British-Columbia as a good instance in point. In the first place, since the wind is blowing from the equator, it conveys a large quantity of heat, in consequence of this alone; but, in the second place, it arrives, as we have said, saturated with aqueous vapor. Now, it is no doubt well known to most of our readers that it takes a vast quantity of heat to convert, say, a pound of water at 212°, or the boiling-point, into a pound of steam at 212°, the heat thus absorbed being quite inappreciable by the thermometer, and being therefore known by the name of latent heat; and the same quantity of heat is thus rendered latent by the formation of vapor at whatever temperature it may be formed; and on the reconversion of this vapor into water, the very same quantity of heat is again evolved, and rendered appreciable by the thermometer. Arriving, then, upon the land, and being condensed and cooled by imparting its heat to it, this air becomes incapable of holding so much aqueous vapor in solution; the excess is of course precipitated as water, and, consequently, gives out the large amount of latent heat which it contained. Passing onwards over the country, the wind encounters the Rocky mountains, and, in consequence of the condensation and cooling it here experiences, gives out its last remains of moisture and warmth, and therefore thenceforth blows over the rest of the continent, as a cold dry wind, communicating neither warmth in winter nor coolness in summer, so that on the east coast we find an extreme climate, in which the cold of winter is most rigorous, and the heat of summer very severe. Passing from west to east over the great continent of the old world, we find the very same phenomena the west coast having a mild equable climate such as is usually enjoyed by islands, and hence called insular; while the east coast has an extreme climate, the range from summer to winter average temperature being very great as, for instance, at Pekin, where the range is as much as 90° Fahrenheit; whereas the range in these kingdoms is only 20 in some parts. Such difference in climate between the east and west sides of a continent is not found within 30° of the equator, since within this limit the cause of the difference is wanting, as the trade-winds blow uniformly towards the equator, at least as far as this latitude. directly upwards, so that here there is perpetual calm; and accordingly, on the approach of one of these hurricanes, those vessels in the North-Atlantic which are sufficiently near always run for Trinidad. It is a curious fact, also, connected with these typhoons, that high land seems to have a peculiar tendency to divert their onward course, and near such land they often re-curve in a peculiar manner, to avoid it; for this reason, the island of Mauritius is much used as a harbor of refuge in the Southern Indian Ocean, as they generally re-curve when passing near it and the Isle of Bourbon. To account for the rotatory motion of these typhoons, various theories have been proposed, the discussion of which would be hardly suitable to these pages; but we may mention that Mr. Redfield, of New-York, considers that this motion, when once begun, from whatever cause it may be, is afterwards kept up by the pressure exercised by the external air on the rotating column, as this external air is drawn in, and in its turn ascends in a spiral manner through the storm, to be in its turn poured out above, for such an ascent of air undoubt Cyclones are a matter of much more importance than other winds, both be cause they are more numerous, and because they follow certain laws, which are capable of investigation, and, indeed, have been largely investigated. It has been computed, from accurate observations, that of the winds which blow in these islands, two thirds come under the head of cyclones. The late terrible storm, so lamentably signalized by the loss of the Royal Charter, was unquestionably a cy-edly takes place in these cyclones, and clone, for, as was no doubt observed by all who read the accounts of it carefully, the wind was from the north-west on the north coast of Wales, while within a very short period of time it was from the southeast on the Kentish coast; and Admiral Elliott's squadron, which stood out to sea from Plymouth on the approach of the storm, actually passed through the center of the hurricane, and after enjoying a brief calm here, was again involved in the storm, but with the wind blowing from exactly the opposite quarter. Dreadful, however, as the effects of such storms are, even in these islands, yet temperate zones enjoy a comparative immunity from them; it is within the tro pics that they assume their most awful and terrific aspect. There, ships are often laid on their beam-ends till their mastheads dip beneath the water; and it is nothing uncommon to see a whole plantation of gigantic sugar-canes cleared in an instant of time, and hurried aloft, roots and all, by the violence of the whirlwind. It has been observed that these cyclones never approach within ten degrees of the equator, the reason of this, of course, being that within these limits the wind moves in but one direction, and that Mr. Redfield considers them to be comparable to chimneys, a comparative vacuum being formed inside by the centrifugal force generated by their rotation. Our readers, must, of course, judge of this theory for themselves, and until this and other such points are settled, we shall content ourselves with merely mentioning a few other well-established and curious facts regarding these cyclones. Whatever be the original cause of the rotation of a cyclone, its direction is invariably in the same hemisphere, being always from right to left in the northern hemisphere, and from left to right, like the hands of a watch, in the southern; and this rotation will sometimes, near the center of the storm, be as rapid as one hundred and twenty miles an hour, though the progressive motion of the storm seldom exceeds fifteen or sixteen, whence it happens that ships sometimes sail into the middle of a storm by merely overtaking it, whereas had they lain to for a time, they would have escaped it. Hence, also, we derive the explanation of the phenomenon termed a ground-swell, or undulatory motion of the surface of the sea, though the wind may at the same time be perfectly quiet, a state of things which generally |