globe, I would inclose its tropical region from thirty degrees north to thirty degrees south, in a glass zone or coating concentric with the globe, and also each of the belts lying between the latitudes of thirty and fifty degrees in like manner, with distinct cases placed respectively in close contact with the tropical glass coating, and divided from it by partitions removable at pleasure; I would fill the tropical case with hot water, and the middle latitude cases, or those embracing the space contained between the latitudes of thirty and fifty degrees in both hemispheres, with cold water; or, which would represent the actual fact still better, a broad ring of heated iron might be fixed round the equator to represent the torrid zone, while the middle or temperate latitudes, both north and south, should be encircled with rings of ice. The water might also be colored in order to render the effects visible. Things being arranged as above described, and the globe being supposed for the present at rest, if the division between the hot and the cold fluids were removed, the cold water would gradually slide along under the hot towards the equator, while the heated water would be carried over the cold towards the poles; and, if nothing else were done, that is to say, if the globe were allowed to remain at rest, a mere circular interchange would take place. The temperate portions of the fluid, on coming into contact with the torrid zone of the globe, and being thereby heated and rendered specifically lighter, would necessarily rise; while the hot portion, on flowing towards the cooling substance in latitudes farther from the equator, would descend to occupy the place of the cold water drawn off to supply the place of the lighter heated water at the equator. A steady current would in this way be produced, running below towards the equator, and at right angles to it, and above towards the poles; this would evidently be the only motion impressed on the fluid as long as the globe stood still.

It is material to remark here, that this motion would be less and less obvious as the currents approached the equator, where the cold fluid would gradually become heated, and have a tendency to rise as well as to flow along, so that their course would be checked, till at length, at the equator, the opposite currents would meet and produce a calm.

While things are supposed to be in this situation, let the globe be put into rapid motion from west to east, we shall say, for the sake of illustration, at the rate of one thousand feet in a minute, while all the circumstances as to temperature remain as before. The cold water would continue to flow just as before, under the hot, towards the equator, where the rarefying cause existed, but it would now come to the equatorial regions, possessed, not only with a motion directly towards the equator, but with the easterly velocity due to that circle of latitude which it had left, or about eight hundred feet in a minute; and if we suppose these equatorial regions to be moving to the eastward at the average rate of nine hundred and fifty

feet in the same interval, the cold water moving at the slower rate would inevitably at its first arrival there be left behind; or, which is the same thing, the surface of the globe would go faster to the eastward than the superincumbent water, and this, in effect, would produce an apparent or relative motion of the water from east to west; or, if the fluid in question were air, we should there have what we call an easterly wind.

This, in its most general sense, is what really takes place with the Trade Winds, and if what I have said be well understood, all the modifications which they undergo will be readily seen to follow.

The cold air, however, (it must be carefully observed,) which comes towards the equator, is acted upon by two forces, or, in other words, is influenced by two sources of motion; first, by that which has been impressed upon it, in a due easterly direction, by the rotation of the earth in the temperate latitudes it has left: and, secondly, by a motion, in the direction of the meridian, towards the equator, and at right angles to it. This last is caused by the air rushing in to fill up the space left by that which has been rarefied by the heat of the torrid zone, as shown in the first experiment where the globe stood still; in which case, it will be remembered, this was the only motion to which the fluid was exposed. The combined effect of these two motions is to produce the southeast Trade Wind in south latitude, and the northeast Trade on the other side of the equator.

When the comparatively slow-moving air of the temperate zone, caused by the rotatory motion of the earth to the east, first comes into contact with the quick-moving or tropical belt of the globe, the difference of their velocities is great compared with the other motion of the air above described, or that directly towards the equator; and consequently the wind blows at the extreme edge of the Trades nearly from the east point. As this cool air, however, is drawn nearer to the equator, and comes successively in contact with parallels of latitude moving faster and faster, this constant action of the earth's rapid easterly motion gradually imparts to the superincumbent air the rotatory velocity due to the equatorial regions which it has now reached; that is to say, there will be less and less difference at every moment between the easterly motion of the earth and the easterly motion of the air in question; while, at the same time, the other motion of the same air, or that which has a tendency to carry it straight towards the equator, having been exposed merely to the friction along the surface without meeting any such powerful counteracting influence as the earth's rotation, will remain nearly unchecked in its velocity. Thus, as I conceive, the Trade Wind must gradually lose the eastern character which it had on first quitting the temperate for the tropical region, in consequence of its acquiring more and more that of the rotatory motion of the earth due to the equatorial regions it has now reached. While this cause operates, there

fore, to destroy the easterly direction of the Trades, their meridional motion, as it may be called, or that towards the equate by remaining constant or nearly so, will become more and more apparent, till at length, when the friction of the earth in its rotatory motion has reduced the velocity of the cool air to the tropical rate, there will be left only this motion towards the equator, which is found invariably to characterize the equatorial limits of both trade winds. This velocity, also, is at length checked, first, by its friction on the surface of the earth; secondly, by the air becoming heated, which causes it rather to rise than to flow along the surface; and thirdly, by the mecting of the two opposite currents -one from the north, the other from the south.

In confirmation of these doctrines, I may state that, in the Trade Winds, the higher clouds are very seldom, if ever, observed to go in the same direction as the wind below. In general, they are seen to move nearly in the contrary direction; and I find it noted in my journal, that on the top of the Peak of Teneriffe, the wind was blowing from the southwest, directly in the opposite direction to the Trade Wind below.

In what has been said above, the quickest moving or equatorial belt of the earth is assumed as being also the hottest and consequently that over which the air has the greatest tendency to rise. This, however, is not the case universally; and where variations in this respect occur, effects very different from those described are the result. The most striking examples with which I am personally acquainted, of this deviation from the general law of the Trade Winds, or that which would obtain, were the earth a uniform mass of water, or land, occur in India and Mexico. That portion of the Pacific Ocean, which stretches from the Isthmus of Panamá to the Peninsula of California, lies between eight and twenty-two degrees of north latitude. Now, the sun's rays strike directly upon the adjacent great territory of Mexico, and, by heating the land violently, cause the air to rise over it. But the vacuum is filled up not only from the northward, but by the comparatively cold air of the equatorial regions in the neighbourhood, This air coming from that part of the globe which revolves quickest, to one which moves more slowly, produces not an easterly, but westerly and southwesterly winds; so that the navigator, who works by what is called the rule of thumb, and takes things for granted, instead of inquiring into them, will be very apt to make sad blunders in his navigation. I confess that I once laid myself open to an accusation little short of this, for which I had less excuse, perhaps, than another man, since, from having long speculated upon these topics, I had in a great measure satisfied myself of the truth of these theories. Yet when I was sent to visit the southwest coast of Mexico alluded to, and was left to my own choice as to the manner of performing the voyage, I miscalculated the probable effect of so vast a heater as Mexico, and expected to find the winds from east or northeast;

and therefore began my voyage at Panamá. I soon learned, however, tó my cost, that, instead of being to windward of my port, I was dead to leeward of it, and I had to beat against westerly winds for many weeks.

After all, however, it is by this union of theory and experience (which is not the worse for being dearly bought), that effectual knowledge can be obtained; and the disasters into which we are led by ignorance must be serious indeed, if they be not more essentially profitable, than mere unobservant success would have been. I mean that our finding things as we expected them is not always a proof that we have reasoned correctly, for had I visited this coast at another season of the year, and found an east wind blowing, I might have called it the northeast Trade, perhaps, and brought away none of the local knowledge, which is now, I trust, well engraved on my mind by the laborious process of rectifying my original error.

The monsoons in India, in like manner, are striking illustrations of this modified part of the theory. When the sun has great northern declination, the Peninsula of Hindostan, the north of India, and China, being heated, the quick-moving equatorial air rushes to the northward to fill up the slowmoving rarefied space, and this supply, being possessed not only with a rapid eastern velocity, but with a motion from the south, produces the southwest monsoon in the Indian Ocean, Bay of Bengal, and in the China Sea. When the sun, on the other hand, goes to the south, the same seas are occupied by air which, coming from regions beyond the northern tropic, = possesses less easterly velocity than the space they are drawn to, which gives them an easterly character; and this combined with their proper motion, if I may so call it, from the north, produces the northeast monsoon. There are numberless other less striking modifications of these principles, which give a high degree of interest to the science of navigation, parrticularly between the tropics ; — but which it is needless to enter into just now. It may however be useful to mention one important case which occurs in the Atlantic, when the sun has high northern declination, and the north of Africa is much heated; the equatorial air is then invited to the north, and a brisk southwest or south-southwest wind blows in the space between the equator and the southern limit of the northeast Trade Wind, which lies then in ten or twelve degrees of latitude, greatly to the astonishment of the inexperienced navigator, who, trusting to his books, expects a wind directly the reverse.

The same reasoning, precisely, will serve to account, not only for the direction, but for the degree of strength with which the winds blow between the Trades and the polar regions,- that is, from 30° to 60°. The heated air which rises over the tropical belt, is carried towards the poles, till it is sufficiently cooled, when it descends, and, by encountering a part of the globe going to the eastward at a much slower rate, produces westerly winds. It must be observed also that, as the lower or cold air of this

range proceeds towards the equator, it encounters, at every stage of its course along the surface, parallels of latitude moving faster and faster to the eastward, and consequently is exposed to more and more friction, by which means the relative difference between its velocity and that of the earth becomes at every moment less and less, till it subsides at length into a calm. But the equatorial air, on the contrary, in its progress towards the middle latitudes, comes constantly to regions of the globe moving with less and less velocity, so that it descends from the high regions of the atmosphere, along which it has passed with less friction to check its easterly motion, than the lower or cold current must have had to contend with, in its passage along the earth's surface. This equatorial air, therefore, comes with scarcely any diminution of its original velocity, into contact with a part of the earth moving more than a hundred miles more slowly to the eastward than itself. Consequently we have furious westerly gales as far as Madeira, on the one side, and the Cape of Good Hope on the other, which lie just beyond the northeast and southeast Trade Winds in the opposite hemispheres.

II. AURORA BOREALIS.

THE aurora borealis is one of the most striking and splendid spectacles in the heavens. In the temperate latitudes it appears as a faint, beautiful, yellow light, like the morning or evening twilight. It generally rises from a kind of dark cloud, or collection of vapors, which runs along from the north to the east and west, through 50°, 100°, and sometimes 150°, with an even edge, elevated 15° or 20° above the horizon. Sometimes it is perpetually changing its altitude, and seems to roll like the sea in a storm. The luminous matter immediately above this cloud is pretty steady and uniform. But from this there are streams that dart up toward the zenith with great rapidity. These are suddenly extinguished and renewed, and continually shift their places; sometimes they rise only a few degrees and with a faint light, at other times they mount in a broad and bright beam to the zenith. Ordinarily the streams do not rise more than 50° or 60°. They often resemble the tail of a comet, being more condensed at the point from which they issue, and growing fainter as they ascend. Sometimes they extend to the zenith, forming a beautiful canopy of luminous wreaths, like the curling flames that meet at the top of an oven.

Besides this variable, undulating light in the north, there is sometimes seen a broad permanent arch from 15° to 20° wide, rising from the magnetic east, and passing near the zenith at right angles to the direction of the streamers. In some instances, it has been observed to have a slow motion, and to throw out small coruscations to the north. It breaks up gradually