The current is, in fact, the difference between the motion of the carriage and that of the air through which it passes. In like manner, the mariner, placed on the watery surface of our swiftly rotating globe, is whirled along insensibly to himself, at a greater velocity than is the bed of air which lies above him, and the sensible effect is that he perceives a strong and equable wind in the opposite direction.

Having thus hastily traced the lower current to the equator, let us now follow the ascending column. Upon rising to a certain altitude, it is there to some extent cooled by parting with its heat into space by radiation, and its upward progress is necessarily arrested; the current is then deflected, and flows toward the poles in a grand stream. As it proceeds it still loses heat, and at about the thirtieth degree of latitude it is so cold as to descend and change places with the lower current from the poles. Proceeding onwards still, it receives heat from its contact with the earth, and again rises to form the upper current, being displaced by the cold and heavy air flowing from the poles. There is thus a sort of atmospheric chain formed. At all places, however, above the latitude of 30°, in consequence of the variations in the amount of heat received from the earth, these currents are very irregular.

Now this upper current-having a different velocity, in consequence of its origin at the swiftly rotating equator, from that of the more northerly, or southerly, slower moving regions which it has now reached-takes on the apparent character of a wind from west to east, and thus appears as a westerly wind; and, when the upper current descends to take the place of the lower at about the thirtieth degree of latitude, it is actually felt as a violent westerly gale. At the poles and within the polar circles, there is a constant steady polar gale toward the equator on every side.

The upper current was long suspected to exist, before its existence was positively known. A curious incident proved the fact. In 1822 an immense volcano burst out in the

island of St. Vincent, and vomited, to an enormous altitude, showers of stones and ashes. Strange to say, although the trade wind blows so strongly in the opposite direction that a circuit of one hundred miles is necessary to enable vessels to reach Barbadoes from St. Vincent, there fell on the island of Barbadoes a quantity of volcanic ashes, which undoubtedly had their origin in the eruption on St. Vincent.

The only explanation of this singular event was-the supposed existence of this upper or back current. The power of the volcano had projected these ashes entirely through the lower current into the upper, and, after being carried by it, they were dropped in Barbadoes. Another curious confirmation of its existence is recorded by Messrs. Humboldt and Bonpland. They set out on an expedition to ascend the peak of Teneriffe; at its base the trade wind was blowing strongly in its customary direction, but, upon reaching the summit, they found that they had actually penetrated through the lower stratum of air and got into the upper, and they now felt a strong wind blowing in a precisely opposite direction!

A variety of local winds, receiving their peculiar modifications from the circumstances in which they originate, are to be found treated of in works upon meteorology. Many of these, together with those here mentioned, appear to be easily explained upon the great principle of the inequalities of temperature. But many circumstances render it at least probable, that other causes of motion in air exist beside, or perhaps superadded to, those of heat and cold.

Mr. Rowell has put forth some curious speculations touching the origin of irregular winds generally. He conceives, that as water in its vaporous condition occupies, when suspended in the air, much more space than when it falls as a raindrop, it must, in the act of its falling, cause a vacuum in the air which must be filled up by a rush of air from the surrounding districts. He supports his views by various calculations as

to the average vacuum per square mile caused by some heavy storms of rain. He also mentions that in the autumn of 1846, while France and other parts of the Continent were deluged with rain, storms of wind swept over England from the north-west and west.-Chemistry of Creation.

CHAPTER XXV.

MOVEMENTS OF THE AIR.

Ir has been thought that electricity, in its various modifications, is in some manner connected with the irregular movements of the air; and it has been remarked as particularly observable, that, in the regions where the magnetic influence of the earth is at the lowest intensity, as in the midst of the Southern Atlantic, storms seldom or never occur, while in those regions where its intensity is greatest, hurricanes and violent tempests are extremely common. If importance is attached to the views of the electrical origin of most meteorological phenomena, it may readily be conceded also that alterations in respect of the electricity of the air may have some direct or indirect influence in setting in motion irregular currents of air. Upon these points there exists at present much ignorance; but, in consequence of the greatly increased amount of attention now paid to the science of storms, it is to be hoped that much of our present obscurity will be in a little time dissipated.

One of the most remarkable and beneficial discoveries hitherto made by meteorological science, is what is called the Rotatory Theory of Storms. By this is meant the remarkable fact, that hurricanes, typhoons, and, it is probable, other violent storms in all regions of the world, have a revolving motion. They do not blow, that is to say, in a straight line, from a point many hundreds of miles distant; but they are vast eddies in the air, which whirl round like the eddies in a

stream of water, or like the water let out of a basin by a plug at the bottom. Besides this revolving movement, these storms have also a movement from place to place, and it is highly remarkable that this changing of position also takes place, not in a straight line, but in a curved direction.

It is possible that, when all the phenomena of these storms are fully developed, the mariner will be able to direct the course of his vessel in such a manner as to escape beyond their destructive influence. A few years ago, the captain of a vessel, by availing himself of the knowledge of this fact, saved his ship and escaped without damage from a violent storm, which might otherwise have endangered, not only property, but life.

Of all the glories of science, none equals that of a welldirected and successful attempt at diminishing the risk of danger to human life; yet, while we owe much to the labours of those who have discovered its important truths, let it not be forgotten that we owe all to that Great Being who, from time to time, permits His creatures to obtain a view of those mighty governing principles with which He orders and directs the course of natural events. Should the inquiry be made as to the immediate connection between the chemistry of nature and the movements of the air, the reply must be that the connection is most intimate. The irregular, capricious winds which constantly agitate the air of temperate regions, fulfil a most important office in the operations of nature.

Powerful as is the influence of the diffusive force of gases in dispersing abroad the noxious emanations of any particular district or region, it may be reasonably doubted whether this force is anything like sufficient of itself to preserve the purity of a densely-populated region. It is true, we might not be sensible of any ill effects from even a three days' calm in our own temperate climate, although such a state of the air seldom lasts for more than a few hours. But this could not long continue without originating very serious evils.

If we imagine a crowd of human beings placed in a hall, however great its magnitude, and the air of such a building entirely without motion, it is easy to foresee the result. The impure emanations from the lungs and bodies of so many human beings, would accumulate in this motionless mass of air to such an extent, as to render it in a lesser or greater period of time altogether unfit to breathe, and death would be the consequence of continuing under its influence.

Such also would be the condition of a great city, over whose hundreds of thousands of inhabitants hung an atmosphere totally without motion; without a breeze to fan the cheek, or a storm to intermix the various parts of air and impurities together. Under such circumstances, a city would be as surely doomed to destruction, as though the windows of heaven were opened and her living multitudes engulphed in a deluge of waters.

It is however, impossible for such a condition as a dead calm to exist in the atmosphere of any region. It is true, that to the senses there may be no apparent motion in the air, and every leaf of the forest may hang unstirred on the branches; but there are invisible movements incessantly occurring in the stillest air. Not only by day, but even in

"The stillness of a moonshine heaven,"

these insensible commotions are constantly taking place. Not only in the open air, but in the closest shut apartment, the airy particles are always in motion. This is easily proved. Let the windows be closed up with shutters, all but a little hole, through which a pencil of sunlight may stream; standing at a little distance, we may perceive that the whole track of the sunbeam is, as it were, animated. Particles of dust are seen incessantly rising, falling, moving now in this, now in that direction, thus plainly indicating that the air in which they float is moved without ceasing.

Some of these insensible and imperceptible motions of the air are due to its elasticity, and to the facility with which its