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law of a regular decrease from the equator towards the poles. So far as this law extends its influence, the equatorial and polar currents will flow on regularly in the upper and lower regions of the atmosphere, and the trade winds will prevail. Beyond this, however, in summer, the surface will be heated beyond the temperature indicated by this law; and consequently, currents will take place beneath, directed on both sides towards the zone in which the deviation from this law is the greatest. If this should be in latitude 40°, the whole zone between that parallel and the trade winds, will experience a current from S. to N., modified into a SW. wind by the circumstances of rotation. In winter, the surface will cool down, first to a temperature indicated by the law of regular decrease, and finally, below it; the course of the currents should therefore change, and the regular trade wind reach to these latitudes. But now another cause begins to act. The upper current of the trade wind, coming from a heated region, will be less dense than it should be in these latitudes consistently with the same law. In such a case, our author shows that the barometer would fall, and a current necessarily set from both sides, to restore the equilibrium of pressure. This would again become a SW. wind at the surface; and would be attended with a counter current from the NE., extended until it assumed, to the highest SW. current, the relation of the lower current of the trade winds to the upper.

That the westerly winds that prevail between the latitudes of 30° and 40°, are not caused by the descent of the upper current of the trade winds, is clear, from the fact of their wide extent. But a stronger proof is derived from a knowledge of the circumstance, that the zone in which this descent really takes place, is as marked as that, where in the great process of aerial circulation the heated air rises in the central region of the trade winds; in the open ocean, at the outer verge of the NE. trade, navigators concur in stating, that there is a space in the North Atlantic, resembling in its phenomena the Variables, which is consequently shunned by them; and to avoid which, they take care not to pass far from the coast of one or the other continent. This space is known by the name of the "Horse Latitudes;" and although we cannot quote authorities, we do not doubt that a similar zone exists in the other oceans. According, then, as the heat of the surface decreases, regularly or irregularly, from the pole to the equator, or as the density of the upper part of the atmosphere is affected by heat, the lower currents will be either from the south, or from the north; and, modified by rotation, will cause, in countries of temperate climates, where local circumstances do not intervene, the prevailing winds to be either NE. or SW. These alternations of wind will cause waves in the atmosphere, and be attended with oscillations of the barometer. In the Unit

ed States, the south-west is a prevailing wind; the north-east. often occurs, but the north-west, as it is usually called, but which is often nearly west, is, in most places, felt perhaps more frequently than either. If, in the southern states, north-east is the most frequent, in the northern, the north-west blows for the greater number of days. In New-York it was determined, from tables accurately kept for several years, that, (after abstracting the land and sea breezes of the summer months,) out of thirtysix days, the wind blew twelve from the north-west, seven from the north-east, twelve from the south-west, and only five from all other points of the compass. This north-west wind is, in fact, the marked characteristic of the climate of that part of North America, which lies east of the Rocky Mountains. It is, generally speaking, cold, clear, and dry; yet in summer, it almost always accompanies thunder-gusts; and in winter, its beginning is attended with the most violent falls of snow. We are to seek for its cause in the peculiar circumstances of the interior of the continent, and of the adjacent ocean. Parallel to the general direction of the coast of North America, from the southern extremity of Florida to the Banks of Newfoundland, runs a remarkable current of water called the Gulf Stream. This, no doubt, arises from the action of the trade winds, which constantly urge the waters of the ocean into the Carribean Sea and the Gulf of Mexico, whence it is discharged by an outlet between Cuba and Florida. Coming from the equatorial regions, it retains a temperature very considerably higher than that of the adjacent waters, and much more so than that of the continent during the winter season. The air above this stream, is consequently heated, and a current would flow from the continent towards the Gulf Stream, attended with a counter current in the opposite direction, until the equilibrium was restored. Hence, in winter, the north-west is by far the most frequent wind. In summer, as the coast of the continent and the valley of the Mississippi are much more heated by the sun than the Rocky Mountains, and the large extent of high land and plains which lie along their eastern base, there will be a tendency in the air of the latter district, after a long continuance of warm weather, expanding and diminishing the pressure of that on the seaboard, to flow towards the coast; this will be accompanied by a corresponding upper current, until the air over the whole extent of country becomes of the same temperature, when the wind will cease, and the general causes that produce SW. or NE. winds, will begin to act according to circumstances. As might be expected from such a cause, the north-west begins to blow with great violence, and produces rapid and sudden falls of temperature. This violence gradually lessens, and the temperature rises until the wind ceases. to blow. A SE. wind is the next in prevalence to these three

great winds of our climate. We may perhaps look for the cause of this, in a condensation that must occasionally take place on the coast of the United States, when a north-west wind has prevailed for some time, and the current that often follows the course of the Gulf Stream, happens to meet at its northern limit with the regular NE. current. These three winds thus blowing at one and the same time, the condensation we have spoken of is the consequence. When the NW. wind is expended, the other two still continue, and hence the surplus air must be discharged in the direction of the continent, and cause a SE. wind.

Besides these, no other winds prevail, at the same time, over any great extent of country, with the exception of the sea and land breezes that temper the heat of summer upon the seacoast. The cause of these is so well understood, that we shall not enter into that trite subject. Our other winds arise from partial causes or local circumstances, and are confined to narrow limits.

In considering the phenomena, and forming a theory of climate, it is not sufficient to have regard merely to an atmosphere of dry gaseous matter. The waters that form so great a portion of the surface of our globe, are constantly giving out aqueous matter to the air, which is again returned by condensation. Aqueous matter, therefore, always exists in the atmosphere. Two different hypotheses have been maintained, by which to account for this. One assumes the water to be chemically combined with the air, as a solution in its menstruum; the other more correctly supposes the water to be present in the form of vapour, and mechanically mixed with the atmosphere. The latter is unquestionably the true theory. Water, which, under the ordinary pressure of the atmosphere, boils, and evaporates rapidly at a temperature of 212°, boils under the exhausted receiver of an air pump at less than 100°. It therefore can exist in the state of vapour at a heat much less than that of ordinary boiling; even ice yields vapour of an elasticity proportionate to its heat. The experiments of Dalton show, that water is capable of rising in vapour at all temperatures, even under the usual pressure of the atmosphere; but that the tension, or elastic force, diminishes far more rapidly than the heat; the one being in geometric, while the other is in arithmetic progression. So far from the air acting as a solvent, it by its pressure retards the evaporation of the water; and it is only when the tension becomes equivalent to the pressure of the atmosphere, that the rapid evolution of vapour, called ebullition, begins to take place.

The quantity of vapour that a given space will take up, depends then upon temperature merely, and not upon the presence of the air. The quantity of evaporation, in a certain time, from

a given surface, depends partly upon the pressure of the atmosphere, and partly upon the quantity of aqueous matter already present in it; and under equal circumstances of pressure and temperature, the quantity evaporated will be in a ratio the inverse of the quantity of water already present. Changes of temperature, sufficient to reduce the mass of air below the point at which the space cooled is capable of supporting the vapour that is present, will cause precipitation; and as at this exact point, a nascent condensation will be just perceptible, and manifest itself by clouding the surface of glass, or other polished substances, this temperature is called the Dew-Point. Mixtures of different masses of air, each containing nearly all the moisture due to its temperature, will also cause precipitation; for as the tension of the vapour varies in geometrical ratio, while the temperature changes in arithmetic progression, the mixture of two equal masses of air produces a temperature, the arithmetic mean of that of the two masses, while the corresponding tension of vapour being no more than the geometric mean, the excess will be at once precipitated. It is in this way that we can account for nearly all the rains that take place, even in our own country. In some others, such is the quantity of moisture present at all times, however clear the atmosphere may appear, that rain occurs upon almost every change of wind. We may here state, that the presence of aqueous vapour does not affect the transparency or refractive power of the atmosphere-indeed it rather increases the former: of this we have an illustration in the fact, that, when distant objects appear more clear and distinct than usual, rain may fairly be anticipated as likely soon to follow.

The condensation and evaporation of water affect the temperature, not only of the surface of the earth, but of its circumambient atmosphere; this produces a very marked change in the direction and continuance of the prevailing winds, and in this way powerfully influences the climate. The phenomena, then, presented by the aqueous portion of the atmosphere, either alone or in mixture with air, are well worthy of being investigated; and this is done by our author, in the second and third parts of his first essay. In the former of these, he treats of the habits of an atmosphere of pure, unmixed aqueous matter; and in the latter, of one composed of a mixture of a homogeneous gas with the vapour of water.

If we assume the terrestrial spheroid to be covered with water every where, at the temperature of 30° of Fahrenheit, the latter would form around it an atmosphere of vapour, of the same degree of heat, whose pressure and consequent tension would be equivalent, at the surface, to a column of two-tenths of an inch of mercury. This tension, and the corresponding temperature, would diminish in ascending from the earth, and they would be

come one-tenth of an inch, and 13°, respectively, at the height of 30,000 feet. In such a state of things, there would be perfect equilibrium, and consequently perfect rest, all over the sphere. No precipitation or farther evaporation would take place, and this atmosphere of vapour would remain transparent and undisturbed. A uniform increase of temperature, affecting the whole surface of the sphere, would vary the quantity and elasticity of the vapour, but its mass would still remain transparent.

In the second modification we before considered-namely, the case of a sphere increasing in heat from the poles to the equatorthe temperature of such an atmosphere would not follow the same law. The elasticity of the whole would be determined by the lowest temperature on the surface; and the water would distil at the equator, with a rapidity sufficient to cause ebullition. It is not necessary that this vapour should pass from the hottest to the coldest point, before condensation began; but the first evolution of vapour in excess, at the equator, would be attended with equal precipitation at the pole. We must, however, leave out of view this rapidity of action, and assume, as actually happens, that the flow of vapour is so much retarded by mechanical resistance, as to enable it to assume the gradation of temperature we have ascribed to the sphere itself. In this case, a circulation, very different from that which occurs in an atmosphere of permanently elastic fluid, would take place. The vapour would flow in mass from the equator towards the poles, and being condensed in its course, would return to the equator in the state of liquid water, causing currents in the ocean. Great evaporation would be constantly going on at that circle, and constant precipitation at all other latitudes.

If the temperature of any given zone should be raised higher than it would be according to our assumption of a regular decrease from the equator towards the poles, the precipitation would cease there, while that in the next lower latitude would be increased; on the other hand, if the temperature of the same zone were diminished, the precipitation would be then increased, while that of the next higher latitude would be lessened.

If, from any cause, the decrease of heat, in rising from the earth, should not follow in some given region, the same law that the temperature of an atmosphere of vapour ought to follow, but should decrease more rapidly, the watery surface will still have a tendency to throw off vapour of a temperature the same as its own; but as the pressure above will be lessened by the change, we have supposed, in the rate at which the temperature decreases, an increased evaporation must ensue below, attended by a precipitation in the region whose temperature we have supposed to be lowered; and a cloud will then be formed. This cloud, however, unless the change of temperature be very great, will not

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