In reference to the 3rd and 4th rules it may be observed, that the quantity of precipitation and lightning are greatly diminished, cæteris paribus, by uniformity of temperature, as in the tropical portions of the sea far from land, and other parts of the world where the winds blow long in one direction; whereas changes of temperature are indispensable to the condensation of atmospheric vapour, and to the evolution of its caloric in the concentrated form of lightning.

CHAPTER II.

Theory of Winds.

Of all the subdivisions of General Philosophy, there is none so little entitled to the name of science as Meteorology.

MASON GOOD.

THAT the reader may comprehend more fully the connexion between caloric, evaporation, and atmospheric electricity, it becomes necessary to present a cursory view of atmospheric currents generally.

The unequal distribution of solar caloric over the earth's surface, together with its annual and diurnal revolutions, determine the periodical movements of the atmosphere which surrounds it.

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It has been long known, that there are three great currents of the aërial ocean, by which it is kept in a state of perpetual circulation: one from the polar regions towards the equator, which is an under current; another from the equator to the poles, which is an upper current; and a third, called the great equatorial current, or trade wind, which blows from east to west around the globe for about 30° on each side of the equator; thus sweeping over the whole extent of the tropical regions, which are upwards of 3000 miles in width.

There is another general wind, which blows from west to east in the middle and higher latitudes, about two-thirds of the year in the northern hemisphere; while in the middle latitudes of the southern hemisphere, where there is little or no land, it is said to be nearly as uniform as the trade wind. Dr. Hadley, and after him Dr. Franklin, attributed these general currents to the following causes. "The air between the tropics, being constantly heated and rarefied by the vertical sun, rises, when its place is supplied by air from the higher and polar latitudes, which, coming from parts that had less diurnal motion, and not suddenly acquiring the swifter motion of the equatorial regions, becomes an east wind; the earth moving from west to east, and slipping under the air."-(See Franklin's Works, vol. iii. p. 236.)

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These general views have been greatly extended by Dr. Dalton and Mr. Daniell. The first of these philosophers observes, that the diurnal motion of the earth at the equator is 1040 miles per hour, and diminishes gradually on to the poles, where it is nothing.-(Meteorological Essays, page 88.) And Mr. Daniell has demonstrated in the most conclusive manner, that from the greater density of the polar atmosphere than the equatorial, its height is proportionably less; so that the expanded air between the tropics must rise and flow towards the poles as an upper current. Before rising, it has acquired the tropical motion of the earth from west to east, which it retains, until descending in higher latitudes, where the earth's diurnal motion is less, it mixes with the lower air, and gives it a westerly direction.

The force and direction of the trade winds are influenced by the proximity of islands and continents. Along the western side of Africa, their direction is reversed. To the distance seaward of about 300 miles, they blow towards the heated land: they are reversed in a similar manner in the Pacific, west of South America. When the sun is in the northern tropic, they extend several degrees farther north than during our winter ; and when the sun is south of the equator, they prevail farther south.

There is a tract corresponding with the iso

thermal division of the globe, which, as will be seen hereafter, is the magnetic equator, where the great polar currents from the north and south meet and neutralize each other. They are characterized by a constant succession of irregular winds and calms, with storms of thunder, lightning, and rain: they are termed by seamen the swamps, or rainy latitudes; and are extremely sultry, owing to the immense quantities of caloric given out during the condensation of so much aqueous vapour.

During winter, in the northern hemisphere, the polar latitudes being deprived of the sun's rays, while the tropical parallels are heated, we have a predominance of northerly winds, the dense polar air pressing towards the tropics, to restore the equilibrium; while the greater velocity of the middle and tropical than that of the northern latitudes, causes a deflection of the polar currents to the south-west and west, making north-east and east winds, which almost uniformly succeed to north winds, and continue blowing until an equilibrium is established between the higher and middle latitudes. In the mean time, under the influence of the solar beams, the land soon becomes more heated than the ocean, even south of it, which causes a south wind until the equilibrium is restored between the sea and land atmosphere.

During summer, in the United States, south

westerly winds predominate, and north-easterly winds during winter, especially if the winter be excessively rigorous. The same thing is true of India, China, and Arabia. They are heated during summer, and the atmosphere over them is rarefied, which causes the air to flow in upon them from the tropical seas, when it is deflected to the east, by passing from latitudes that move rapidly, to those which move more slowly. This is what seamen term the south-west monsoon.

When the sun is south of the equator, the air moves from the northern land, which is cooled down, towards the equator, that has a swifter motion, thus causing the north-east monsoon, which corresponds with the north-east wind of the North American cold season. It was supposed by Volney, that the prevailing south-west wind of the Mississippi valley was a recoil of the tropical trade wind, deflected by the Andes of Mexico; but the fact which he states, of its crossing the Alleghany Mountains, and advancing north-eastward, as far as Montreal and Quebec, is sufficient to prove, that it must be owing to a cause far more extensive and general in its operation than mountain ranges.

During summer in North America, the land is greatly heated, and the air presses from the Atlantic ocean, especially during the day, causing a sea breeze; when from the south, it is changed into a south-west wind, by passing from