INTRODUCTION. THE HE AIR, if it be once set in motion, either with moderate velocity or as a storm, may preserve the direction of that motion unchanged, or else blow successively from different points of the compass. In speaking of atmospherical currents, we distinguish permanent winds from those which are variable. In the same way, ordinary language has drawn a distinction between different classes of storms: those whose direction is constant being called gales; those which have a rotatory motion hurricanes, or, as Piddington calls them, cyclones; and lastly, rotatory storms of smaller dimensions being termed trombs. We have succeeded, by the help of the Law of Gyration,' in referring the Trade-winds (whose direction. is never changed), the Monsoons (whose direction is changed periodically), and the so-called Variable Winds of higher latitudes, to one common general principle, which Hadley had first applied to explain the origin of the Trade-winds. It is, therefore, not unnatural for us to expect that in the more violent disturbances of the atmosphere, certain general conditions will exist, which are common to them all, as well in their origin as in their subsequent course, although the appearances which they present may exhibit considerable variety. Instead of following this natural course of inquiry, identity has been sought, not in the original conditions which produced the phenomena, but in the phenomena themselves. The consequence of this error has been that a very animated discussion has arisen; one party maintaining that all storms are cyclones, while the other asserts that the rotatory motion is never developed on a large scale. An error which is constantly refuted, and yet as often re-appears, is the belief that the meteorological phenomena of the temperate zone are only modifications, on a smaller scale, of those which appear in the torrid zone. The real state of the case is, that the torrid zone presents the simplest form of the phenomena, whose more general characters are exhibited in higher latitudes. This assertion is found to be the case as regards the movements of the atmosphere, as well in their mean direction, as in the extraordinary increase in their intensity, which we call a storm. In the case of the former, we find that the Law of Gyration is the general rule observable in the temperate zones, while its exceptional cases, the Trade-winds and Monsoons, are peculiar to the torrid zone. In the case of the latter, we find that the existence of both gales and cyclones in the temperate zone may be demonstrated, whereas the cyclonical class of storm predominates to such an extent in the torrid zone, that the other class sinks into comparative insignificance beside it. A general theory of the wind ought to explain the reason why the primary causes, which affect the whole of the atmosphere, should produce exclusively certain definite and exceptional effects in the torrid zone. If we inquire in what cases a vane, which is exposed to the action of air in motion, will preserve its direction. unchanged, we find that they are three in number: 1. When the air flows from all sides towards one fixed point at which it rises, or flows away, on all sides, from one fixed point where it sinks.* 2. When the air circles about one fixed point, either from left to right, or from right to left. In this case the direction of the vane at the place of observation indicates the tangent to the whirlwind, which exists at that fixed point; whereas, in the former case, it corresponds with a radius, along which the influx or efflux of the air takes place. 3. When large masses of air move from one point on the earth's surface to another, and consequently the individual particles move in parallel rectilinear paths. In the first case which we have just discussed, the paths are diverging and converging right lines, while in the second they are concentrical circles. The third case of motion, viz., that in parallel rectilinear paths, can only take place under the following conditions : a. If the places from which the air starts and at which it arrives be in the same latitude, no change in the distance between them, or in the velocity of the wind, can have any effect on the vane; i.e., the direction of the vane is unaltered, however the extent in longitude or the velocity of the current may vary (according to Hadley). * The land and sea-breezes of small islands in hot climates, which are felt when there is no prevailing wind, are examples of this. If, during the day, the land assumes a higher temperature than the sea, the air in contact with it rises, and the cooler sea-air flows in below. The cold air sinks over the surface of the sea, as it does in the shade of a passing cloud on a hot summer's day. During the night the land loses its acquired temperature more rapidly than the sea, so that the surface of the latter becomes at last warmer than that of the land, and a counter current from the land to the sea sets in. This circular motion in a vertical plane may be compared to the revolutions of a wheel. If there be no difference of temperature there is no motion; if there be a difference, motion ensues, first in one direction, then in the other. There are two periods of rest every day, viz., those at which the direction of the rotation changes. |