Even very limited local movements of the atmosphere are modified by the same cause. It is a remark as old as Bacon, and afterwards confirmed by Mariotte in France, Sturm in Germany, and Toaldo in Italy, and since by " many other writers both in Europe and North America, that the wind has a decidedly preponderating tendency to veer round the compass according to the sun's motion, that is, to pass from north through north-east, east, south-east, to south, and so on round in the same direction through west to north ; that it often makes a complete circuit in that direction, or more than one in succession (occupying sometimes many days in so doing); but that it rarely veers, and very rarely or never makes a complete circuit, in the contrary direction." According to Sir John Ilerschel, Pro fesser-Dove was the first to show that this tendency is a direct con sequence of the rotatory motion of the earth ; it has therefore been denominated " Dove's law of rotation of the wind," to which we shall shortly return.
The heated equatorial air, while it rises and flows over towards the poles, carries with it the rotatory velocity due to its equatorial situa tion into a higher latitude, where the earth's surface has less motion. Hence, as it travels northward or southward, it will gain continually more and more on the surface of the earth in its diurnal motion, and assume constantly more and more a westerly relative direction ; and when at length it returns to the surface, in its circulation, which it must do more or less in all the interval between the tropics and the poles, it will act on it by its friction as a powerful south-west wind in the northern hemisphere, and a north-west in the southern, and restore to it the impulse taken up from it at the equator. " We have here the origin of the south-west and westerly gales so prevalent in our latitudes, and of the almost universal westerly winds in the North Atlantic, which are, in fact, nothing else than a part of the general system of the re-action of the trades, and of the process by which the equilibrium of the earth's motion is maintained under their action." The only winds of a regular character which remain to be noticed are the Land and sea breezes which occur diurnally on the coasts and in the islands of the tropical regions, and the periodical winds which are observed to prevail in some parts of Europe. The first are probably caused by the inequality of the sun's action on the land and water ; and both, by the tendency of the atmosphere to preserve a state of nearly uniform density. During the day the land acquires a tempera ture higher than that of the neighbouring ocean : the atmosphere above it consequently becomes rarefied, and from about 9 A.m. the air from the sea flows towards the land, to occupy the partial vacuum there produced. In proportion as the heat of the land goes on in creasing, the force of the sea-breeze also increases, and this continues till 2 or 3 nat. After that time the temperature over the land di minishes more rapidly than over the sea, as the heat more readily escapes by radiation from the land than from the water, and about sunset the breeze from the sea ceases. During the night, the land continuing to cool, the air over the sea becomes comparatively warmer and more rarefied, and a breeze from the land, takes place : this wind augments in force till near sunrise, when the temperature of the earth begins to increase, and about 9 a.sr. the wind blows from the sea as at
first. These land-breezes diverge in every direction towards the coasts of the tropical islands from the high lands in their interior. Mr. Red field modifies the hypothesis above stated by assuming that when the stratum of air lying on the surface of land which ascends towards tho interior of a country becomes rarefied by the sun's heat, it is forced by an excess of pressure at its lowest part to move up the slope ; and during the night the stratum of air on this inclining surface acquiring greater density, its gravity causes it to descend towards the sea. (' Amer. Journal of Science,' vol. xxxiii., No. 1.) The Eteeian winds (so called from irricrfaz,"annual") is a designation formerly given only to those which every summer blow during six weeks over the countries bordering the Mediterranean ; but it has since been applied to other periodical winds, as those which blow on the coast of Holland. They commence in the Levant about the middle of July, rising at 9 A.m., and continuing during the day-time only : the direction of the current of air is from north-east to south-west ; and it is probably caused by the rarefaction of the atmosphere nearly under the tropic of Cancer, in consequence of the heat of the sun at that season. I'liny states that, in Spain and Asia, the etesian winds blow from the east; and lie adds that they also take place in winter, when they are called Ornithian winds : these arc, however, said to be more gentle than the others, and to continue during nine days only. (Smyth's ' 3Iediterranean; p. 270.) It may be observed, in addition to what has been said respecting the trade and other regular winds, that those which prevail in the tempe rate zones are probably the results of currents proceeding about the earth from the tropical regions. Professor Dove suggests (in a paper published in Poggendorff's Annalen; of which a translation appeared in the Philosophical Magazine' for September, 1837) that when the sun is on the meridian of any place, as London, situated beyond those regions, the currents of heated air which proceed from the point verti cally under him must arrive at that place from the south earlier than at any other place eastward or westward of it on the same parallel of latitude. But in proportion as the sun becomes successively vertical at different points westward of the meridian of London, the currents of air, in describing great circles of the sphere, arrive later, and in a direction from the westward of south ; and when, during the summer, he is vertically over a point about 60 degrees west of London (that is, In the evening), they arrive nearly from the west. At midnight, when the sun is on the meridian under the horizon, the current of air passing over the north pole is felt as a north wind ; and after this time the currents coming from points having less than 180 degrees of longitude eastward are felt as easterly winds, which become due east in the morning when the sun is about 60 degrees eastward of the meridian.