The production of winds, we have already stated, is mainly attribut able to the unequal distribution of heat in the atmosphere. The attractive action of the sun and moon on that fluid, though producing atmospheric) tides, which are believed to have been recognised by barometrical observations, could only produce a tropical wind with a velocity of 4 miles per day, which would be evidently inappreciable amongst the numerous disturbances arising from temperature ; but the heating action of the sun in tropical climates produces some well known winds in the following manner.
On account of the annual accumulation of heat from the solar rays between the tropics, two currents of air from the north and south rush forward to occupy the place of the rarified air of this region. Now since the earth in its diurnal rotation moves from west to east, these currents appear to deflect to the west on account of the increased velocity of the parallels of latitude near the equator, which have greater radii than the arctic parallels, the apparent excess of motion towards the west being the excess of the space described in rotation by the equinoctial above that described by the tropical and even polar circles; the velocities perpendicular to the equator, being nearly equal and con trary, produce no sensible wind, but those parallel to it produce a wind directly west and enduring, called the " trade-wind," with this excep tion, that the northern hemisphere being the warmer, the resultant is a few degrees north of the equator, crossing the Atlantic from Africa to Brazil, and the Pacific from Panama to the Philippine Isles, and the Indian seas from Sumatra to Zanguebar. But when the continents stretch into the torrid zone, the local accumulation of heat being great, two opposite periodical winds, known by the name of monsoons, are produced, approaching the north tropic in summer, and the south in winter, and blowing in the Arabian and Indian Sala north-west from April to October, and in the contrary direction during the rest of the year : the reverse occurs south of the equator.
Now as the cold air rushes into the tropical regions, expelling the warm air of that locality between 28' on each side, the warm air forms a counter-current, still possessing equatorial velocity, and produces in both hemisphere. exceeding the above latitude a westerly and genial brease, which is felt during three-quarters of the year from Newfound land to the west of England, and from South America to the Cape of Good Hope. On the same principle the easterly, or rather north easterly winds originate in the polar currents, and are piercingly cold In *general- The great radiation of arid plains produces many local winds, such as the sirocco from Africa to Italy, and the destructive simoom of Arabia, &c., but they do not depend on general atmospheric
causes.
The greater number of the several subjects the knowledge of which constitutes the science of 3leteorology, will be found distributed throughout this division of the' English Cyclopedia' in alphabetical order. But it may be useful to the reader and student to refer to the more important of them in this place, arranged under the heads of the principal branches of the science. That of the form, general nature, and extent of the gaseous envelope of the earth is treated of under AT MOSIIIEME, and partially returned to in the article COMETS, Physical Cbustittstion of. In the finstaaamed article, and also under Ata,, its c-hunical nature and relations are described, and its agency in changing the coalition of the bodice composing the earth's surface and em i-loyed in human constructions, under ATMOSPHERIC tarsus:fez. The articles Ac mons-ran, Tuirzfuress, and Tatanonrrett, belong, the first sutirely,and the latter in part, to its relations to heat; while some consequences of its action on light are described in those of Ar.ntal. Pease/cerise ; liertscrioN and RETRACTION, ATMOSPHERIC; and Twi t-ions. Tug subject of the prefigure of the atmosphere and its variations, and their connection with the weather, are explained under Ilasiourrts ; lionsou MP LIQUIDS; and MANOMETER. The consequences of the changes Induced by variations of temperature in the aqueous constituent of the air, forming so Important a branch of practical Ileteorukary, are described under the following heads : Ctocus ; Law Dzw-rotar ; Evaronarios ; Ilan.; IIOAR-PROST ; IITOROSIE rza and HTOROMSTRT • MIST; !Lox; RAIN-GAUGE; SNOW; THAW. The phenomena depenient upon the mechanical affections of the air, and the philosophy of winds, storms, and cyclones, are treated of in the articles AlknO-Drasones ; AIR; ANEMOMETER ; ATMOSPHERE; CALMS ; FLUIDS, ELASTIC ; MONSOON ; SAMIELI ; SIROCCO ; TORNADO; TYPHOON; WHIRLWIND; WIND. The electrical part of meteorology will be found under ELECTRICAL LIGHT; ELECTRICITY, ATMOSPHERIC; and LIGHTNING; a certain subject relating to this branch of the subject being also noticed under BRFATn-FIOURES. CLIMATE and WEATHER, and the special subjects of DUST, ATMOSPHERIC; IONIS FATE'S; METEORS, IGNEOUS or LUMLSOUS; and POLAR LIGHTS, are discussed under those heads.