METEOROLOGY. 



KETBOB& 



is important in thi* place to indicate the bearing of the coincidence 

 upon y> } rr furmulA, expressing th diminution of heat from the 

 equator la the pal*. Sir J. Hvnchel bom tbt if a aerie* of curve* 

 be tncni by calculation from aa equation derived frutn and equivalent 

 to UM general equation of the optical curves in question, for tempera- 

 lore* varying from a certain mean temperature thermometricilly indi- 

 cated, and from each other, by equal tbennometric interval*, these 

 in reapeet of their magnitude*, distance* from their foci and 



the pole, a* well M in their general gradation* of flexure, will coincide 

 or nearly ao, with a aerie* of iaochromatic curve* tqvtdutaut i .'/,.. 

 refer* oj <Mt* Thia latter aerie*, if laid down on a map, will divide 

 the meridian! in the polar region* in a lucceauon of point* distributed 

 over them according to a certain law of progression. By calculation 

 from the** data, he obtain* equation*, of which he taya : " Now it ia 

 remarkable that thia ia precisely the form in which Mayer endeavoured 

 'to exprea* empirically the decrement of temperature in proceeding 

 from the equator towards the pole, from such observations a* could be 

 obtained about the middle of the bat century." The .details of thia 

 curioua subject are given by Sir J. Hencbel in bis article Meteorology, 

 ' Encyc, Brit. 1 pan. 192 to 194. 



The production of wind*, 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 froui temperature ; but 

 the heating action of the sun in tropical climates produces some well- 

 known winds in the following manner. 



On account of the nnnl 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 ratified 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 ; 

 tile 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-wiud," with this excep- 

 tion, that the northern hemisphere being the warmer, the resultant ia 

 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 monsoom, are 

 produced, approaching the north tropic in summer, and the south in 

 winter, and blowing in the Arabian and Indian seas 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 aa 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 

 tireete, which ia felt during three-quarter* of the year from Nev. 

 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 current*, and are piercingly cold 

 ia general The great radiation of arid plain* produce* many local 

 wind*. *ucb a* the lirocco from Africa to Italy, and the destructive 

 simoom of Arabia, Ac., but they do not depend on general atmospheric 



The gil*r number of the several subjects the knowledge of w -hich 

 eonatitut** the science of Meteorology, will be found distributed 

 throughout this division of the ' English Cyclopwdia 1 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 head* of the 

 principal branch** of the science. That of the form, general nature, 

 and extent of the gaMOO* envelop* of the earth U treated of under AT- 

 Moarunut, and partially returned to in the article COMKTS, Pkyrical 

 CtaMtihtfiea of. In the first-named article, and also under Am, its 

 riiaminsl nature and relation* are described, and iu agency in changing 

 the condition of the bodies composing the earth'* surface and em- 



ATnosrHtxic Ixri.crxct. The 



article* AcnxouETEB, TutruiATUBX, and THIBMOUETEK, belong, the 

 Bnt**Xir*ry,and the latter in part, to it* relation* to heat; while some 

 eonoqa^ce* of it* action on light are described in those of ABRIAL 

 raaaricTivz ; RtrLicnox and RETRACTION, ATMOSPHERIC; and Ti- 

 UOHT. Tiir (object of the procure of the atmosphere and it* 

 variation., and their connection with the weather, are explained under 

 BaMMKTM ; BOILI xu or LIO.CIM ; and MANOMETER. The consequence* 

 of the change* induced by variation* of temperature in the aqueous 

 onetrtuent of the air, funning ao important a branch of practical 

 MvUorulugy, are described under the following bead*: CLOUDS; 

 Dw; l>l>oiXT; Kva ro*ATio* ; HAIL; HoaHii... r llrr.aoMC- 

 TIB and HTOBOMITBI; MIST; RAM; RAU.-CAUOE ; SNOW; THAW. 

 The phmomsn* d*p*nJ*nt upon the mechanical affection* of the air, 



and the philosophy of wind*, storms, and cyclone*, are treated of in 

 the article* AflMKDVaUim ; AlR ; ANEMOMETER ; ATMOsraEU ; 

 CALMS I.ASTIC ; MONSOON; SAMIELI ; SIROCCO; TOHXADO; 



TvrmxiN ; WIIIHI.WINH; WIND. The electrical part of meteorology 

 will be found under ELECTRICAL Li > KICITY, ATMOSFUXBIC ; 



and Ln.in M.M; ; a certain subject relating to thia branch of the 

 subject being also noticed under Bai i. CLIMATE and 



WKATIIER, and the apodal subject* of DUST, ATMOSPUERIC : 



-; METEORS, IGNEOVS or Liuixois; and PULAII LIGHTS, are 

 discussed under those heads. 



METEORS, IGNEOUS OR LUMINOUS. The following is a con- 

 densed statement of the present condition of knowledge in the 

 department of meteorology relating to these objects, which may 

 now not improperly be termed meteoric astronomy, and which is con- 

 nected in a remarkable manner with coamical science, and also with 

 almost every branch of physics and of the htudy of inorganic nature. 



The spaces through which the bodies of the solar system and the 

 comets revolve about the sun, appear also to be traversed by celestial 

 bodies comparatively minute, but in number incalculable; which, in 

 common with the smaller true planets of the system have received 

 from certain astronomers. and physicists the appellation of Asteroids, or 

 star-like bodies the word star being taken u> its universal and ancient 

 sense of any luminous object seen in the heaven*.. [ASTEROIDS.] 

 Zone* of these bodies, consisting of countless myriads of them, we 

 have much reason to infer, revolve about the sun in nearly th.-ir ,./. n 

 planes, or in some plane which they include ; and certain astronomers 

 consider that some of them have been proved to be minute satellites of 

 the earth, as was believed, indeed, early in the scientific history of this 

 subject. 



These smaller asteroids, when they approach within a certain com- 

 paratively small distance from the earth, are, or subsequently become, 

 the objects termed shooting-stars, fire-balls, and igneous meteors ; new 

 series of phenomena being successively presented by them as they 

 come nearer to the earth's surface, and especially in many instances, in 

 consequence of their finally passing through a great extent of the 

 atmosphere in an oblique direction, so as to experience the effect* of its 

 increasing density, both physical and chemical, throughout a trni 

 of great length. But in this general statement it is not pretended to 

 deny that objects and phenomena very different iu their origin and 

 nature, as well from each other as from those now under review, may 

 probably be included also under the popular or only half-scientific desig- 

 nations of meteors and shooting-stars. 



The appearance of these meteors is in many cases attended or suc- 

 ceeded by the fall of solid bodies, either stones or metallic iron, or iu 

 some cases both mingled together in the same block, forming a series 

 of bodies collectively termed Meteorites (or AEROLITES, in NAT. HIST. 

 Drv.), consisting, mineralogically, of two principal groups, graduating 

 into ev;h other, namely, meteoric stones, and meteoric iron ; and 

 which have also received the appellation of Uetcuric Soclct in the classifi- 

 cation of the mineral aggregates to which the term ruck has in modern 

 science been extended. 



The visible meteor, when observed at those distances which must 

 be within the atmosphere, must consist of flame or gaseous i 

 in an ignited or incandescent state, and undergoing combustion : l.ut 

 arising, a* may be inferred from Sir H. Davy'* researches on flan. 

 from the combustion of matter which under ordinary circumstance* 

 can exist in a gaseous state at the surface of the earth ; but from 

 that of matter which i* there solid, consisting, doubtless, of the 

 metallic or other combustible bases which meteorites are found to 

 contain. Among these, it may be remarked, are sulphur and phos- 

 phorus, both which Davy particularised a* capable of com! 

 in air rarefied to a degree equal to that of the regions of the 

 atmosphere in which meteors had been observed to display their phe- 

 nomena. No particular stress, however, can be laid upon this 

 circumstance, as meteorite* contain other combustible bases which 

 very probably have the same property, including the metal magnesium, 

 an element of almost every meteoric stone, in considerable pru[>ortion*. 

 The extraordinary intensity of light also which attends the cinni 

 of magnesium in oxygen, may be connected with the brilliant ] 

 mena of the meteors. 



The luminous extensions in the directions opposite to that of the 

 meteor's motion*, ordinarily called tails, as observed in many, if not in 

 all igneous meteors, are manifestly referable to the elonp . 

 mass of flame constituting the visible meteor, by the resistance of the 

 atmosphere to their progress through it with planetary velocity, as well 

 a* in some degree by the adhesion of the air to it ; the passage from 

 the intense white light of the head or body and proximate portions of 

 the tail, to the red light of the distant and extreme portions of the 

 latter, being attributable to the cooling down of the flame during its 

 course, and in proportion to it* distance from the most intensely heated 

 part of the meteor. 



It would appear, both from theory and observation, that the figure 

 of the meteors must approach in their nmrse more or less closely to 

 that of the solid of least resistance. The meteorites which fill, or are 

 cart down from them, when unbroken, especially those consisting of 

 metallic iron, retain approximately the form which had thus been 

 imparted to the meteors of whirli they were the inirl i. l,y thr resist- 

 ance of the air, a form resembling that of the more perfect volcanic 



