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Show 86 vVINDS. NoTE XXXIII. fqueezed forward by an addition to the atmofphere behind them, as appears by the rifing of the barometer. TRADE-WINDS. A column of heated air becomes lighter than before, and will therefore afcend, by the preffure of the cold air which furrounds it, like a cork in water, or like heated fmoke in a chimney. Now as the fun paffes twice over the equator for once over either trop ic, the equator has not time to become cool; and on this account it is in general hotter at the line than at the tropics; and therefore the air over the line, except in rome few inftances hereafter to be mentioned, continues to afcend at all feafons of the year, pn:ffcd upwards by regions of air brought from the tropics. This air thus brought from the tropics to the equator, would confl:itute a north wind on one fine of the equator, and a fouth wind on the other; but as the furface of the earth at the equator mo-.,es quicker th:1n the furface of the earth at the tropics, it is evident that a region of air brcugh t from either tropic to the equator, and which had previoufly only acquired the velocity of the earth's furface at the tropics, will now move too flow for the earth's furface at the equator, and will thence appear to move in a direCtion contrary to the motion of the earth. Hence the trade-winds, though they conflfl: of regions of air brought from the north on one (Ide ofthe line, and from the fouth on the other, will appear to have the diagonal direCtion of north-eaft and fouth-eafi winds. Now it is commonly believed that there are fuperior currents of air paffing over thefe north-earl: and fouth-eaft currents in a contrary clireCl:ion, and which defcending near the tropics produce vertical whirlpools of air. An important quefl:ion here again prefents itfelf, ff/hat becomes of the moijlure w!Jich t!Jis heated air ought to depojit, as it c~ols in the upper rtyions of the alrtwjphere in its journty to the tropics? It has been {hewn by Dr. Prieftley and Mr. lngenhouz that the green matter at the bottom of cifl:crns, and the frefl1 leaves of plants immerfed in water, give out conflderable quantities of vital air in the fun-fuine; that is, the perfpirable matter of plants (which is water much divided in its egrefs from their minute pores) becomes clecompofed by the fun's light, and converted into two kinds of air, the vit:1l and inflammable airs. The moifl:ure contained or diffolved in the afccnding heated air at the line mufl: exifl: in great tenuity; and by being expofed to the great light of the fun in that climate, the water may be decompofed, and the new airs fprcad on the atmofphere from the line to the poles. I. From there being no con!bnt depofition of rains in the ufual courfe of the tradewinds, it would appear that the water rifing at the line is decompofed in its afcent. 2. From the obfervations of M. Bougner on the mountain Pinchinca, one of the Cordelieres immediately under the line, there appears to be no condenfible vapour above three or four miles high. Now though the atmofphere at that height may be cold to a very conflderable degree; yet its total deprivation of condenfible vapour would feem to fhew, that its •vatcr was decompofed; as there are no experiments to evince that any degree of cold hitherto known has been able to deprive; air of its moifl:ure; and great NoTE XXXIII. WINDS. abundance of fnow is Jcpofited ti·om th . - l 87 c au t lat flows to tl l - expo fed to no greater degrees of colcl - - - le po ar reg1ons thougl1 't- - - - In Its JOurney thither h ' I IS mdes height m the atmofphere at the line. t an probably cxifl:s at four 3· The hygrometer of Mr. SaulTilre a1(i . - h . I I . o pomtcc 1 to drynefs I fi :ur; t c nng e lair of which it was co ('- n d as lea cended into rarer 1 llllllcLC COiltr Cl' f ' Effais fur l'Hygromet. P· 143• ' a lllg rom deficiency of moilture. Frr om thcf_e obfervations it appears e1' tl 1er t 11 at rare and cold - _ . to 1aturate 1t than dcnfe air . or th:Jt tl .11 . air lequi re· more moilture - - . , lC n!Ol lUle be omes 1 {' 1nto <llr, as 1t afcends into thefe cold d . tccompo cd and converted T an rare regions of the at li h 4· here feems fome analoo-y bet h · _ · tno P ere. o ween t e Clrcumflanc f . b . generated iu the cold parts of the atmof h b h . e o air eing produced or p ere ot Jt the line and at the poles. MONSOONS AND TORN A DOES. t. In the Arabian and Indian feas · d . r: :uc Win s, which blow (i 1 11X mot:ths the other and ::Ire called M (i IX mont 1s one way, and fi - ' on oons; by the accid t I d'r fi . and ea 1t happens, that in fome plac I . en a 11p0 1t1ons of land es t Ie air near the tro - . C warmer when the fun is verticJ! over -t h h . PIC IS .ippofed to become t ' t an at t c b ne Th . . . h fequc:ntly afccnds preffed upon one fide b tl I . . e air m t efe places con- (j y 1e nort 1-eafl regwns f . d 1-de nby the fouth-weil: regions of air F h _ 0 air, an on the other c • or as ( e air brought fro l fi h vwu y obtained the velocity of the earth' li .c m t 1e out has pre- ' s 1111ace at the line ·1 c t1 earths furface near the tropic where 't . ' 1 moves 1aner than the h . I now arnvcs, and becomes li h fl w tie the air from the north becomes a . I fl - a out -wen wind, - not t l-ean wmd as befo I . d wmds do not fo quietlv join and afccnd th h re exp ame . Thefe two . . as e nort -eaft and fouth e fl: . d h' meet at the line with equal w:. umth and ve I OC-lty an d wc rm the t d - .a d wtnb s, w ICh meet in contrary direCtions before they afcend d b fi ra e-wJn s; ut as they lance each other a rota to t. . ' an cannot e uppofed accurately to ba- ::'~'~;~.~:~~~:~~:~ea~a~:i~:::l:~:;~;:1~~FE:~~~~~~:::.~'i~~~;~~;~~:~: c:an ln t:e v>eil: or fand from the deferts of the eal1, in lcfs violent deo-rces they only miX to?et er the _two currents of north-eail: and fouth-weft air, and J~roduce by th' means tnceffant ra1 h · f 1 15 li h _ ' ns, as t c a1r o t 1e north-caft acquires fome of the heat r 0 tl out -weil: w1nd a 1 - d- N tr m 1e 0 th r ' _s exp allle Jn otc XXV. This circum(bnce of the eddies produced li y e moniOon-wmds wa~ fee b M B . A . lr . · , n Y r. n1ce m byffima; he relates that for many uccelllve mornmgs at the , f l - f commr.ncement o t 1e ramy monfoon he obfervc~ J d o appare tl li · II I' . . ' '"a c ou l h 11 Y ma < 1menfiou whirling round with great r~pidity, :~nd in a few minutes t le eavens became cove ed . h d k I d . C . r w!l ar c ou s with confequent great rains. See Note 011 anto III. ]. 129• h 2. B_ ut it is noton1y at th e PI ·a ce w h ere t1 1 e at·r :1 (ic ends at the northern extremity of 1 e rallly monfoon, and where it forms tornadoes, as obferved above by Mr. Bruce but fol ver a great tract .o f cotI n t ry {Ie vera 1 de grees t. n lc ngth 1. n cenain parts as in the Ara'b ian eah, a P_erpetual ram for fcveral months dcfcends, fimilar to what happens for weeks to-get er 111 our own cl't ma t e 1·1 1 a 1e 1rs d egree Jun·n g the fouth-wefl winds. Another im- |