Page:Cyclopaedia, Chambers - Volume 2.djvu/1025
Eaft, than that neater the Poles Thus, as the Earth
turns Eaftward, the Particles of the Air near the Equinoc- tial being exceeding light, are left behind, fo that in re- fpefl of the Eatth's Surface, they move Weftwards, and become a conftant Eafierly Wind.
This Opinion feems confirmed by this, that thefe Winds are found only between the Tropicks, in thofe Parallels of
Latitude where the diurnal Motion is fwiftell —But
the conftant Calms in the Atlantick Sea, near the Equator, the wdterly Winds near the Coal! of Guinea, and the pe- riodical weiterly Monfoons under the Equator in the Indian Seas, declare the InfufHciency of this Hypothefis.
Eefides, the Air being kept clofe to the Earth by the Principle of Gravity, would in time acquire the fame degree of Velocity, that the Earth's Surface moves with, as well in rcfpcc~l of the diurnal Rotation, as of the annual about the Sun, which is about thirty times iwitter
Dr. Halley, thetefore, fublbtutes another Caufe, capable of producing a like ccnlfant EfFcft, not liable to the fame Objections, but agreeable to the known Properties of the Elements of Water and Air, and the Laws of the Motion
of fluid Bodie:. Such a one is the Action of the Sun's
Beams upon the Air and Water, as he patTcs every day over the Ocean, confider'd together with the Quality of the Soil, and the Situation of the adjoining Continents.
According to the Laws of Staticks, the Air which is lefs rarefied or expanded by Heat, and confequently more pon- derous, tnuft have a Motion towards thofe Pans thereof which are more rarefied and lefs ponderous, to bring it to an Equilibrium ; alfo the prcfence of the Sun continually Shifting to the Weft ward, that part towards which the Air tendsby reafon of the Rarefaction made by hisgreateft Me- ridian Heat, is with him carried Weftward, and confe- quendy the tendency of the whole Body of the lower Air is that way.
Thus a general Eafierly Wind is formed, which being imprefs'd upon all the Air of a vatt Ocean, the parts im- pel one the other, and fo keep moving till the next return of the Sun, whereby fo much of the Motion, as was loft, is again reftored ; and thus the Eafterly Wind is made perpetual.-
From the fame Principle it follows, that this Eafterly Wind ihould on the North Side of the Equator be to the Northwards of the Eaft, and in South Latitudes to the ■•'Southwards thereof; for near the Line, the Air is much rnore rarefied, than at a : greater diftance from ir 5 becaufo the Sun is twice in a Year vertical there i and at no time diftant above 25 r Degrees : Ar which diftance, the Heat being as the Sine of the Angle of Incidence, is but little fhort of that of the perpendicular Ray ; whereas under the Tropicks, tho' the Sun ftay long Vertical, yet he is a long time 47 Degrees oft"; which is a kind of Winter, wherein the Air fo cools, as that the Summer Heat cannot warm it to the fame degree with that under the Equator. Wherefore the Air towards the Northward and Southward being lefs rarefied, than that in the middle, it follows, that from both Sides, it ought to tend towards the Equator. See Heat.
This Motion compounded with the formerly Eafterly Wind, accounts for all the Phenomena of Ak general Trade- Wind's ; which, if the whole Surface of the Globe were ., .£isi > .-.would undoubtedly blow quite round the World, as
rtW found to do in the Atlantick, and the Ethiopick
Ocean?. But feeing fo great Continents do interpofc, and I
break the Continuity of the Oceans, regard muft be had to - the Nature of the Soil, and the Petition of the high Moun- tains, which are the two principal Caufes of the Variation of the Wind from the former general Rule ; for if a Country lying near the Sun prove to be flat, fandy, and low Land ; fuch astheDefarts of Libya are ufually reported to be ; the Heat occafion'd by the Reflections of the Sun's Beams, and the retention thereof in the Sand, is incredible to thofe who have not felt it; whereby the Air being exceedingly rarefied, it is tiecetfary, that this cooler, and more denfe Air, mould run thitherwards to reflore the Equilibrium.
This is fuppofed to be the Caufe, why near the Coatt of Guinea, the Wind always fets in upon the Land, blowing "Weiterly inftead of Eafterly ; there being fumcient reafon to believe, that the inland Paits of Africa are prodigioufly hot, fince the Northern Borders thereof werefo intemperate as t'opive the Antients caufe to conclude, That all beyond the Tropicks was uninhabitable by excefs of Heat.
From the fame Caufe it happens, that there are fo con- flant Calms in that part of the Ocean call'd the Rains ; for this Tracl being placed in the Middle, between the Wefter- ty Winds blowing on the Coaft of Guinea, and the Eafterly tfrade-Winds blowing to the Weftwards thereof 5 the ten- dency of the Air here, is indifferent to either, and fo ftands in Equilibria between both 5 and the weight of the incum- bent Atmofphere being diminilhed by the continual con- trary Winds blowing from hence, is the reafon that the Air here holds not the copious Vapour it receives, but lets it fall in fo frequent Rains.
But, as the cool and denfe Air, by reaf.m of its greater Gravity, pretTes upon the hot and rarefied, it isdemonltrable, that this latter mull defcend in a continued Stream, as fa ft as it rarefies; and that being afcended, it mult difperfe it- felf to prefcrve the Equilibrium ; that is, by a contrary Current the upper Air mull move from thofe Parts whero thegreateft Heat is: fo by a kind of Circulation, the North- Eaft "Trade-Wind below, will be attended with a South- Weflerly above ; and the South-Eatt with a North-Weft Wind above. SccCurrent, Uudzk- Current, Sjfc
That this is more than a bare Conjecture, the atmoft in- fbntaneous Change of the Wind to the oppoiite Point,which is frequently found in pafling the Limits oj the Trade-Winds, feems to affurc us ; but that which above all confirms this Hypothefis, is the Phenomenon of the Monfoons, by this means, moil eafily fclved, and without it hardly expli- cable. See Monsoon.
Suppofing therefore fuch a Circulation as above 5 it is to be confider'd, that to the Northward of the Indian Ocean, there is every where Land, within the ufual Limits of the Latitude of 50, viz Arabia, Terfia, India, &c. which, for the fame reafon as the Mediterranean Pans of Africa, are fubject- to unfufirerable Heats, when the Sun is to the North, pafling nearly Vertical ; but yet are temperate c- nough, when the Sun is remov'd towards the other Tro- pick, becaufe of a Ridge of Mountains at fome diftance within the Land, faid to be frequently in Winter cover'd with Snow, over which the Air, as it pafTes, muft needs be much chill'd Hence it happens, that the Air com- ing, according to the general Rule, out of the North-Eait, in the Indian Sea, is fometimes hotter, fometimes colder, than that which, by this Circulation, is re turn *d out of the South-Weft; ar.d by confequence, fometimes the Under- Current, or Wind, is from the Nortb-Eall, fometimes from the South- Weft.
Tnat this has no other Caufe, is clear from the Times, wherein thefe Winds fet, viz. mA$fil% when the Sun begins to warm thofe Countries to the N T orth, the South- Weft Monfoons begin, and blow during the Heats till Oc- tober, when the Sun being rerir'd, and all things growing cooler Northward, and the Heat increafing to the South, the North-Eaft enter, and blow all the Winter, till April again. And it is undoubtedly from the fame Principle, that to the Sourhward of the Equator, in part of the In- dian Ocean, the North-Weft Winds fucceed the Southr- Eaft, when the Sun draws near the Tropic of Capricorn* See Tide.
But, the Induftry of fome late Writers having brought the Theory of the Production and Motion of Winds to ffjmewhat of a Mathematical Demonftration; we /hall here give it the Reader in that form.
Zaws cf the Trodutlicn, Sec. of Winds.
If the Spring of the Air be weaken'd in any place, more than in the adjoining places; a Wind will blow thro* the place where ihe Diminution is. See Elasticity.
For, fince the Air tndeavours,by its elaftic Force, to ex- pand itfelf every way j if that Force be lefs in one place than another ; the Nifus of the more, againft the jgfaAaf- tic, will be greater than the Nifus of the latter a*din%,tjie
former The lefs elaftic. Air, therefore, will rdj&vw-Sl
lefs force than it is urg'd, by trrp more clai^B--£\*Gd , nfe- quently, the lefs elaftic will be driven out. .g^a^pj^re^ and*" the more elaiiic will fucceed, ■ \ /.^^"w'*""
If, now, the Excefs of the Spring* of ;ihe" more elaftic, above that of the lefs elaftic, be fuch as to occafion a little alteration in the Barofcope ; the Motion both of the Air expellM, and that which fucceeds it, will become fehfible.
2. Hence, fince the Spring of the Ai.r .increafes, as the comprefling Weight increafes ; and coniprefs'd Air is den- fe r than Air lefs comprefsM : All Winds blow into rarer Air out of a place til I'd with a denfer.
3. Wherefore, fince a denfer Air is fpecifically heavier than a rarer; an extraordinary Lightncfs of the Air in any place, muft be attended with extraordinary Winds or Storms.
Now, an extraordinary Fall of the Mercury in the Baro-
meter, .'-Viewing an extraordinary Lightnefs of the Atmo- fphere; 'tis no wonder, ; f th.it foretels Storms.
4. If the Air be fuddenly condens'd in any place, its Spring will be fuddenly diminish 'd : Hence, if this Dimi- nution be great enough to aftvet the Barometer, there will a Wind blow thro' the condens'd Air.
5. But fince it cannot be fuddenly condens'd, unlefs it have before been much rarefy'd ; there will a Wind blow thro' rhe Air, as it cools, after having been violently beared.
6". In like manner, if Air be fuddenly rarefy'd, its Spring is fuddenly increas'd ; wherefore, it will flow thro* the contiguous Air, not acted on by the rarefying Force—