£gg|§ T I O N*
there is every where, land within the ufeal limits'; ,o£ the
latitude of 30°, viz. Arabia, Psrfia, India? &c. which
for the fame reafon-as the mediterranean parts o f Africa,
are fubjeft to unfuffetable heats when the fun is to the
north, palling. ne,arly- vertical, .but yet are temperate
enough whentthe fun is removed towards the other tropic,
Ivi I N T R O D
north- èaft win’d- on the north fide o f the equator. A n d I
in like manner, therp’’ mu ft be a fouih-eaft ntipd on the
feqth fide. The air which is thus continually moving
from the poles towards the equator, being rarefied when I
it pomes, there, afcends to the top ó f Hip atmofphere, I
and then return» back to the, poles. This folutioa is
given by ,Mr. in tbe Phil. Trattf. vols ,39.
' 267. The periodical winds are fuppofed to be owing I
to the,cqurfe of,the fun northward and fbuthward of.tbe I
eqpptqr/ Dr. H alley explains them fhus : feeiqgj
tn j t fo great Continents dq mterpofe and break the continuity
o f the Ogean, regard mutt be had to the nature I
o f the foil, and the pedition, o f the h ig h mountains,
which' I fupjiofe the two principal caMes o f .thefcycral I
variations o f the winds from the general rule : for-if a
country lying: near the fun prove, to be flat, fandy, low
land, fuch as the Defarts o f Libya are ufually reported to 1
be, the heat occalioned b y the refle&ion o f the fun’ s I
beams, and the. retention thereof in the fand, is incredible
.to thofe that have never felt i t ; whereby the air
being exceedingly rarefied, it is neceffary that the cooler
and more denie air flionldrun thitherward to reftore the I
equilibrium. This I take to bp the caufe, why near
the coaft o f Guinea, the wind always, fets in upon the
.land, blowing-wefterly in (lead o f eafterly, there being I
fufficient reafoirtpbelieve, that the inland parts ofjlfrica
are prodigioufly hot, fince the northern borders thereof
were fo intemperate, as to give the ancients.eaufe to conclude,
that all beyond the tropic was made uninhabitable
b y excefs o f heat. From, the. fame caufe it happens,
that there are fo"~confl:ant calms in that’ part o f the
Ocean called the Rains. F o r this trail being placed in
the middle, between tlie wefterly wónds blowing on the
coaft of, Guinea, and the eafterly trade winds blowing to
the weftw-ards thereof, the tendency o f the air here is indifferent
to either, and fo Hands in equilibrio between
b o th ; and the weight o f the incumbent atmofphere
being diminifhed b y the continual contrary winds blowing
from hence, is the reafon that the air here holds n ot!
thé'.copious vapours which it receives, but lets them fall
in fo frequent rains.
2.68, A s the cold and denfe air, b.y reafon o f its greater
gravity, preffes upon the hot and rarefied, ’ tis demon-
ilrative that this latter muft afeend in a continual dream
as fall as it is rarefied, and that being- afcended,: it muft
difperfe itfelf to preferve the equilibrium, that is, b y a
I contrary current, the upper air muft move .from thofe
parts where .the greateft heat i s : fo b y a kind o f circulation,
the N. E . trade wind below, will be attended
with a S . W . above, and the S. E . below noth a N. W .
wind above. An d that this is more than a bare conjee,
ture, 'thé almóft inftantaneons change o f the wind to the
oppofite point, which is’ frequently found in palling the
limits' o f the trade winds, fee ms to, aifiire u3; but that
which-above all confirms this liyppthefis, is this, that the
phenomenon o f the monfoons is,: b y this means, moll
' ealily folved, and without it, hardly explicable, ■ Siip-
poling therefore, fuch a- circulation as above, ’ tis to be
confidered, that to, the northward o f the ItfflanWMm
becaufc, o f a ridge, o f t mountains at* fome diftauce
tijupA thé,lai},<i>ukld tq he, frequently.in wiaferu,QOvered,
with fnow, over which the air, as it paffes, muft needs
be much chilled. Heqee it Ggmea jo pafs, that the air
ccuping, according Co the general rgle, opt o f the N. E .
in,the Indian Scas,yis fqntetii£te% hottef^fom^tilhês'.eolidjer '
than that which,"by tliis^circulation,-is returned,out' of.
S,,W . a,spsgle af'frqm.fe6fri^ïw®^injj31qfe)wind^^?t
m,’ viz in when | l e J» <wmrm thofev
countries to the’ north, th eS . W . monfoon begins, aidET
blows llhrrng the Heats t i l l Oahber, wfttn’ tht,*fun.o-.iog
-retired jand aK t r i g s ' grhiwin^i coldst" SorthvPam, and
the heat increafiiigto the "feutli, ,tile N. E . m u h differ
and blow aft*the.winierytiU..^ein7 again; .
, "260.' A n d a t is undoubtedly from the fame principle,'
th a t to. enl'^iith v fetd pf'tnqpequator, in-part o'flfliq
Indian Ostóij,'the FF W . winds^fucceeu fnC ,W,E. when
thédÉn? draws near the tröpie IfGdprècoIhf'- dlïit -ï’fniiü:,
, confefs, that in this latter occurs a’ difficulty nor -Ael? to
i -bt'aOeaóirfeed'fOr, wli-icli is, why this changejofdhé mon-,
■ ftfeiA-Ahifkld-'-be any more iflftthia'Oêean, thin in the
Ifaiiie'Iatifiitiés, m'cthe-' fdhiopic, where ï« l| P i s n uHtrftg
Imore berfain-tlrapta.S , I L a w i n d year.
I *' znp. ’T h hktAjfi v i ' h i d to lO u u v c , why thp
limits o f the trade.winds flioadd be fbtefl-aboutl E e j i 1*
o f latitude allround the globe, and tnat'they iltould fo 1 fêldoih tratifgrefs of fal 1 fh o lto f thofe bounds ; as alfo,
that in the Indian Sea,,oojy tliL\.nc>rtherapart' |hipu'l(f be
Ifubjecl'to tbc changeable mopfobus, and ip tiui fputireru
there lllfeulirbe'a èonflant fy .tfA ’
I '2 7 Ï .' Ther’é'may perjraps -be fii'md'ca^itj's o fthrlé pq-
I riodical winds, whteb we-'can lipt fee aiVpgeuiér a folutlon
I o f j .b o t if all thelfirSumftanCes o f f^tlidtpiy heat,'cql.ii,
I & c. weit known, thcVc 1» no icJftm lg
they might be .miiouiitcd lot hou the prmt ij Ii, hu e
delivered..
27c. Wind o u r Ihe f me place, al d fiere it,'•IftuiJtS,
ate found'tg-blow in.'(Efferent d rehii ns ; f 1 we tec
.clouds at" différent altitudes. .móving.Tin different, direc-‘
tions ; and experiments with air balloons prove the
I Jamei - ..
I 273. W e may rurthei ohferve in rc-fpeft to the-dired-
tion in which winds blow, that i f a current fet off in anyone
direfriori, north eaffc. for inltanee.'-'and move in a
great circle, i t will riot continue-to- move on that point o f
the compafs, becaufe a great1- circle • will not meet all the
meridiansJ it the fame angle. TliiS-'circumftance therefore’.
-Ihould enter.into our confideration, in ellimating
the direction o f the wind. " High .'.mountains lire alfo
. obferved to icEaïïge its direélion. On the lake o f Geneva
there are only two winds, that is, either up 1' '' ■
valley. A n d the like is known to happen.at other filch
I plabeij.
I N T R O D U C T I O N;
1 a y4.' T he cinjlaut .in i, periodical winds blow only at
Tea 1 at-iand, the -win'd is always •uaridlle.
: 2yy. Befides the. winds a 1 rea'dy- inen ti0ncd, there,are
others called^ land and fea breezes. 5j®lfe|aii| over the
lain . w day than the air over the
fea, a cufreri.t .of.air w ilf fet' in iroiu the fea to the, land
b y oyer the fea,bei®g ■ hotter than that
over i!ui Ir.r.d at n ight, the current at night will her from
•the.-land to th e i!T e a .T h is is'.very ■ reniarkabfoteiifiamda
i 1 between the .-tropics- , Mri C:LA'BrE';;exem-j|liftes
‘this,>b y ■ «tlgffollo.wing'. ex penment.In.'thbismiddletofjfe:
■ yelel of, water, place a water-plat e of warm water, the
water in ffie'^effeli-reprefentingithe ocean, and the plate,
iSsilland.rarefyingthe air over it. Then hold a'lighted
caiidl'e, o.yer: tlieoKold vva.ten, an<1 blow it opt,, and -the: j
fmokc will .mo've towards the platei .yBut if the,plate be -
“bvand1 the 1 ' 1 yteid warm, the' finoke will.1
move ill the contrary (lireirior.. • The ica breezes ili the
W eft .Indies .begin to ajipeai- aboiit.-g in the -morning, ..in
curl upon the- waterfaj p on Hi gi the'rmd'r.ci;;.
i^nppfldfet; igradpaily. till 110011, and dies away at 4 oi|
5. in the ffiehiibeii.. Ab ou t ,6' 1 it change ,
to. a landrbredz^ whicp^feivva'.from the land .to ',fife fear;:
and tails till S. -in thc.inorning . '
- 276. D r. D erh am, from repeated obfervations upon
the,' mol-iou o f iiglit downy feathers; .-.found .that the1.
greafeS velocity of t'u wind wa notrlab 1 6 mile in
an hpur. Bu t-M r. B rice jullly. obferves,-iJSiit' fuel«
experiments mn'ft:be fubjeai.tb great u ;"..v the
feathei cannot proceed in 1 til ght line he thercfdre
eftimate the velocity by mean of th fiiadow o f a ploud
oyer the'earfb, by which hejound, that in'a great ftorm,
th'i iyiffd moves 63 riiiles in 'an hqurT;: when J
frefli gale, at the -rate o f 21 miles in an Ii6ur';-and:m a"
finall' breeze, at' Ithe'rate o f about 10 miles iri-an hour :'
bu t :this';m6thb|; takes for granted that the clouds -move as
■ fail as' the. wind. l t is profeble that the velocity .is foiiic-
thingmore than is here-dated. Mr. Rouse makes^the
velocity- b f a; hurricane which tears up frees, &p. to be
i s o &&& in Sffi hearv'ii' V'
. 277. Therg'are certain lakes which,’at times, are agitated
during a calm -feafon, b/ptom'c'd'nkhbwi' 'ca-flt 'p
and- the plnenomenon is called a bottom windIs Mr.- D.al'-
ton, in iiis Mrtcdrolngicul Obfervations, informs us, that!
Mr. CitpsTHwiiTE has been pretty, affiduona
fefpedling tliefe phienomena, and in
;bbferv.iihg '-any icircumftances which ;might lead to a1 dif-
covety o f the caufe' ; but nothing has .yet occurred to
him, thalSprpmifcs*to throw any Kg-ht’-Upon the'»Bjtfe1&: |
m Qbfervations made on Derwent Lake. >'
■ t. A p r il 30. , 18 A . M . till noon, the lake pretty
'.•much,
Align'd 9. A t 8 A M. the lake in very great agita-
'tion ; white breakers on large wavesT&cirwitlfe'ntiwind.
•Augiift 29. A t .9 A . M.'a finall bottom wind.
I W m m M n
■ June 26'; A t 8 P. M a bottom wind' on the lake.
OAober -11. A t 8 P. M a bottom Wind on the lake.
§ Dcccmlicr 1. A t 9 P„ M. a ftrong bottom wind on
the-lake.'
m K KS B m S 1 1 K l l p M W m $ * %
■ "OStober' 28. A t 1 ,P -M . a bottom wind ; the water
. much r.giliitcd. j -
278. -In,many parts of the world, more particularly in
-tb#i Weft Indies, they are attacked'- by hurricanes ; th e ft
ii'apficn '-there -in' the rainy feafbnj principally in the
jmbnl|gi'ip'f^Aug-u.flf;. deftroying all the produce o f the.
gro.ll'hd'-;: ptearing u-p -trees ; blowing down buildings ;
and imiiiilaling large trails o f ilie-.cqiinti-y. T hey arc
. - -and very :v'i6leri.t.flor.ms of wind, rain, thunder and
'hgh'S^Jalt-tended with a great fwelling o f the fea, and
Kw etime^^illi eai tliqmkes ■ There ace figns by which
the inhabitants are warned 'of their approach. "• T he y
come oneiifrer; at'tlfe quartefj'ipr at' the full change o f
thelmphbp'iTf they conre, onpaththeifull" cliange,-then at
the preceding change, .the fi<y is, troubled, the fun more
red than ufual, thette is a dead' calmibelow, and the tops
v of-the-.mpunlains are > _ ‘ 1 thofe imifts-.w-hieh ufually.
liov^ria&outthem-. In the caveiirisip'fithe earth,-and' in
vvells, iyou .hear a ;6qiiqwsrnmblii^®und, like the rufiling
o f a great wind.' I v ' feem much
larger than ufualf and afe fuiTOunded-v/ith a fort o f burs ;
the bbiith-weft ficy has a'-blaefe ahdfmehkdmg appearance ;
the .feahertTitsA' ftrdng fmell-, - and" rifes into vail waves,
often without any wind. The wind itfelf now forfakes
its ufual ealliem heady' ftream, and fiiift-s .about- t.ojthe
weft,,’ from whence it ribnieiime;;,-'with intermifiions,
Bfews-vio'lently arid irregularly for about;-/- hoar.-; atatime-o
■ 279. Then quality o f air .depends in a great meafure
upon the foil over udlieK-itbpi-ffes; '/ The fandy deferts
o f Africa-and ^ , burning beat and (blading
quality to tbe air palling oyerbt-hem1 A t Gorte, in the
fiver Senegal, there: is an eakeriy wind from' the inland
parts; -witfriwM'cM thofe who are fuddehl.y met by it in
the face'are fcorched, as by a blaft from a .furnace. A t
Falkland ^Iflands an extraordinary blalling wind is'-felt,
- But its duration list feldom ' above- 24 hours. I't cuts
rlown the herbage, as if-fires hadbecn made under them;
the leaves are parched'' up and ..crumble into dull-;
fowlsk-reTeized with cramps," and never, recover; men
' are .opp-refied with a Hepped, p.erfpiration, heavinefs. a t tire
brea ft, kid fore .throat, bnt tliey recover with proper
care. But the moil dreadful winds are thofe at theade-
\ ’ mv.sor, mortifying, wind.
T he camels -perceive- their approach, and are laid te
make an umiftial noifelarid cover their nofes in the fand.
T o . efcape their 'effe&ptravellefs' -.throw tlvemfelves as
clofe as poflible on -cheg.rottrid;'affifevait till.it haspaffed
over,'.whjcheis;.cbmmbn'ly:in a few minutes. Thus feme
t'fcape.; ■ but thofe who; die", 'have their limbs mortified-
l f this wind -meet- with a. (hower-of rain',' it'is;fa.'d to be
'-deprived o f its noxious,quality. I t is alfb fa il; that it
-f;5 1 «ever