Air. In otiher words, vapours ‘ are either dènfe-and heav^Ipr in a.
rarer flatê; now fpdciöüsfeotobinatións, now {mailer andi more c«n-
- traded; ' differently fhap’d, their furfaces .either- doping f on -horizontal,
concave at one time and convex at ! another. Thefc
ufievenneffes- of the ■ vapoürous contents t of the • Atmofphere gmuft
dilpofe the'{paces of Air which• lie betwixt and on every fide:of
them into equal irregularities; fometimes intovnarrow gutsJand ftraits,
fometimes into wider and more extended ehanels; noW perhaps
feme hundreds of leagues long, and fometimes not a mile; here a
tall column of Air depends, there it is fcompreffed into a fpreading
oblate dilk, all caufedfoy the different fizes; {hapes, .-and Lubdance of
the Clouds and Vapours, and making thé Atmofphere fomewhat like
Earth which it invefts, full, if I may fo eXprefs myfelf; ■ of fleeting
mountains, hills, plains, valleys, ftraits, and expanfcs. v ^hisds^hq
general form o f the Atmofphere; andiweré i^pbfSble rfor.the fortman
eye to comprehend this éxtenfivé -proved, and -tcraote5 thè’ïuccéG-
iive alterations made in the Air by repletion and vafcuity,: hydfext
and cold, we fhould bef«o more furprifed to.. fee fuch a multifarious
fluid perpetually in motion, refilled, protruded,; .com-
denfed, expanded, retarded, or accelerated,, in its flifféVéöt! parti,
than we are tof obfervè thed various ‘‘ëdcfteYf ifbrrents^jrfwift
ftreams, and ftiller "pools.; of a. large river.J ^dFor. in^aince,kki
the latter cafe, if we fee! the current ftrohgand fwirt, :we .attrfo
bute the velocity to the narrowing or fhelvihg:!cMnël,‘ .for «perhaps
to the additional influx of extraordinary Rain or Snow; but n
we find it impetuous and irrefiftible; we toriolud& the'banks li r e :
given way, and the Water, ftruggling. tor’ défcerid, .rtflfoes fo;ifoaf
place where there is leaf! refiflance. As; théfePSre the oeaufes .of
dired flfeams and their different velocities, the Oppofltion cm little
iflands and projecting banks in a river are ;the caufes dEi delayefl.
and Crooked currents. In like manner,the. ïpaees' between - thé
floating vapours are fb many ehanels through which the Air: paffes;
ever ftruggling to maintain an equilibrium, and tending to!any_fpace
which is lefs replete in one part than in another. Wind is nothing
more than Air in motion ; rarefadions and vacuities in the Atmofphere
are the immediate caufes of this motion, and are produced
frequently, we may fay continually, by feveral different! caufes. The
Sun, by concurrent circumftances in- land, water, and vapour,
lightens and difperfes the Air from one place, and at one time,
more than at another. Inflammable exhalations and their explofions
fhall warm and thin: the Air in particular places. A cloud or portion
of Vapour full of eledrical matter, palling near a cloud or region
of land more deftitute of eledrical matter, will fhed ftreams of fire
upon the lefs eledric body, and thereby excite violent motions.
Great
Great fall's of !^.ain and Dew fhafl make (y^ay for the Air- t o expand,
[become rarer, and caufe. an indraught pf. that which is heavier.
Gondenfatiotis p f the Air in ont^place will fometimes produce rarity
in another-;, a body ofVapours intercept and obftruét the cqmr
mUBiea|tiQ0:sO-f the.. feveral airy pa|ts!:of . th,e Atmofphere,, aiidconfe-
tquently,prevent an equilibrium. Now, whsre-^eyer the Air is thrown
into, a flate of iprefadfon, the|^p. vacuity is produced, and tflf
adjacent Air flows, as Water to thp^bfeacbj o fq {cJam, and „the flood
is-either violent or not, a« the fpaee through \yhich it paffes, is fhaped,
lafling a; the quantity qf - the fluid in Apd as the extent
of the vacuity is, which is Jp, b e replenifhed, If the,,vacuity he
Ipacfous, the flow will be, plentiful; (obftrudions in ^ yyay being
allowed for) if the chanel through which the influx runs be long,
narrow, and! funnel-like,[jlfoe velodÉtjy syifl bé greats'and vice versa;
bnt-fjf- a large quantity of conden^l Air chances at this .time» to
prefs forward-towards this largp-vacuity, the paorion of/the Air will
be impetuous, or what we call a Storm«, Ifl .on flip other hand,
the rarefadions in particular diftrids be gentle, and there is room
for; denferAir tq fbqepfd without violence, the mprion afloris gentle;
and where.’no extraordinary rarefadions are produced, and tfie
pours are equally difperfed, a.Galm enfues. , If the Vapours afliime
the fhape- of an oblate difk, pveripreading as a-canopy.a jyitfc
^hfiW©fghttvund; 'CQntintftty t'fif' thé inerifobeflt,Anr in this dxffoiél
for a time, and to a pertain degree,. tfufpended, the Mercury, flpfos'
in the Barometer, and
this dilk fhall go one way, towards any vacuity wjhfek1MI ematep*
ftofh tendency, and the under current of Air, influenced by another
mtefaflion, mall go on in a (flffemnt, perhaps oppöfitc.dheöjion,
there being no communication betwixt the currents above and be--,
lo y the difk of vapours, fufflqient to. determine them tapnepoint.
Thus again, by the .fall or even the recefs ,of a great body o f vapours in
One place out of our fight, the pir.ov-^ QW fields before condens’d,
and keeping the IVfercury high, extends itfelf into the jpa^i^y ;^ t^
wind blows*‘as we fay, and the Mercury fi|ls in a f^tene fky tq put
fhrprize. By the riling of a like foody pf vapours, and accumulat-.
ing the air of our horizon, the Mercury rifes: to a domly and even,
rainy Iky. When the Wiod $ violent, -the perpendicular preflhre of
the air is much leffened by thip yplocity of the hprizontal procefs »(as
a wheel that runs fwiftly makey-not fo deep an- ,imprefli0fl. 35 when
flowjy moved) and the Mercury,fells. , When the air|^s; fullefl.of
vapours,, the Mercury falls, tlre;preflure of'the atmpfph«:e depending
ne|f|o®ly on the weight o f lhe i • but pq thé agility^
and elafticity of the-column of air which is. broken and intercepted
by fuch a quantity of moifture floating between, coodenfing and
E f ; w X) ' ^ " ready.