OF CORNWALL. 9 
Great falls of Rain and Dew {hall make way for the Air to expand, 
become rarer, and caufe an indraught of that which is heavier. 
Condenfations of the Air in one place will fometimes produce rarity 
in another; a body of Vapours may intercept and obftruCt the com- 
munication of the feveral airy parts of the Atmofphere, and confe- 
quently prevent an equilibrium. Now, where-ever the Air is thrown 
into a Rate or rarefaction, there a vacuity is produced, and the 
adjacent Air flows as Water to the breach of a dam, and the flood 
is ci flier violent or not, as the fpace through which it pafies is fhaped, 
aiung as the quantity of the fluid fet in motion, and as the extent 
Oi tne vacuity is, which is to be replenifhed. If the vacuity be 
pacious, the flow will be plentiful; (obftruCtions in the way being 
allowed for) if the chanel through which the influx runs be long, 
narrow, and funnel-like, the velocity will be great, and vice versa; 
at r A a birge quantity of condenfed Air chances at this time to 
prexs forward towards this large vacuity, the motion of the Air will 
^ impetuous, or what we call a Storm. If, on the other hand, 
the rarefactions in particular diftriCh be gentle, and there is room 
for denfer Air to fucceed without violence, the motion alfo is gentle ; 
and where no extraordinary rarefactions are produced, and the Va- 
poun are equally difperfed, a Calm enfues. If the Vapours affume 
t e nape or an oblate difk, overfpreading as a canopy a wide extent, 
the weight and continuity of the incumbent Air is in this diftrid 
for a time, and to a certain degree, fufpended, the Mercury finks 
j.f >ar ? 11 jf ter ’ and at the fame time the current of the Air above 
t is difk mall go one way, towards any vacuity which fhall create a 
frefh tendency, and the under current of Air, influenced by another 
rarefaction, fhall go on in a different, perhaps oppofite direction, 
there ^ being no communication betwixt the currents above and be- 
0 w the difk of vapours, fufficient to determine them to one point. 
1 Uls agam, by the fall or even the recefs of a great body of vapours in 
one place out of our fight, the air over our heads before condens’d, 
and keeping the Mercury high, extends itfelf into the vacuity; the 
wind blows as we fay, and the Mercury falls in a ferene fky to our 
.ur prize. . y the riling of a like body of vapours, and accumulat- 
ln P air ° our _ oiazon j the Mercury rifes in a cloudy and even 
ramy. xv. nen the Wind is violent, the perpendicular preflure of 
ne an is much leffened by the velocity of the horizontal procefs (as 
n U ee tl5at runs Swiftly makes not fo deep an impreflion as when 
lowl y mo f d ) and the Mercury falls. When the air is fulleft of 
vapours, the Mercury falls, the prefibre of the atmofphere depend- 
ln ^ T a ° nl y on die we igbt of the fluid, but on the agility, 
i‘ n r C t uC1l 7 or tbe column of air which is broken and intercepted 
sy wn a quantity of moifture floating between, condenfing and 
D readv 
