| ThesPret adh = 
Whether alfo thofe'feams, which feem to. iffve out of the Earth, and 
mix. with the Air (and fo to precipitate fome aqueous Exhalations,where- 
with *tisimpregnated ) ‘may not be by fome way detefted before they produce 
the effet, feems hard todetermine ; yet fomething of this kind Lam able to. 
difcover,by an Inftrument I contriv’d to fhew all the minute variations in 
thepreffure of the Air; by which Iconftantly find,that before, and during 
the time of rainy weather, thepreffure of the Air is lef3, andin dry wea- 
ther , but efpecially when an Eaftern Wind _¢ which having paft over 
vaft tratis of Land is heavy with Earthy Particles). blows, it 1 much 
more, though thefe changes are varied according to very odd Laws. 
_ The Inftrument isthis.. .I prepare a pretty capaceous Bolt-head A B, with 
a {mall {tem about two foot and a half long D C3 upon the end of thisD 
I put ona fmal! bended Glafs,or brazen Syphon DEF (openat D,E andF, 
but to be clofed with cement at F and E, asoccafion ferves) whofeftemF. _ 
fhould be about fix or eight inches long,.but the bore of it not above half'an 
inch diameter,and very even; thefeI fix very ftrongly together by the help 
of very hayd'Cement, and then fit the whole Glafs ABC DEF into a long 
Board, or Frame,in fuch manner,that almoft half the head AB may lye buri- 
ed ina concave Hemifphere cut into the Board RS; thenI placeit foon 
the Board RS, asis expreft inthe firftFigure of the firft Scheme and fix 
it very firm and fteady in that pofture, fo as that the weight of the Aéreury. 
that is afterwards to be put into it,may not inthe leaft fhake or ftir it 5 then 
drawing a line XY on the Frame RT, fo that it may divide the ball into 
two equal parts, or that it may pafs, as twere, throughthe center of the 
ball. 1 begin from that, and divide allthe reft of the Board towards UT 
into inches, and the inches between the 25 and the end E(which need not be 
above two or three and thirty inches diftant from the line X Y) I fubdivide 
into Decimals; then {topping the end F with foft Cement, or foft Wax, Tin- ° 
vert the Frame, placing the head downwards, and the OrificeE upwards ; 
and by it, witha {mall Funnel, I fill the whole Glaf§ with Quickfilvers then 
by {topping the fmall Orifice E with my finger, I oftentimeserect and invert 
e whole Glafs and Frame,and thereby free the Quickfilver and Glafs from 
all the bubbles or parcels of lurking Air; then inverting it as before,Ifillit 
top full with clear and well ftrain'd Quickfilver, and having'made ready a 
{mall ball of pretty hard Cement, by heat made very foft,I prefsitintothe 
hole Ey and thereby ftop it very faft; andtofecure thisCementfrom flying __ 
out afterward,I bind over it a piece of Leather, thatis{pread over inthe in- 
fide with Cement, and wound about it whilft the Cement is hot: Having 
thus faftned it; ] gently erect again the Glafs after this manner : I firft let the 
Frame down edge-wayes, till the edge R V touch the Floor, orly horizon- 
tal ; and thenin that edging pofture raife the end RS; this Ido, thatif 
there chance to be any Air hidden in the {mall Pipe E, it may afcendinto the 
Pipe F, and not into the PipeD C: Having thuserectedit, andhungit by __ 
the hole Q, or fixt it perpendicularly by any other means, openthe endF, 
a 
