[no] 
Auvergne^ callM le puy de Domme. So that the Rarefaftlon 
and Vapours feem not to have altered confiderably, the 
Gravity of the under parts of the Air ; and much above 
the hight where thefe Experiments were made, do few Va- 
pours alcend, and the cold is fuch that the Snow lies conti- 
nually, jfb that for the more elevated parts of the Sphere of 
Air there is much lefs reafon to doubt. 
But now we have had occafion to mention the difference 
there is between the hight of the Mercury at one time, fi'om 
the hight thereof at another, it may not be unacceptable to 
offer at fbme reafons for the faid difference, which, at leaft to 
my felf, feem to have fbme appearance of truth \Jirfi then tis 
undoubtedly demonftrable that the hight of the Cylinder of 
Mercury y is equal to the weight of the whole incumbent Air^ 
and conlequently that that whole is fbmtimcs a fifteenth 
more than at other times, which cannot otherwife be, but 
by the accefs of new matter when tis heavy, and its dimi- 
nution when tis light ; that Hypothefts therefore that fhews 
how the Air fliall be encreafed or diminiOied, in any par- 
ticular place, will give a reafon for the greater and lelTer 
hight of the Mercury in the Barofcope : but to direO: us in the 
choice of the feveral caufes, which may be affigned for the 
encreafe and decreafe of the Air, twill not be unneceffary to 
enumerate fome of the principle obfervations made upon the 
Barometer y moft whereof are fufBciently known already to 
all thoft that are curious in thefe matters. 
The/r/?is, that in calm weather when the Air is enclined 
to Rain, the Mercury is commonly low. 
2. That in Serene good fetled weather the Mercury is ge- 
nerally high. 
J. That upon very great Winds tho' they be not accom- 
panied with Rain the Mw/^r^ finks loweft of all, with re- 
lation to thepoiat of the Compafs the Wind blows upon. 
4. That ceteris paribus the greatefl: hights of the Mercury 
are found upon Ealterly and North- eafl:erly winds. 
5. That in calm frofty weather the i/krcury generally 
ftandshigh. 6. 
