found by fevera! experiments tryed here, and 2it Tlorem^^ 
by the Academia del Cimento. Nor can the Rarefaftion pro- 
ceed in in^nitum ; for fuppofing the Spring whereby it de- 
lates it felf, occafioned by what texture of parts you pleafe, 
yet muft there be a determinate magnitude of the natural 
ftate of each Particle, as we fee it is in Wool! and the hke^ 
whole bodies being comprefTabie into a very fmall fpace, 
have yet a determinate bulk which they cannot exceed^ 
when freed from all manner pf prelTure. 
Thefe objeftions being true do difturb the Geometrical 
accuracy of thefe conclufions, drawn from the fpecifick gra- 
vity of the Air obferved at any time ; but the Method here 
fhewn will compute by a like calculation, the hights of 
the Quicksilver, and the Rarefadions of the Air from any 
alTigned highc of the Barometer at the Earths Surface, and 
any fpecifick Gravity given. As to the Condenlation and 
Raretafibion by heat and cold, and the various mixture of 
Aqueous and other Vapours, thefe two objeftions ieem gene- 
rally to compenfate each other, for when the Air is rarifi- 
ed by heat the Vapours are raifed moft copioufly, fo that tha 
the Air properly fb called, be expanded and coniequently 
lighter, yet the interflices thereof being crouded full of Va- 
pours of much heavier matters, bulk for bulk the weight 
of the Compofitum may continue much the lame ; at leait a 
moft Curious experiment made by the Ingenious Mr. Joh^r 
Cdfivell of Oxford upon the top of Smwdon hill in Caermrvan- 
Sheir^ feems to provt that the firft Inches of Mercury have 
their portions of Air near enough to what I now determine; 
for the hight of the hill being 1 240 yards or very near it, he 
found the Mercury to have fubfided t02 5,6 inch. or4 inch, be- 
low the mean Altitude thereof at the level of the Sea, 
(which is a greater difference than has been found in any of 
our formerExperiments,)and the {pace anfwering to 4 inch, 
by my calculation fliould bei288yards ;andit agrees as well 
with the Obfervations in the Appendix to Mr. Pafca/Ps 
Book, deC Eciuilibre des Liqueurs y made on the high hill in 
P 2 Ah- 
