of being compreffed into the 180000th part of the fpace it 
would naturally take up, when free from prelTure j Now 
what texture or compofition of parts fhall be capable of this 
great expanfion and contradion^feems a very hard queftion ; 
and which, I fuppofe, is fcarce fufRciently accounted for, 
by the comparing it to Wool, Cotton^ and the hke fpringy 
bodies. 
Hitherto I have only confidered the Air and Atmofpherc^ 
as one unaltered body, as having conftantly at the Earth's 
fiirface the Sooth part of the weight of Water, and being 
capable of rarefa6tion and condeniation in infimtum \ nei- 
ther of which Hypotheses are rigidly true : for here in Eng- 
Imd \\s notoriouily known, that the weight of the whoh 
Atmofphere is various , being couaterpoiled fbmetimes by 
281 inches ofMermrj^ and at otiier times by no lefi than j o 
fo that the under parts being preffed by about a 1 5th part, 
lefs weight, the fpecifck grd,Vity of the i//> upon that (core 
will fbmetimes be a 1 5th part lighter than another ; Befides 
heat and cold does very confiderably dilate and contrail' 
the Air, and confequently alter its gravity, to which add the 
mixture oi effluvia or fleams rifing from allmoft all Bodies, 
which affimulating into the form of Air are kept fiiipended 
therein, as Salts difTolved in Liquors or Mettles in corroding 
Menfirua^ which bodies being all of them very much Hea- 
vier than ^/>^. their particles by their a^dmixture muft needs 
encreafe the weight of that Air they lie incorporated withal, 
after the iame manner as melted Salts do augment the vS'pe- 
cifick Gravity of Water. The other confideration is that 
the Rarefaction and Condeniation of the Air is not precifely 
according to the proportion here laid down, for tho expe- 
riment very nearly agrees thereto, as may be ften in the 
58 th Chapter of Mr. Hooks Micrographie^ yet are the Con- 
denfations not poffible beyond certain degrees, for being 
compreffed into an 800th part of the fpace it takes up here, 
its confiftence would be equally denJte with that of Water, 
wliich yeilds not to any force whatldever, as hath been 
found 
