mixture may not alter the Air s fpecifical gravity. Or 3^ 
We may poffibly come to a nearer refolution of this by 
confiderjng the influence which the Heat and Cold have 
upon the Air's fpring. The Air you know has this pecu* 
liar propertie which is not (o much obferv'd of other flu- 
ids, that it is indu d with Elafl:icity as well as gravity, and 
therefore we are to confider what influence the change of 
its fpring may have upon the change of its weight, and 
it feems evident that the encreafe. of its fpring doth di- 
minifh its weight, and theleflening of its fpring willen- 
creafeit; for upon the encreafe of the Air's fpring, the 
Air is rarefied, and fo a leffer portion of it prefTes upon 
the fubjacent fluid, but when it is lefTen d the Air is con- 
denfed, and fo a greater portion of it prefTes upon the 
fubjacent body. For example let us fuppofe the {pringy 
particles of Air, to be like to thefpringy hairs of Wooll, 
or the fpring of a Watch; and that many Millions of 
rows of them goto makeup the Cylinder of Air which 
from the top of theAtmofphere prefles upon*the Mercury 
in the Barofcope and keeps it fufpended to the height of 
3 o Inches 5 let us fuppofe this Air rarified, fo that all its 
fpringy particles expand therafclves, and therefore fhut 
off of this Cylinder fome Thoufands of thoferows,- this 
Cylinder being now made up of a farlefTer number of 
thofe rows of particles, muft needs have a lefler prefTure 
upon the Mercur , fo that itwillfubfide perhaps to 29. 
And thus it continues till the Air's fpring be weakncd, 
and fo the particles crowded again into narrower room. 
Now if this be found to hold in the Theory, Experience 
feems very well to anfwer it^ fori have hitherto obferv- 
ed that in Cold weather and fharp Frofts the Mercury 
rifes highefl in the Barofcope,and if the forreign meafures 
agree with ours it is ufually higher here then in France 
ox Italy. 
I (hall here after all f ubjoy n two or three obfervations 
which may ferve to confirm what has been faid. The 
firft 
