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ferve tb put usia the right rode in our enquiries about th^ 
Variations of the weather. 
And firft the Air being a fluid, we are to take notice of 
thofe properties which it has in common with othet flu- 
ids, and thofe wherein it differs ftom theni, in f6 fi^. as 
they eoncern this matter. It agreed witlx all cxthet fti*ids 
in this that it Gravitates^j and again It has^this peculiar 
property (which is not (omuch obierv'd of otHer fluids^ 
that its fpecifcal Gravity is not always the fame. Now 
you know according to the certain rules of tbc ballancing 
of fluids amongfl:themfelves, every ftuid fpecifically ligh* 
ter then another will afcend and emerge above it, and 
every fluid fpecifically heavier will delcend and fubfide 
below. Thus if a Glafle in whofe bottom there is oyl bet 
fiU'd with water, the oyl will emerge above,but if the 
water be drawn out with a Siphon, and the Air ^a fpecifi^^ 
cally lighter fluid,) fucceed^the oyl will fubfide again.; 
Now there is fome certain proportion betweenthe fpeci-r 
fical Gravities of the fluid of Air, and of that which ^f- 
cends in Vapours and falls down again into rain j and if 
this proportion were ftill the fame, 'tis like we would; 
have no commixture of thofe fluids, but the Vapoursi 
would cither always float above or always flay below./ 
But this proportion of their fpecifical gravity is frequent-! 
ly changed j for ic is known that water when warm andv 
tepid is lighter then when it is cold (^and yet perhaps toOr 
thefmall particles of it ("if figured according to ^/^^^<»r*r 
^^x his con jedfaie,) are more capable of afcending inVa^ 
pours in froft then at another feafon, as being more cx^ 
tended, and fo having a larger portion of the fluid om 
Air to bear them up,- ) and the daily obfervations af tbel| 
different heights of the Mercmry in the Barofcope do 
make appear that the Atmofpheres gravity is not always 
the fame. And now from thefe known properties may 
be ealily deduced a ilatical account of the riling of Va* 
pours, their being carried in the Air ia Clouds and their 
laliing down again into rain* For 
