For if we may be allowed to fuppofe that wliefi the At- 
mofphere isheavieit, therc fcfom^ inch proportion be- 
tween its fpecifical. gravity, and that of the fluid of Va- 
pours, as there is between water andoyl^ the Vapours 
according to the known laws of fluids muft needs afcend, 
and fo'Iong-as this proportion continues they muft needs 
float above in the Ait : but when the Atiliofpheres gra- 
vity is chaiig'd, the Vapours muft fall down again. 
Thus you fee how eafily this may be accounted for, and 
bow reafonable this fuppofition is frorn the known pro- 
perties qfthefe fluids. 
Now fet us fee whethet this accords with obfervation 
and experience. And firft as to the afcent of Vapours, I 
do not know any determiriate Inftrument that will In- 
dicate their afcent as certainly as the Barofcope does the 
change of the Air s weight f for our common Hygrofcopes 
^re not very exad, and befides I fuppofe their change by 
moifture fliews rather the falling then the rifing ofVapoursj 
yet there are two or three bbfervations which leem certain 
Indications of their afcent ; as firft, if the Horizon and 
the remote Hills leem Smoakie and inconfpicuous,fo that 
nothing can be feen at any diftance, and that tho' the 
Heavens be not cloudy, but clear, and tho* there be no 
Fog nor yet any Cap of Clouds upon the Hills ('which do 
rather indicate the falling of Vapours,) again if when 
you look to any diftant part of the Country round about 
you, it appear all in an undulating motion (and this you 
know our Countrie People call by the name of Summer 
Colts in the Air,^ thisieems to be a fign of the plentiful! 
rifing of Vapours, for this is only occafioned by looking 
through an unequall waving Medium, which makes fre- 
quent inflexions of the beams of light, as you lee any 
object feems to have a tremulating motion in all its parts 
when you look upon it through Smoak. Another Indi- 
cation of the afcent of Vapours feems to be the copious 
rifmg of Steams above waters, marifh grounds and fens, 
B b 2 which 
