the Heavens are much emptier of Air than any _Vacuum_ we
can make below. For since the Air is compress'd by the Weight of the
incumbent Atmosphere, and the Density of Air is proportional to the
Force compressing it, it follows by Computation, that at the height of
about seven and a half _English_ Miles from the Earth, the Air is four
times rarer than at the Surface of the Earth; and at the height of 15
Miles it is sixteen times rarer than that at the Surface of the Earth;
and at the height of 22-1/2, 30, or 38 Miles, it is respectively 64,
256, or 1024 times rarer, or thereabouts; and at the height of 76, 152,
228 Miles, it is about 1000000, 1000000000000, or 1000000000000000000
times rarer; and so on.

Heat promotes Fluidity very much by diminishing the Tenacity of Bodies.
It makes many Bodies fluid which are not fluid in cold, and increases
the Fluidity of tenacious Liquids, as of Oil, Balsam, and Honey, and
thereby decreases their Resistance. But it decreases not the Resistance
of Water considerably, as it would do if any considerable part of the
Resistance of Water arose from the Attrition or Tenacity of its Parts.
And therefore the Resistance of Water arises principally and almost
entirely from the _Vis inertiae_ of its Matter; and by consequence, if
the Heavens were as dense as Water, they would not have much less
Resistance than Water; if as dense as Quick-silver, they would not have
much less Resistance than Quick-silver; if absolutely dense, or full of
Matter without any _Vacuum_, let the Matter be never so subtil and
fluid, they would have a greater Resistance than Quick-silver. A solid
Globe in such a Medium would lose above half its Motion in moving three
times the length of its Diameter, and a Globe not solid (such as are the
Planets,) would be retarded sooner. And therefore to make way for the
regular and lasting Motions of the Planets and Comets, it's necessary to
empty the Heavens of all Matter, except perhaps some very thin Vapours,
Steams, or Effluvia, arising from the Atmospheres of the Earth, Planets,
and Comets, and from such an exceedingly rare AEthereal Medium as we
described above. A dense Fluid can be of no use for explaining the
Phaenomena of Nature, the Motions of the Planets and Comets being better
explain'd without it. It serves only to disturb and retard the Motions
of those great Bodies, and make the Frame of Nature languish: And in the
Pores of Bodies, it serves only to stop the vibrating