elocity, arises from the _Vis inertiae_ of
the Parts of the Fluid.

Now that part of the resisting Power of any Medium which arises from the
Tenacity, Friction or Attrition of the Parts of the Medium, may be
diminish'd by dividing the Matter into smaller Parts, and making the
Parts more smooth and slippery: But that part of the Resistance which
arises from the _Vis inertiae_, is proportional to the Density of the
Matter, and cannot be diminish'd by dividing the Matter into smaller
Parts, nor by any other means than by decreasing the Density of the
Medium. And for these Reasons the Density of fluid Mediums is very
nearly proportional to their Resistance. Liquors which differ not much
in Density, as Water, Spirit of Wine, Spirit of Turpentine, hot Oil,
differ not much in Resistance. Water is thirteen or fourteen times
lighter than Quick-silver and by consequence thirteen or fourteen times
rarer, and its Resistance is less than that of Quick-silver in the same
Proportion, or thereabouts, as I have found by Experiments made with
Pendulums. The open Air in which we breathe is eight or nine hundred
times lighter than Water, and by consequence eight or nine hundred times
rarer, and accordingly its Resistance is less than that of Water in the
same Proportion, or thereabouts; as I have also found by Experiments
made with Pendulums. And in thinner Air the Resistance is still less,
and at length, by ratifying the Air, becomes insensible. For small
Feathers falling in the open Air meet with great Resistance, but in a
tall Glass well emptied of Air, they fall as fast as Lead or Gold, as I
have seen tried several times. Whence the Resistance seems still to
decrease in proportion to the Density of the Fluid. For I do not find by
any Experiments, that Bodies moving in Quick-silver, Water or Air, meet
with any other sensible Resistance than what arises from the Density and
Tenacity of those sensible Fluids, as they would do if the Pores of
those Fluids, and all other Spaces, were filled with a dense and
subtile Fluid. Now if the Resistance in a Vessel well emptied of Air,
was but an hundred times less than in the open Air, it would be about a
million of times less than in Quick-silver. But it seems to be much less
in such a Vessel, and still much less in the Heavens, at the height of
three or four hundred Miles from the Earth, or above. For Mr. _Boyle_
has shew'd that Air may be rarified above ten thousand times in Vessels
of Glass; and