[Sidenote: All Figures sink or swim, upon bathing or not bathing
of their tops.]
_All Figures, therefore, of whatever greatnesse, may go, and not
go, to the Bottom, according as their Sumities or Tops shall be
bathed or not bathed._
And this Accident being common to all sorts of Figures, without
exception of so much as one. Figure hath, therefore, no part in the
production of this Effect, of sometimes sinking, and sometimes again
not sinking, but only the being sometimes conjoyned to, and sometimes
seperated from, the supereminent Air: which cause, in fine, who so
shall rightly, and, as we say, with both his Eyes, consider this
business, will find that it is reduced to, yea, that it really is the
same with, the true, Naturall and primary cause of Natation or
Submersion; to wit, the excess or deficiency of the Gravity of the
water, in relation to the Gravity of that Solid Magnitude, that is
demitted into the water. For like as a Plate of Lead, as thick as the
back of a Knife, which being put into the water by it self alone goes
to the bottom, if upon it you fasten a piece of Cork four fingers
thick, doth continue afloat, for that now the Solid that is demitted
in the water, is not, as before, more grave than the water, but less,
so the Board of Ebony, of its own nature more grave than water; and,
therefore, descending to the bottom, when it is demitted by it self
alone into the water, if it shall be put upon the water, conjoyned
with an Expanded vail of Air, that together with the Ebony doth
descend, and that it be such, as that it doth make with it a compound
less grave than so much water in Mass, as equalleth the Mass already
submerged and depressed beneath the Levell of the waters Surface, it
shall not descend any farther, but shall rest, for no other than the
universall and most common cause, which is that Solid Magnitudes, less
grave _in specie_ than the water, go not to the bottom.
So that if one should take a Plate of Lead, as for Example, a finger
thick, and an handfull broad every way, and should attempt to make it
swimme, with putting it lightly on the water, he would lose his
Labour, because that if it should be depressed an Hairs breadth beyond
the possible Altitude of the Ramparts of water, it would dive and
sink; but if whilst it is going downwards, one should make certain
Banks or Ramparts about it, that should hinder the defusion of the
water upon the said Plate, the which Banks should rise so high, as
that they might be able to contain as much water, as should weigh
equally with the said Plate, it would, witho{u}t all Question, descend
no lower, but would rest, as being sustained by vertue of the Air
contained within the aforesaid Ramparts: and, in short, there would be
a Vessell by this means formed with the bottom of Lead. But if the
thinness of the Lead shall be such, that a very small height of
Rampart would suffice to contain so much Air, as might keep it afloat,
it shall also rest without the Artificiall Banks or Ramparts, but yet
not without the Air, because the Air by it self makes Banks sufficient
for a small height, to resist the Superfusion of the water: so that
that which in this case swimmes, is as it were a Vessell filled with
Air, by vertue of which it continueth afloat.
I will, in the last place, with an other Experime{n}t, attempt to
remove all difficulties, if so be there should yet be any doubt left
in any one, touching the opperation of this [E]Continuity of the Air,
with the thin Plate which swims, and afterwards put an end to this
part of my discourse.
[E] Or rather Contiguity,