[Sidenote: Solids of any Figure & greatnesse, that naturally
sink, may by help of the Air in the Rampart swimme.]
_That it is possible to fo{r}m Solid Bodies, of what Figure and
greatness soever, that of their own Nature goe to the Bottome;
But by the help of the Air contained in the Rampart, rest without
submerging._
[Illustration]
The truth of this Proposition is sufficiently manifest in all those
Solid Figures, that determine in their uppermost part in a plane
Superficies: for making such Figures of some Matter specifically as
grave as the water, putting them into the water, so that the whole
Mass be covered, it is manifest, that they shall rest in all places,
provided, that such a Matter equall in weight to the water, may be
exactly adjusted: and they shall by consequence, rest or lie even with
the Levell of the water, without making any Rampart. If, therefore, in
respect of the Matter, such Figures are apt to rest without
submerging, though deprived of the help of the Rampart, it is
manifest, that they may admit so much encrease of Gravity, (without
encreasing their Masses) as is the weight of as much water as would be
contained within the Rampart, that is made about their upper plane
Surface: by the help of which being sustained, they shall rest afloat,
but being bathed, they shall descend, having been made graver than the
water. In Figures, therefore, that determine above in a plane, we may
cleerly comprehend, that the Rampart added or removed, may prohibit or
permit the descent: but in those Figures that go lessening upwards
towards the top, some Persons may, and that not without much seeming
Reason, doubt whether the same may be done, and especially by those
which terminate in a very acute Point, such as are your Cones and
small Piramids. Touching these, therefore, as more dubious than the
rest, I will endeavour to demonstrate, that they also lie under the
same Accident of going, or not going to the Bottom, be they of any
whatever bigness. Let therefore the Cone be A B D, made of a matter
specifically as grave as the water; it is manifest that being put all
under water, it shall rest in all places (alwayes provided, that it
shall weigh exactly as much as the water, which is almost impossible
to effect) and that any small weight being added to it, it shall sink
to the bottom: but if it shall descend downwards gently, I say, that
it shall make the Rampart E S T O, and that there shall stay out of
the water the point A S T, tripple in height to the Rampart E S: which
is manifest, for the Matter of the Cone weighing equally with the
water, the part submerged _S B D T_, becomes indifferent to move
downwards or upwards; and the Cone _A S T_, being equall in Mass to
the water that would be contained in the concave of the Rampart _E S T
O_, shall be also equall unto it in Gravity: and, therefore, there
shall be a perfect _Equilibrium_, and, consequently, a Rest. Now here
ariseth a doubt, whether the Cone _A B D_ may be made heavier, in such
sort, that when it is put wholly under water, it goes to the bottom,
but yet not in such sort, as to take from the Rampart the vertue of
sustaining it that it sink not, and, the reason of the doubt is this:
that although at such time as the Cone _A B D_ is specifically as
grave as the water, the Rampart _E S T O_ sustaines it, not only when
the point _A S T_ is tripple in height to the Altitude of the Rampart
_E S_, but also when a lesser part is above water; [for although in
the Descent of the Cone the Point _A S T_ by little and little
diminisheth, and so likewise the Rampart _E S T O_, yet the Point
diminisheth in greater proportion than the Rampart, in that it
diminisheth according to all the three Dimensions, but the Rampart
according to two only, the Altitude still remaining the same; or, if
you will, because the Cone _S {A} T_ goes diminishing, according to
the proportion of the cubes of the Lines that do successively become
the Diameters of the Bases of emergent Cones, and the Ramparts
diminish according to the proportion of the Squares of the same Lines;
whereupon the proportions of the Points are alwayes Sesquialter of the
proportions of the Cylinders, contained within the Rampart; so that
if, for Example, the height of the emergent Point were double, or
equall to the height of the Rampart, in these cases, the Cylinder
contained within the Rampart, would be much greater than the said
Point, because it would be either sesquialter or tripple, by reason of
which it would perhaps serve over and above to sustain the whole Cone,
since the part submerged would no longer weigh any thing;] yet,
nevertheless, when any Gravity is added to the whole Mass of the Cone,
so that also the part submerged is not without some excesse of Gravity
above the Gravity of the water, it is not manifest, whether the
Cylinder contained within the Rampart, in the descent that the Cone
shall make, can be reduced to such a proportion unto the emergent
Point, and to such an excesse of Mass above the Mass of it, as to
compensate the excesse of the Cones Specificall Gravity above the
Gravity of the water: and the Scruple ariseth, because that howbeit in
the descent made by the Cone, the emergent Point _A S T_ diminisheth,
whereby there is also a diminution of the excess of the Cones Gravity
above the Gravity of the water, yet the case stands so, that the
Rampart doth also contract it self, and the Cylinder contained in it
doth deminish. Nevertheless it shall be demonstrated, how that the
Cone _A B D_ being of any supposed bignesse, and made at the first of
a Matter exactly equall in Gravity to the Water, if there may be
affixed to it some Weight, by means of which i{t} may descend to the
bottom, when submerged under water, it may also by vertue of the
Rampart stay above without sinking.
[Illustration]
[Illustration]
Let, therefore, the Cone _A B D_ be of any supposed greatnesse, and
alike in specificall Gravity to the water. It is manifest, that being
put lightly into the water, it shall rest without descending; and it
shall advance above water, the Point _A S T_, tripple in height to the
height of the Rampart _E S_: Now, suppose the Cone _A B D_ more
depressed, so that it advance above water, only the Point _A I R_,
higher by half than the Point _A S T_, with the Rampart about it _C I
R N_. And, because, the Cone _A B D_ is to the Cone _A I R_, as the
cube of the Line _S T_ is to the cube of the Line _I R_, but the
Cylinder _E S T O_, is to the Cylinder _C I R N_, as the Square of _S
T_ to the Square of _I R_, the Cone _A S T_ shall be Octuple to the
Cone _A I R_, and the Cylinder _E S T O_, quadruple to the Cylinder _C
I R N_: But the Cone _A S T_, is equall to the Cylinder _E S T O_:
Therefore, the Cylinder _C I R N_, shall be double to the Cone _A I
R_: and the water which might be contained in the Rampart _C I R N_,
would be double in Mass and in Weight to the Cone _A I R_, and,
therefore, would be able to sustain the double of the Weight of the
Cone _A I R_: Therefore, if to the whole Cone _A B D_, there be added
as much Weight as the Gravity of the Cone _A I R_, that is to say, the
eighth part of the weight of the Cone _A S T_, it also shall be
sustained by the Rampart _C I R N_, but without that it shall go to
the bottome: the Cone _A B D_, being, by the addition of the eighth
part of the weight of the Cone _A S T_, made specifically more grave
than the water. But if the Altitude of the Cone _A I R_, were two
thirds of the Altitude of the Cone _A S T_, the Cone _A S T_ would be
to the Cone _A I R_, as twenty seven to eight; and the Cylinder _E S T
O_, to the Cylinder _C I R N_, as nine to four, that is, as twenty
seven to twelve; and, therefore, the Cylinder _C I R N_, to the Cone
_A I R_, as twelve to eight; and the excess of the Cylinder _C I R N_,
above the Cone _A I R_, to the Cone _A S T_, as four to twenty seven:
therefore if to the Cone _A B D_ be added so much weight as is the
four twenty sevenths of the weight of the Cone _A S T_, which is a
little more then its seventh part, it also shall continue to swimme,
and the height of the emergent Point shall be double to the height of
the Rampart. This that hath been demonstrated in Cones, exactly holds
in Piramides, although the one or the other should be very sharp in
their Point or Cuspis[77]: From whence we conclude, that the same Accident
shall so much the more easily happen in all other Figures, by how much
the less sharp the Tops shall be, in which they determine, being
assisted by more spacious Ramparts.
[77] Natatio{n} easiest effected in Figures broad toward the top.