(S.118ff)
[Dialog]
SALV. [...] Seeing that the difference of Velocity in Moveables of
divers Gravities is found to be more and more, as the Mediums are
more and more Resisting: And withall, that in a Medium of
Quicksilver, Gold doth not only go to the bottom more swiftly
than Lead, but it alone descends in it, and all other Metals and
Stones move upwards therein, and flote thereon; whereas between
Balls of Gold, Lead, Brass, Porphiry, or other grave matters, the in−
equality of motion in the Air shall be almost wholly insensible, for
it is certain, that a Ball of Gold in the end of the descent of an
hundred yards shall not out−strip one of Brass four Inches: seeing
this, I say, I have thought, that if we wholly took away the
Resistance of the Medium, all Matters would descend with equall
Velocity.
SIMP. This is a bold speech, Salviatus, I shall never believe
that in Vacuity it self, if so be one should allow Motion in it, a lock
of Wooll would move as swiftly as a piece of Lead.
SALV. Fair and softly, Simplicius, your scruple is not so ab−
struce, nor I so incautelous, that you should need to think that I
was not advised of it, and that consequently I have not found a re−
ply to it. Therefore, for my explanation, and your information,
hearken to what I shall say. We are upon the examination of
what would befall Moveables exceeding different in weight in a
Medium, in case it should have no Resistance, so that all the diffe−
rence of Velocity that is found between the said Moveables ought
to be referred to the sole inequality of Weight. And because on−
ly a Space altogether void of Air, and of every other, though te−
nuous and yielding Body, would be apt sensibly to shew us what
we seek, since we want such a Space, let us successively observe that
which happeneth in the more subtill and lesse resisting Mediums,
in comparison of that which we see to happen in others lesse subtill
and more resisting: for if we should really find the Moveables
different in Gravity to differ lesse and lesse in Velocity, according
as the Mediums are found more and more yielding; and that,
finally, although extreamly unequal in weight, in a Medium more
tenuous than any other, though not void, the difference of Velo−
city discovers it self to be very small, and almost unobservable, I
conceive that we may, and that upon very probable conjecture,
believe, that in a Vacuum their Velocities would be exactly equal.
Therefore let us consider that which hapneth in the Air; wherein
to have a Figure of an uniform Superficies, and very light Matter,
I will that we take a blown Bladder, in which the included Air
will weigh little or nothing in a Medium of the Air it self, because
it can make but very small Compression therein, so that the Gravi−
ty is only that little of the said film, which would not be the thou−
sandth part of the weight of a lump of Lead of the bigness of
the said Bladder when blown. These, Simplicius, being let fall
from the height of four or six yards, how great a space, do you
judge, that the Lead would anticipate the Bladder in its descent?
Assure your self that would not move thrice, no nor twice as fast,
although even now you would have had it to have been a thou−
sand times more swift.
SIMP. It is possible that at the beginning of the Motion, that
is, in the first five or six yards this might happen that you say; but
in the progresse, and in a long continuation I believe, that the Lead
would leave it behind, not only six, but also eight and ten parts of twelve.
SALV. And I also believe the same: and make no question,
but that in very great distances the Lead will have passed an hun−
dred miles of way, ere the Bladder will have passed so much as one.
But this, Simplicius, which you propound, as an effect contrary to
my Assertion, is that which most especially confirmeth it. It is
(I once more tell you) my intent to declare, That the difference of
Gravity is in no wise the cause of the divers velocities of Movea−
bles of different Gravity, but that the same dependeth on exteri−
our accidents, & in particular, on the Resistance of the Medium, so
that, this being removed, all Moveables move with the same de−
grees of Velocity. And this I chiefly deduce from that which but
now you your self did admit, and which is very true, namely, that
of two Moveables, very different in weight, the Velocities more and
more differ, according as the ^{*} Spaces are greater and greater that
they passe: an Effect which would not follow, if it did depend on
the different Gravities: for they being alwaies the same, the pro−
portion betwixt the Spaces would likewise alwaies continue the
same, which proportion we see still successively to encrease in the
continuance of the Motion; for that the heaviest Moveable in the
descent of one yard will not anticipate the lightest the tenth part
of that Space or Way, but in the fall of twelve yards will out−go
it a third part, in that of an hundred will outstrip it 90/100.
SIMP. Very well: But following you step by step, if the dif−
ference of weight in Moveables of different Gravities cannot
cause the difference of proportion in their Velocities, for that the
Gravities do not alter; neither then can the Medium, which is
supposed alwaies to continue the same, cause any alteration in the
proportion of the Velocities.
SALV. You wittily bring an instance against my Position, that
it is very necessary to remove. I say therefore, that a Grave Body
hath, by Nature, an intrinsick Principle of moving towards the
Common Center of heavy things, that is to that of our Terrestrial
Globe, with a Motion continually accelerated, and accelerated
alwaies equally, scilicet, that in equal times there are made equal
^{*} additions of new Moments, and degrees of Velocities: and this
ought to be understood to hold true at all times when all acciden−
tal and external impediments are removed; amongst which there
is one that we cannot obviate, that is the Impediment of the Me−
dium, which is Repleat, when as it should be opened and latterally
moved by the falling Moveable, to which transverse Motion the
Medium, though fluid, yielding and tranquile, opposeth it self
with a Resistance one while lesser, and another while greater and
greater, according as it is more slowly or hastily to open to give
passage to the Moveable, which, because, as I have said, it goeth
of its own nature continually accelerating, it cometh of conse−
quence to encounter continually greater Resistance in the Medi−
um, and therefore Retardment, and diminution in the acquist of
new degrees of Velocity; so that in the end, the Velocity arriveth
to that swiftnesse, and the Resistance of the Medium, to that
strength, that ballancing each other, they take away all further
Acceleration, and reduce the Moveable to an Equable and Uni−
form Motion, in which it afterwards continually abides. There is
therefore in the Medium augmentation of Resistance, not because
it changeth its Essence, but because the Velocity altereth where−
with it ought to open, and laterally move, to give passage to the
falling Body, which goeth continually accelerating. Now the
observing, that the Resistance of the Air to the small Moment or
Impetus of the Bladder is very great, and to the great weight of
the Lead is very small, makes me hold for certain, that if one should
wholly remove it, by adding to the Bladder great assistance, and
but very little to the Lead, their Velocities would equalize each
other. Taking this Principle therefore for granted, That in the
Medium wherein, either by reason of Vacuity, or otherwise, there
were no Resistance that might abate the Velocity of the Motion,
so that of all Moveables the Velocities were alike, we might con−
gruously enough assign the proportions of the Velocities of like
and unlike Moveables, in the same and in different, Replear, and
therefore Resisting Medium' s. And this we might effect by study−
ing how much the Gravity of the Medium abateth from the Gra−
vity of the Moveable, which Gravity is the Instrument wherewith
the Moveable makes its Way, repelling the parts of the Medium
on each Side: an operation that doth not occur in void Mediums ;
and therefore there is no difference to be expected from the di−
verse Gravity: and because it is manifest, that the Medium abateth
from the Gravity of the Body by it contained, as much as is the
weight of such another mass of its own Matter, if the Velocities of
the Moveables that in a non−resisting Medium would be (as hath
been supposed) equal, should diminish in that proportion, we
should have what we desired. As for example; supposing that
Lead be ten thousand times more grave than Air, but Ebony a
thousand times only; of the Velocities of these two Matters, which
absolutely taken, that is, all Resistance being removed, would be
equal, the Air substracts from the ten thousand degrees of the
Lead one, and from the thousand degrees of the Ebony likewise
abateth one, or, if you will, of its ten thousand, ten. If there−
fore the Lead and the Ebony shall descend thorow the Air from
any height, which, the retardment of the Air removed, they would
have passed in the same time, the Air will abate from the ten
thousand degrees of the Leads Velocity one, but from the ten
thousand degrees of Ebony' s Velocity it will abate ten: which is
as much as to say, that dividing that Altitude, from which those
Moveables departed into ten thousand parts, the Lead will arrive
at the Earth, the Ebony being left behind, ten, nay, nine of those
same ten thousand parts. And what else is this, but that a Ball of
Lead, falling from a Tower two hundred yards high, to find how
much it will anticipate one of Ebony of lesse than four Inches?
The Ebony weigheth a thousand times more than the Air, but that
Bladder so blown, weigheth only four times so much; the Air
therefore from the intrinsick and natural Velocity of the Ebony
subducteth one degree of a thousand, but from that, which also in
the Bladder would absolutely have been the same, the Air sub−
ducts one part of four: so that by that time the Ball of Ebony
falling from the Tower, shall come to the ground, the Bladder
shall have passed but three quarters of that height. Lead is twelve
times heavier than Water, but Ivory only twice as heavy; the
Water therefore, from their absolute Velocities which would be
equal, shall abate in the Lead the twelfth part, but in the Ivory
the half: when therefore, in the Water, the Lead shall have de−
scended eleven fathom, the Ivory shall have descended six. And,
arguing by this Rule, I believe, that we shall find the Experiment
much more exactly agree with this same Computation, than with
that of Aristotle. By the like method we might find the Veloci−
ties of the same Moveable in different fluid Mediums, not compa−
ring the different Resistances of the Mediums, but considering the
excesses of the Gravity of the Moveable over and above the Gra−
vities of the Mediums: v. gr. ^{*} Tin is a thousand times heavier than
Air, and ten times heavier than Water; therefore dividing the ab−
solute Velocity of the Tin into a thousand degrees, it shall move
in the Air, (which deducteth from it the thousandth part,) with nine
hundred ninety nine, but in the Water with nine hundred only;
being that the Water abateth the tenth part of its Gravity, and
the Air the thousandth part. Take a Solid somewhat heavier than
Water, as for instance, the Wood called Oake, a Ball of which
weighing, as we will suppose, a thousand drams, a like quantity
of Water will weigh nine hundred and fifty, but so much Air will
weigh but two drams,: it is manifest, that supposing that its abso−
lute Velocity were of a thousand degrees, in Air there would re−
main nine hundred ninety eight, but in the Water only fifty; be−
cause that the Water of the thousand degrees of Gravity taketh
away nine hundred and fifty, and leaves fifty only; that Solid there−
fore would move well−near twenty times as fast in the Air as Wa−
ter; like as the excesse of its Gravity above that of the Water is
the twentieth part of its own. And here I desire that we may con−
sider, that no matters, having a power to move downwards in the
Water, but such as are more grave in Species than it; and conse−
quently many hundreds of times, more grave than the Air, in
seeking what the proportions of their Velocities are in the Air and
Water, we may, without any considerable errour, make account
that the Air doth not deduct any thing of moment from the abso−
lute Gravity, and consequently, from the absolute Velocity of such
matters: so that having easily found the excesse of their Gravi−
ty above the Gravity of the Water, we may say that their Velo−
city in the Air, to their Velocity in the Water hath the same propor−
tion, that their total Gravity hath to the excesse of this above
the Gravity of the Water. For example, a Ball of Ivory weigh−
eth twenty ounces, a like quantity of Water weigheth seventeen
ounces: therefore the Velocity of the Ivory in Air, to its Velocity
in Water is very neer as twenty to three.