Galilei, Galileo - The systems of the world

Galilei, Galileo, The systems of the world, 1661

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Now this acceleration of motion is never made, but when the
moveable in moving acquireth it; nor is its acquiſt other than an
approaching to the place deſired, to wit, whither its natural in
clination attracts it, and thither it tendeth by the ſhorteſt way;
namely, by a right line. We may upon good grounds therefore
ſay, That Nature, to confer upon a moveable firſt conſtituted in
reſt a determinate velocity, uſeth to make it move according to

a certain time and ſpace with a right motion. This preſuppoſed,
let us imagine God to have created the Orb v. g. of Jupiter, on
which he had determined to confer ſuch a certain velocity, which
it ought afterwards to retain perpetually uniform; we may with
Plato ſay, that he gave it at the beginning a right and accelerate
motion, and that it afterwards being arrived to that intended de

gree of velocity, he converted its right, into a circular motion,
the velocity of which came afterwards naturally to be uniform.

Right motion by
nature infinite.

Motion by a right
line naturally im
poſſible.

Nature attempts
not things impoſſi
ble to be effected.

Right motion might
perhaps be in the
firſt Chaos.

Right motion is
commodious to
range in order,
things ous of or
der.

Mundane bodies
moved in the be
ginning in a right
line, and after
wards circularly?
according to Plato.

* Thus doth he co
vertly and modeſt
ly ſtile himſelfe
throughout this
work.

A moveable be
ing in a ſtate of
reſt, ſhall not move
unleſs it have an
inclination to ſome
particular place.

The moveable ac
celerates its moti
on, going towards
the place whither
it hath an inclina
tion.

The moveable paſ
ſing from reſt, go
eth thorow all the
degrees of tardity.

Reſt the inſinioe
degree of tardity.

The moveable doth
not accelerate, ſave
only as it approach
eth nearer to its
term.

Nature, to intro
duce in the move
able a certain de
gree of velocity,
made it move in a
right line.

Vniform velocity
convenient to the
circular motion.

SAGR. I hearken to this Diſcourſe with great delight; and I
believe the content I take therein will be greater, when you have
ſatisfied me in a doubt: that is, (which I do not very well com
prehend) how it of neceſſity enſues, that a moveable departing

from its reſt, and entring into a motion to which it had a natural
inclination, it paſſeth thorow all the precedent degrees oſ tardity,
comprehended between any aſſigned degree of velocity, and the
ſtate of reſt, which degrees are infinite? ſo that Nature was not
able to confer them upon the body of Jupiter, his circular moti
on being inſtantly created with ſuch and ſuch velocity.

Betwixt reſt, and
any aſſigned degree
of velocity, infinite
degrees of leſs ve
locity interpoſe.

Nature doth not
immediately con
fer a determinate
degree of velocity,
howbeit ſhe could.

SALV. I neither did, nor dare ſay, that it was impoſſible for
God or Nature to confer that velocity which you ſpeak of, imme
diately; but this I ſay, that de facto ſhe did not doit; ſo that the
doing it would be a work extra-natural, and by confequence mi
raculous.

SAGR. Then you believe, that a ſtone leaving its reſt, and en
tring into its natural motion towards the centre of the Earth, paſ
ſeth thorow all the degrees of tardity inferiour to any degree of
velocity?

SALV. I do believe it, nay am certain of it; and ſo certain,
that I am able to make you alſo very well ſatisfied with the truth
thereof.

SAGR. Though by all this daies diſcourſe I ſhould gain no
more but ſuch a knowledge, I ſhould think my time very well
beſtowed.

SALV. By what I collect from our diſcourſe, a great part of
your ſcruple lieth in that it ſhould in a time, and that very ſhort,
paſs thorow thoſe infinite degrees of tardity precedent to any ve
locity, acquired by the moveable in that time: and therefore be
fore we go any farther, I will ſeek to remove this difficulty, which

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