Galilei, Galileo Dialogues on two world systems 1661, tr. Salusbury, Thomas

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12

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