Galilei, Galileo Dialogues on two world systems 1661, tr. Salusbury, Thomas
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7
is its motion. Having confirmed these motions, he proceeds saying,
that of natural bodies some being simple, and some composed of
them (and he calleth simple bodies those, that have a principle
of motion from nature, as the Fire and Earth) it follows that
simple motions belong to simple bodies, and mixt to the com­
pound; yet in such sort, that the compounded incline to the part
predominant in the composition.

Local motion of
three kinds, right,
circular, & mixt.

Circular, and
streight motions
are simple, as pro­
ceeding by simple
lines.

Ad medium, à me­
dio, & circa medi­
um.

SAGR. Pray you hold a little Salviatus, for I find so many
doubts to spring up on all sides in this discourse, that I shall be
constrained, either to communicate them if I would attentively
hearken to what you shall add, or to take off my attention from
the things spoken, if I would remember objections.

SALV. I will very willingly stay, for that I also run the same
hazard, and am ready at every step to lose my self whilst I sail be­
tween Rocks, and boisterous Waves, that make me, as they say, to
lose my Compass; therefore before I make them more, propound
your difficulties.

The definition of
Nature, either im­
perfect, or unseaso­
nable, produced by
Aristotle.

SAGR. You and Aristotle together would at first take me a
little out of the sensible World, to tell me of the Architecture,
wherewith it ought to be fabricated; and very appositly begin to
tell me, that a natural body is by nature moveable, nature being
(as elsewhere it is defined) the principle of motion. But here I
am somewhat doubtfull why Aristotle said not that of natural bo­
dies, some are moveable by nature, and others immoveable, for
that in the definition, nature is said to be the principle of Motion,
and Rest; for if natural bodies have all a principle of motion,
either he might have omitted the mention of Rest, in the definiti­
on of nature: or not have introduced such a definition in this place.
Next, as to the declaration of what Aristotle intends by simple
motions, and how by Spaces he determines them, calling those sim­
ple, that are made by simple lines, which are onely the right, and


circular, I entertain it willingly; nor do I desire to tenter the
instance of the Helix, about the Cylinder; which in that it is in e­
very part like to it self, might seemingly be numbred among sim­
ple lines. But herein I cannot concurre, that he should so re­
strain simple motions (whilst he seems to go about to repeat the
same definition in other words) as to call one of them the motion
about the medium, the others Sursum & Deorsum, namely up­
wards and downward; which terms are not to be used, out of the
World fabricated, but imply it not onely made, but already in­
habited by us; for if the right motion be simple, by the simplicity
of the right line, and if the simple motion be natural, it is made on
every side, to wit, upwards, downwards, backwards, forwards, to
the right, to the left, and if any other way can be imagined, pro­
vided it be straight, it shall agree to any simple natural body; or