Definitions

Definitions

1. A body which is moved either touches another or it does not touch. It touches if no intermediate space is given. Touching, if it is moved, either passes by or strikes against (impingit).

2. It passes by if, by its continued motion, it would move nothing of the other from its place.

3. If it would move something, it impels; and if the other is also moved, it will strike against it; however, the use of these words is almost promiscuous.

4. It strikes against the whole (toti impingit) if, by its continued motion, it drives the other whole from its place.

5. It strikes against a part (parti impingit) if otherwise. Furthermore, it strikes in various ways, either by reason of the line of motion or of the term.

6. The line of motion is that which the center of the moved thing describes; the line of impact is that which the center of the striking thing describes, or of that part of it which is about to follow into the place of the receiving thing. Whence sometimes the line of impact from the whole can be straight, and from a part, curved; although, where there is no need for distinction, in the following I shall also use the naming of the line of motion for the line of impact.

7. The measure of the line of motion is either the line of motion itself, if it is straight, or, if it is oblique, the straight line made from the oblique turned backward, at the end, which is turned forward….

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…unmoved, extended. The line of motion of the striking thing is compared either to the line of motion of the other or to its center. If the line of motion of the striking thing is compared with the line of motion (8.) of the receiving thing, that is, of that which is struck, then either the lines of motion of the striking and receiving thing are joined at their ends; or the end of the line of motion of the striking thing touches not an end, but another point of the line of motion of the receiving thing. In this case, the striking thing is said (9.) to run into (incurrere) the receiving thing, and the receiving thing only passes by (praetervehitur). In the former case, both are striking and are said (10.) to concur (concurrere); the concurrence is either (11.) an occurrence (occursus), when the measure of the line of continued motion of one falls into the side of the coming (adventus) of the other, or (12.) an accursus, when this does not happen.

Occurrence is either (13.) direct (rectus), if the measure of the produced line of motion makes a right angle to the side of coming of the other, or coincides with the measure of the line of motion of the other, or (14.) oblique, if otherwise. Accursus is either (15.) direct, if the measure of the line of motion is parallel to the side of coming of the other, or makes a right angle to the measure of the line of motion of the other; or (16.) oblique, if otherwise. (17.) The side of coming or that from which it comes, is the straight line from which (as from a plane) the measure of the line of motion exits perpendicularly toward the impact. Furthermore, if the line of motion of the striking thing is compared to the center of the receiving thing, the impact is either (18.) central, if the produced line of motion of the striking thing falls into the center of the receiving body…

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…or (19.) eccentric, if otherwise. (20.) It is said to graze (radere), which at the moment of contact does not attempt to drive the whole touched body from its place (whether it is passing by or striking, but eccentrically); finally, if the boundaries of the striking and receiving things are compared, they are either surfaces on both sides, or on one part a point or a line.

(21.) One body is constituted by parts which are to remain contiguous to themselves for some time. (22.) Parts cohere of which, when one is moved, the others are moved. (23.) Flexion is a mutation concerning rectitude and curvature. (24.) A face is every extremity of a thing which can be touched by another in one region, or which can be cut off entirely by a straight line. (25.) Hardness is a cohesion not superable by a small motion. (26.) A simple figure is one of which any face is closed by one line or surface. (27.) One line or surface is that which can be made by one motion.

(28.) Motion is one, prior and posterior, if the continuation is made spontaneously, or through itself, even if no extrinsic impulse is added. (29.) A wheel-shaped body is that which can be moved so that it does not leave its place—that is, so that it comes into no part of itself where some part of it has not already been—such as the motion of the Celestial Orbs believed by the ancients, and which can only exist in a plenum.