10. Between the degrees of hard and soft are those things which we call tough, tough being that which may be bent without being altered from what it was; and the bending of a line is either the adduction or diduction of the extreme parts, that is, a motion from straitness to crookedness, or contrarily, whilst the line remains still the same it was; for by drawing out the extreme points of a line to their greatest distance, the line is made strait, which otherwise is crooked. So also the bending of a superficies is the diduction or adduction of its extreme lines, that is, their dilatation and contraction.
Dilatation & contraction suppose mutation of the smallest parts in respect of their situation.
11. Dilatation and contraction, as also all flexion, supposes necessarily that the internal parts of the body bowed do either come nearer to the external parts, or go further from them. For though flexion be considered only in the length of a body, yet when that body is bowed, the line which is made on one side will be convex, and the line on the other side will be concave; of which the concave, being the interior line, will, unless something be taken from it and added to the convex line, be the more crooked, that is, the greater of the two. But they are equal; and, therefore, in flexion there is an accession made from the interior to the exterior parts; and, on the contrary, in tension, from the exterior to the interior parts. And as for those things which do not easily suffer such transposition of their parts, they are called brittle; and the great force they require to make them yield, makes them also with sudden motion to leap asunder, and break in pieces.
All traction is pulsion.
12. Also motion is distinguished into pulsion and traction. And pulsion, as I have already defined it, is when that which is moved goes before that which moveth it. But contrarily, in traction the movent goes before that which is moved. Nevertheless, considering it with greater attention, it seemeth to be the same with pulsion. For of two parts of a hard body, when that which is foremost drives before it the medium in which the motion is made, at the same time that which is thrust forwards thrusteth the next, and this again the next, and so on successively. In which action, if we suppose that there is no place void, it must needs be, that by continual pulsion, namely, when that action has gone round, the movent will be behind that part, which at the first seemed not to be thrust forwards, but to be drawn; so that now the body, which was drawn, goes before the body which gives it motion; and its motion is no longer traction, but pulsion.
Such things as being pressed or bent restore themselves, have motion in their internal parts.
13. Such things as are removed from their places by forcible compression or extension, and, as soon as the force is taken away, do presently return and restore themselves to their former situation, have the beginning of their restitution within themselves, namely, a certain motion in their internal parts, which was there, when, before the taking away of the force, they were compressed, or extended. For that restitution is motion, and that which is at rest cannot be moved, but by a moved and a contiguous movent. Nor doth the cause of their restitution proceed from the taking away of the force by which they were compressed or extended; for the removing of impediments hath not the efficacy of a cause, as has been shown at the end of the [3rd article] of chap. XV. The cause therefore of their restitution is some motion either of the parts of the ambient, or of the parts of the body compressed or extended. But the parts of the ambient have no endeavour which contributes to their compression or extension, nor to the setting of them at liberty, or restitution. It remains therefore that from the time of their compression or extension there be left some endeavour or motion, by which, the impediment being removed, every part resumes its former place; that is to say, the whole restores itself.
Though that which carrieth another be stopped, the body carried will proceed.
14. In the carriage of bodies, if that body, which carries another, hit upon any obstacle, or be by any means suddenly stopped, and that which is carried be not stopped, it will go on, till its motion be by some external impediment taken away.
For I have demonstrated (chap. VIII, [art. 19]) that motion, unless it be hindered by some external resistance, will be continued eternally with the same celerity; and in the [7th article] of chap. IX, that the action of an external agent is of no effect without contact. When therefore that, which carrieth another thing, is stopped, that stop doth not presently take away the motion of that which is carried. It will therefore proceed, till its motion be by little and little extinguished by some external resistance: which was to be proved; though experience alone had been sufficient to prove this.