------------------------------------------------------------------------ [Illustration: _Vol. 1. Lat. & Eng._ C. XXI. _Fig. 1-5_ ] ------------------------------------------------------------------------ ------- CHAPTER XXII. OF OTHER VARIETY OF MOTION. 1. Endeavour and pressure how they differ.—2. Two kinds of mediums in which bodies are moved.—3. Propagation of motion, what it is.—4. What motion bodies have, when they press one another.—5. Fluid bodies, when they are pressed together, penetrate one another.—6. When one body presseth another and doth not penetrate it, the action of the pressing body is perpendicular to the superficies of the body pressed.—7. When a hard body, pressing another body, penetrates the same, it doth not penetrate it perpendicularly, unless it fall perpendicularly upon it.—8. Motion sometimes opposite to that of the movent.—9. In a full medium, motion is propagated to any distance.—10. Dilatation and contraction what they are.—11. Dilatation and contraction suppose mutation of the smallest parts in respect of their situation.—12. All traction is pulsion.—13. Such things as being pressed or bent restore themselves, have motion in their internal parts.—14. Though that which carrieth another be stopped, the body carried will proceed.—15, 16. The effects of percussion not to be compared with those of weight.—17, 18. Motion cannot begin first in the internal parts of a body.—19. Action and reaction proceed in the same line.—20. Habit, what it is. [Sidenote: Endeavour and pressure how they differ.] 1. I have already (chapter XV. art. 2) defined endeavour to be motion through some length, though not considered as length, but as a point. Whether, therefore, there be resistance or no resistance, the endeavour will still be the same. For simply to endeavour is to go. But when two bodies, having opposite endeavours, press one another, then the endeavour of either of them is that which we call pressure, and is mutual when their pressures are opposite. [Sidenote: Two kinds of mediums in which bodies are moved.] 2. Bodies moved, and also the mediums in which they are moved, are of two kinds. For either they have their parts coherent in such manner, as no part of the moved body will easily yield to the movent, except the whole body yield also, and such are the things we call _hard_: or else their parts, while the whole remains unmoved, will easily yield to the movent, and these we call _fluid_ or _soft_ bodies. For the words _fluid_, _soft_, _tough_, and _hard_, in the same manner as _great_ and _little_, are used only comparatively; and are not different kinds, but different degrees of quality. [Sidenote: Propagation of motion, what it is.]