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air, is about the commonest of all motions. And yet in the observation and inquisition hereof men have shown a strange supineness and negligence; nor is it
small loss that is entailed by miscarriage in the invesigation of the nature and power of this motion ; seeing it is of use in infinite ways, and as the life and soul of artillery, engines, and the whole business of mechanics. Now most inquirers, when they have pronounced this notion to be violent, and distinguished it from natural notion, think they have done. And it is indeed the peculiar manner and discipline of Aristotle and his school, to teach men what to say, not what to think ; ind how to discharge themselves by affirming or denying, not how to explain and satisfy themselves in chought. Others use a little more diligence, and tak. ng up the position that two bodies cannot be in the ame place, conclude that the stronger impels, and the weaker gives way; that this giving way or flight, if the force applied be small, does not continue after the cessation of the first impulse, as in protrusion ; but that if the force be great, it continues for a time even after the removal of the impelling body, till it is gradually diininished, as in throwing. And these again, after another inveterate habit of the same school, catch at the beginnings of things, but do not trouble themselves about their process and end; as if every beginning implied the rest; and hence, in a kind of
premature impatience, they break off the inquiry. For upon the point that bodies yield at the instant of the stroke, they have something to say ; but why, after the impelling body has been removed, and the necessity for the disarrangement of the bodies has thereby absolutely ceased, the motion should still continue, they say
nothing, nor do they clearly understand themselves. Others, more diligent and perseverant in inquiry, having observed the power of the air in winds, and the like, which is so great that it can even blow down trees and towers, imagined that the force which carries and accompanies projectiles after the first impulse should be attributed to the air collecting itself and rushing in behind the body moved, by which force the body is carried forward like a ship in the water. And these certainly keep to the point, and carry their speculation to its issue ; yet they fail of the truth. Now the case is really this. The principal motion seems to be in the parts of the body projected, which being too subtle to be perceived by the eye, and men not being attentive enough but passing the matter by with a light observation, is not observed. But to an accurate observer it is manifest that hard bodies are most impatient of pressure, and have, as it were, a very acute perception thereof; so that when forced ever so little out of their natural position, they strive with great velocity to free themselves and return to their former state.
And to do this, all the parts, commencing with the part struck, thrust and press one another forward, just like an external force ; which produces a continuous and intense
; (though invisible) trepidation and commotion of the parts. And this we see in glass, sugar, and brittle things of the like nature; which, if they be cut or divided with any sharp iron instrument, directly and almost instantaneously break to pieces in other places untouched by the stroke of the instrument; which plainly proves that the motion of pressure is communicated to the neighbouring parts; which motion, working all round, and making trial everywhere, causes
fracture in that part, where from the predisposition of the body the union was weakest ; and yet this very motion, while it disturbs and penetrates every part, does not show itself to the eye until there is an open fracture or solution of continuity. Again, we see if a piece of iron wire, or a stick, or a quill (or such like bodies as are flexible and yet elastic) be bent, and held by both ends between the finger and thumb, it immediately leaps away. Now the cause of this motion is proved manifestly not to lie in the extreme parts of the body, which are held fast by the fingers; but in the middle, which bears the violence; to relieve which this motion is set at work. But in this example it plainly appears that the cause of motion they derive from the impulse of the air is excluded; for there is no percussion to set the air in action. And this is also shown in the trivial experiment of squeezing a fresh and slippery plum-stone between the fingers, gradually increasing the pressure, and so shooting it out. For in this example likewise, compression takes the place of percussion. But the most evident effect of this motion is seen in the perpetual revolutions or rotations of projectiles in their flight; for they go forward, but their progress is in spiral lines, — that is, revolving as they
go. And certainly I have felt some doubt as to this spiral motion, so rapid as it is and yet so free and as it were familiar to things, whether it did not depend on some higher principle. But I think the cause of this effect is the same that I am now speaking of, and no other. For pressure of a body at once excites a motion in the parts or particles to extricate and free themselves in any way they can. And hence the body is not only driven in a straight line, and so flies forward ; but it tries all round, and therefore revolves; for both motions help to set it free. And in solid bodies this is something subtle and abstruse ; in soft bodies it is evident, and almost palpable. For as wax or lead, and similar soft bodies, on being struck with a hammer, give way not only forwards, but on all sides ; so hard or resisting bodies fly both in a right line and round about. For the corporeal yielding in soft bodies and the local yielding in hard proceed on the same principle ; and it is in the change of shape of a soft body that we can best perceive what the passion of a hard body is when it escapes and flies. Meantime, I would not be understood to deny that, besides this motion (which is the principal thing), some part of the work is also to be attributed to the conveyance of the air, by which the principal motion may be assisted, impeded, turned, and directed. For of this too the power is not inconsiderable. And this explanation of violent and mechanical motion (which has hitherto escaped observation) is as the fountain of practical operation.
On the cause of Motion in Fire-arms, that it has only been
inquired in part, and that not the principal one. The cause of fire-arms, and the explanation of so powerful and noble a motion is imperfect, and deficient in the most important part. For they say that gunpowder, when converted into flame and rarefied, dilates itself, and fills a larger space; and hence follows, — as otherwise either two bodies would be in one place, or there would be a penetration of dimensions, or the form of the element would be destroyed, or the situation of the parts would be contrary to the nature of the whole (for these are the phrases used), — the expulsion or breaking out of the opposing body. And there is something in what they say. For this appetite and passion of matter has likewise some part in this kind motion. Nevertheless they are wrong in too hastily referring the matter to this necessity of dilatation of the body, without distinctly considering that which in nature precedes. For that the body of the powder after it is turned into flame should occupy a larger space, is indeed necessary; but that the body of the powder should catch flame, and that too with such rapidity, is not so, but depends on the preceding conflict and relation of motions with one another. For no doubt but that the solid and heavy body, which is driven out or removed by a motion of this kind, resists sedulously before it gives way; and if it be stronger it gains the victory ; that is, the flame does not drive out the bullet, but the bullet smothers the flame. Therefore if in place of gunpowder you take sulphur, camphor, or like things, which themselves soon catch fire, and if (since compactness of bodies is an impediment to kindling) you make them up into grains of powder mixed with some portion of the ash of juniper, or some other very combustible wood, yet (if there be no nitre) that rapid and powerful motion will not follow, but the motion of kindling will be hindered and restrained by the mass of the resisting body, and will not develop itself or take effect. But the truth of the matter is this. You will find that the motion here inquired is double and compound. For besides the motion of kindling, which is principally in the sulphur of the powder, there is another stronger and more violent. This proceeds from