414 THOUGHTS ON THE NATURE OF THINGS. permeates the whole, come into view, till a visible break or divulsion of continuity takes place. Again, we observe, if we happen to bend and compress between the thumb and forefinger the two ends of a wire, or bit of cane, or the harder part of a pen, (or similar bodies which unite flexibility with a certain degree of elasticity,) they anon spring from the hand. The cause of which motion is evidently discernible not to be in the extremities compressed by the fingers, but in the middle part, which is the seat of forcible pressure, to relieve itself from which, the motion cornes into play. And in this instance it is clearly shown, that the alleged cause of motion, the impulsion of -the air, is inadmissible. For here there is no concussion to let in a rush of air. This is also proved by a slight experiment, when we press the fresh and smooth ball of a plum, drawing the fingers gradually together, and in this manner let it go. For in that instance also compression is substituted for percussion. But the most conspi cuous effect of this interior motion is in the revo lutions and gyrations of missiles while flying. The missiles, indeed, proceed onwards, but they majie their progression in spiral lines, that is, by straight-lined and rotatory motion together, and indeed this curvilinear motion is so fleet, and at the same time so easy, ard somehow so familiar to things, as to excite a doubt in my mind whe ther it does not depend on some higher principle. Yet I think that the cause of this fact is no other than the same we are now handling. For the concussion of the body occasions an excessive impetus in all its parts and particles, to effect in some way or other their extrication and freedom. The body, therefore, not only acts and flies forth in a straight line, but strives to move from every point in it at once, and, therefore, whirls round ; for in both ways it somewhat relieves itself of its impulse. Now this, which in the harder solids is a somewhat recondite and latent property, is in the softer ones evident, and, so to speak, palpable. For as wax, and lead, and similar soft bodies, when struck with a mallet, give way not only in the line of percussion, but laterally every way; so, in like manner, hard or resisting bodies move in a straight line and periphery at once. For the retrocession of soft bodies in their substance, and of hard ones in their place, is the same in its prin ciple, as is evidently seen in the structure of the soft body, and in the affection of the hard one, exhibited in its flight and volant path. Meantime let none think that besides this motion, (which is the cardinal point,) I do not ascribe a certain de gree of effect to the conveyance of the air, which is capable of assisting, obstructing, modifying, and reguiating the principal motion ; for its power is far from being inconsiderable. And this doc trine of violent or mechanical motion (which has oeen hitherto unknown) is, as it were, the foun tain-head of practical mechanics. Of the cause of Motion in Fire-arms^ whir.h has been hitherto investigated only in part, and that part comparatively unimportant. The theory of fire-arms of a motion so power ful and so remarkable, is imperfect, and, in the more important part, defective. For it is said in explanation that the gunpowder, after having been converted into flame and volatilized, expands and occupies more space; whence it follows, that as two bodies cannot exist in the same space, other wise a jumbling of their dimensions would ensue, or the elementary form be destroyed, or a preter natural arrangement of the internal parts of the body be the effect, (for this is what they say,) that the impeding body is ejected or broken. And what they say contains something. P or this ten dency is both an affection of "matter, and an ingre dient in the motion itself. Yet they err in this, that in their over hasty way of determining, they jump at once to the necessary consequence of thn dilatation of a body, and do not accurately consider what comes first in the order of nature. For thai the substance of the gunpowder, after having been converted into flame, must occupy a larger space, is doubtless a thing of necessity ; but that the sub stance of the gunpowder should be inflamed at all, and that so instantaneously, is not determined by a like necessity, but depends on an antagonism, and comparative force of motions. For there is not a doubt, that the compact and heavy body which is expelled or dislodged by this motion, offers considerable resistance before it gives way, and, if it happen to be the stronger, is victorious, that is to say, the flame, in that case, does not cast out the ball, but the ball stifles the flame. If, therefore, instead of gunpowder, you take sul phur, asphaltum, or the like, bodies which are also quickly inflammable, and (as the closeness of particles in bodies hinders ignition) reduce them to a grain like gunpowder, mixing up with it a small quantity of the ashes of the juniper, or some other very combustible wood ; yet, should the nitre be wanting, that rapid and powerful motion does not follow : the motion to perfect inflamma tion is impeded and fettered by the resisting body, so that it cannot fully expand and take effect. I For, besides the motion of inflammation, which chiefly arises from the sulphureous part of the I gunpowder, there is yet another powerful andvio- I lent motion in the case. This ie caused by the j crude watery ether, which is extricated from the nitre in part, but chiefly from the charcoal, and which not only itself dilates, as exhaled essences are wont to do, on the application of heat, but at the same time (which is the principal circum stance) by a motion of extreme rapidity, flies off and breaks forth from the heat and flame, thus distending and opening passages for the inflam mation to follow. Of this motion we see the
