CHAPTER VII.. Problems of Motion Perpendicular, and Oblique;

PROBLEMS OF MOTION PERPENDICULAR, AND OBLIQUE; OF PRESSION AND PERCUSSION; REFLECTION AND REFRACTION; ATTRACTION AND REPULSION.

A. If a bullet from a certain point given, be shot against a wall perpendicularly, and again from the same point obliquely, what will be the proportion of the forces wherewith they urge the wall? For example, let the wall be A B, a point given E, a gun C E, that carries the bullet perpendicularly to F, and another gun D E, that carries the like bullet with the same swiftness obliquely to G; in what proportion will their forces be upon the wall?

B. The force of the stroke perpendicular from E to F will be greater then the oblique force from E to G, in the proportion of the line E G to the line E F.

A. How can the difference be so much? Can the bullet lose so much of its force in the way from E to G?

B. No; we will suppose it loseth nothing of its swiftness. But the cause is, that their swiftness being equal, the one is longer in coming to the wall than the other, in proportion of time, as E G to E F. For though their swiftness be the same, considered in themselves, yet the swiftness of their approach to the wall is greater in E F than in E G, in proportion of the lines themselves.

A. When a bullet enters not, but rebounds from the wall, does it make the same angle going off, which it did falling on, as the sun-beams do?

B. If you measure the angles close by the wall their difference will not be sensible; otherwise it will be great enough, for the motion of the bullet grows continually weaker. But it is not so with the sun-beams which press continually and equally.

A. What is the cause of reflection? When a body can go no further on, it has lost its motion. Whence then comes the motion by which it reboundeth?

B. This motion of rebounding or reflecting proceedeth from the resistance. There is a difference to be considered between the reflection of light, and of a bullet, answerable to their different motions, pressing and striking. For the action which makes reflection of light, is the pressure of the air upon the reflecting body, caused by the sun, or other shining body, and is but a contrary endeavour; as if two men should press with their breasts upon the two ends of a staff, though they did not remove one another, yet they would find in themselves a great disposition to press backward upon whatsoever is behind them, though not a total going out of their places. Such is the way of reflecting light. Now, when the falling on of the sun-beams is oblique, the action of them is nevertheless perpendicular to the superficies it falls on. And therefore the reflecting body, by resisting, turneth back that motion perpendicularly, as from F to E; but taketh nothing from the force that goes on parallel in the line of E H, because the motion never presses. And thus of the two motions from F to E, and from E to H, is a compounded motion in the line F H, which maketh an angle in B G, equal to the angle F G E.

But in percussion (which is the motion of the bullet against a wall,) the bullet no sooner goeth off than it loseth of its swiftness, and inclineth to the earth by its weight. So that the angles made in falling on and going off, cannot be equal, unless they be measured close to the point where the stroke is made.

A. If a man set a board upright upon its edge, though it may very easily be cast down with a little pressure of one’s finger, yet a bullet from a musket shall not throw it down, but go through it. What is the cause of that?

B. In pressing with your finger you spend time to throw it down. For the motion you give to the part you touch is communicated to every other part before it fall. For the whole cannot fall till every part be moved. But the stroke of a bullet is so swift, as it breaks through, before the motion of the part it hits can be communicated to all the other parts that must fall with it.

A. The stroke of a hammer will drive a nail a great way into a piece of wood on a sudden. What weight laid upon the head of a nail, and in how much time will do the same? It is a question I have heard propounded amongst naturalists.

B. The different manner of the operation of weight from the operation of a stroke, makes it incalculable. The suddenness of the stroke upon one point of the wood takes away the time of resistance from the rest. Therefore the nail enters so far as it does. But the weight not only gives them time, but also augments the resistance; but how much, and in how much time, is, I think, impossible to determine.

A. What is the difference between reflection and recoiling?

B. Any reflection may, and not unproperly, be called recoiling; but not contrariwise every recoiling reflection. Reflection is always made by the reaction of a body pressed or stricken; but recoiling not always. The recoiling of a gun is not caused by its own pressing upon the gunpowder, but by the force of the powder itself, inflamed and moved every way alike.

A. I had thought it had been by the sudden re-entering of the air after the flame and bullet were gone out. For it is impossible that so much room as is left empty by the discharging of the gun, should be so suddenly filled with the air that entereth at the touchhole.

B. The flame is nothing but the powder itself, which scattered into its smallest parts, seems of greater bulk by much, than in truth it is, because they shine. And as the parts scatter more and more, so still more air gets between them, entering not only at the touchhole, but also at the mouth of the gun, which two ways being opposite, it will be much too weak to make the gun recoil.

A. I have heard that a great gun charged too much or too little, will shoot, not above, nor below, but beside the mark; and charged with one certain charge between both, will hit it.

B. How that should be I cannot imagine. For when all things in the cause are equal, the effects cannot be unequal. As soon as fire is given, and before the bullet be out, the gun begins to recoil. If then there be any unevenness or rub in the ground more on one side than on the other, it shall shoot beside the mark, whether too much, or too little, or justly charged; because if the line wherein the gun recoileth decline, the way of the bullet will also decline to the contrary side of the mark. Therefore I can imagine no cause of this event, but either in the ground it recoils on, or in the unequal weight of the parts of the breech.

A. How comes refraction?

B. When the action is in a line perpendicular to the superficies of the body wrought upon, there will be no refraction at all. The action will proceed still in the same straight line, whether it be pression as in light, or percussion as in the shooting of a bullet. But when the pression is oblique, then will the refraction be that way which the nature of the bodies through which the action proceeds shall determine.

A. How is light refracted?

B. If it pass through a body of less, into a body of greater resistance, and to the point of the superficies it falleth on, you draw a line perpendicular to the same superficies, the action will proceed not in the same line by which it fell on, but in another line bending toward that perpendicular.

A. What is the reason of that?

B. I told you before, that the falling on worketh only in the perpendicular; but as soon as the action proceedeth farther inward than a mere touch, it worketh partly in the perpendicular, and partly forward, and would proceed in the same line in which it fell on, but for the greater resistance which now weakeneth the motion forward, and makes it to incline towards the perpendicular.

A. In transparent bodies it may be so; but there be bodies through which the light cannot pass at all.

B. But the action by which light is made, passeth through all bodies. For this action is pression; and whatsoever is pressed, presseth that which is next behind, and so continually. But the cause why there is no light seen through it, is the unevenness of the parts within, whereby the action is by an infinite number of reflections so diverted and weakened, that before it hath proceeded through, it hath not strength left to work upon the eye strongly enough to produce sight.

A. If the body being transparent, the action proceed quite through, into a body again of less resistance, as out of glass into the air, which way shall it then proceed in the air?

B. From the point where it goeth forth, draw a perpendicular to the superficies of the glass, the action now freed from the resistance it suffered, will go from that perpendicular, as much as it did before come towards it.

A. When a bullet from out of the air entereth into a wall of earth, will that also be refracted towards the perpendicular.

B. If the earth be all of one kind, it will. For the parallel motion, will there also at the first entrance be resisted, which it was not before it entered.

A. How then comes a bullet, when shot very obliquely into any broad water, and having entered, yet to rise again into the air?

B. When a bullet is shot very obliquely, though the motion be never so swift, yet the approach downwards to the water is very slow, and when it cometh to it, it casteth up much water before it, which with its weight presseth downwards again, and maketh the water to rise under the bullet with force enough to master the weak motion of the bullet downwards, and to make it rise in such manner as bodies use to rise by reflection.

A. By what motion (seeing you ascribe all effects to motion) can a loadstone draw iron to it?

B. By the same motion hitherto supposed. But though all the smallest parts of the earth have this motion, yet it is not supposed that their motions are in equal circles; nor that they keep just time with one another; nor that they have all the same poles. If they had, all bodies would draw one another alike. For such an agreement of motion, of way, of swiftness, and of poles, cannot be maintained, without the conjunction of the bodies themselves in the centre of their common motion, but by violence. If therefore the iron have but so much of the nature of the loadstone as readily to receive from it the like motion, as one string of a lute doth from another string strained to the same note, (as it is like enough it hath, the loadstone being but one kind of iron ore), it must needs after that motion received from it, unless the greatness of the weight hinder, come nearer to it, because at distance their motions will differ in time, and oppose each other, whereby they will be forced to a common centre. If the iron be lifted up from the earth, the motion of the loadstone must be stronger, or the body of it nearer, to overcome the weight; and then the iron will leap up to the loadstone as swiftly, as from the same distance it would fall down to the earth; but if both the stone and the iron be set floating upon the water, the attraction will begin to be manifest at a greater distance, because the hindrance of the weight is in part removed.

A. But why does the loadstone, if it float on a calm water, never fail to place itself at last in the meridian just north and south.

B. Not so, just in the meridian, but almost in all places with some variations. But the cause I think is, that the axis of this magnetical motion is parallel to the axis of the ecliptic, which is the axis of the like motion in the earth, and consequently that it cannot freely exercise its natural motion in any other situation.

A. Whence may this consent of motion in the loadstone and the earth proceed? Do you think, as some have written, that the earth is a great loadstone?

B. Dr. Gilbert, that was the first that wrote anything of this subject rationally, inclines to that opinion. Descartes thought the earth, excepting this upper crust of a few miles depth, to be of the same nature with all other stars, and bright. For my part, I am content to be ignorant; but I believe the loadstone hath been given its virtue by a long habitude in the mine, the vein of it lying in the plane of some of the meridians, or rather of some of the great circles that pass through the poles of the ecliptic, which are the same with the poles of the like motion supposed in the earth.

A. If that be true, I need not ask why the filings of iron laid on a loadstone equally distant from its poles will lie parallel to the axis, but on each side will incline to the pole that is next. Nor why by drawing a loadstone all along a needle of iron, the needle will receive the same poles. Nor why when the loadstone and iron, or two loadstones, are put together floating upon water, will fall one of them astern of the other, that their like parts may look the same way, and their unlike touch, in which action they are commonly said to repel one another. For all this may be derived from the union of their motions. One thing more I desire to know, and that is; what are those things they call spirits? I mean ghosts, fairies, hobgoblins, and the like apparitions.

B. They are no part of the subject of natural philosophy.

A. That which in all ages, and all places is commonly seen (as those have been, unless a great part of mankind be liars) cannot, I think, be supernatural.

B. All this that I have hitherto said, though upon better ground than can be had for a discourse of ghosts, you ought to take but for a dream.

A. I do so. But there be some dreams more like sense then others. And that which is like sense pleases me as well in natural philosophy, as if it were the very truth.

B. I was dreaming also once of these things; but was wakened by their noise. And they never came into any dream of mine since, unless apparitions in dreams and ghost be all one.

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