OF GRAVITY AND GRAVITATION.
_B._ What books are those?
_A._ Two books written by two learned men concerning gravity. I brought
them with me, because they furnish me with some material questions about
that doctrine; though of the nature of gravity, I find no more in either
of them than this, that gravity is an intrinsical quality, by which a
body so qualified descendeth perpendicularly towards the superficies of
the earth.
_B._ Did neither of them consider that descending is local motion; that
they might have called it an intrinsical motion rather than an
intrinsical quality?
_A._ Yes. But not how motion should be intrinsical to the special
individual body moved. For how should they, when you are the first that
ever sought the differences of qualities in local motion, except your
authority in philosophy were greater with them than it is? For it is
hard for a man to conceive, except he see it, how there should be motion
within a body, otherwise than as it is in living creatures.
_B._ But it may be they never sought, or despaired of finding what
natural motion could make any inanimate thing tend one way rather than
another.
_A._ So it seems. But the first of them inquires no farther than, why so
much water, being a heavy body, as lies perpendicularly on a fish’s back
in the bottom of the sea, should not kill it. The other, whereof the
author is Dr. Wallis, treateth universally of gravity.
_B._ Well; but what are the questions which from these books you intend
to ask me?
_A._ The author of the first book tells me, that water and other fluids
are bodies continued, and act, as to gravity, as a piece of ice would do
of the same figure and quantity. Is that true?
_B._ That the universe, supposing there is no place empty, is one entire
body, and also, as he saith it is, a continual body, is very true. And
yet the parts thereof may be contiguous, without any other cohesion but
touch. And it is also true, that a vessel of water will descend in a
medium less heavy, but fluid, as ice would do.
_A._ But he means that water in a tub would have the same effect upon a
fish in the bottom of the tub, as so much ice would have.
_B._ That also would be true, if the water were frozen to the sides of
it. Otherwise the ice, if there be enough, will crush the fish to death.
But how applies he this, to prove that the water cannot hurt a fish in
the sea by its weight?
_A._ It plainly appears that water does not gravitate on any part of
itself beneath it.
_B._ It appears by experience, but not by this argument, though instead
of water the tub were filled with quicksilver.
_A._ I thought so. But how it comes to pass that the fish remains
uncrushed, I cannot tell.
_B._ The endeavour of the quicksilver downward is stopped by the
resistance of the hard bottom. But all resistance is a contrary
endeavour; that is, an endeavour upwards, which gives the like endeavour
to the quicksilver, which is also heavy, and thereby the endeavour of
the quicksilver is diverted to the sides round about, where stopped
again by the resistance of the sides, it receives an endeavour upwards,
which carries the fish to the top, lying all the way upon a soft bed of
quicksilver. This is the true manner how the fish is saved harmless. But
your author, I believe, either wanted age, or had too much business, to
study the doctrine of motion; and never considered that resistance is
not an impediment only, or privation, but a contrary motion; and that
when a man claps two pieces of wax together, their contrary endeavour
will turn both the pieces into one cake of wax.
_A._ I know not the author; but it seems he has deeplier considered this
question than other men; for in the introduction to his book he saith,
“that men have pre-engaged themselves to maintain certain principles of
their own invention, and are therefore unwilling to receive anything
that may render their labour fruitless;” and, “that they have not
strictly enough considered the several interventions that abate, impede,
advance, or direct the gravitation of bodies.”
_B._ This is true enough; and he himself is one of those men, in that he
considered not, that resistance is one of those interventions which
abate, impede, and direct gravitation. But what are his suppositions for
the questions he handles?
_A._ His first is, that as in a pyramid of brick, wherein the bricks are
so joined that the uppermost lies everywhere over the joint or cement of
the two next below it, you may break down a part and leave a cavity, and
yet the bricks above will stand firm and sustain one another by their
cross posture: so also it is in wheat, hailshot, sand, or water; and so
they arch themselves, and thereby the fish is every way secured by an
arch of water over it.
_B._ That the cause why fishes are not crushed nor hurt in the bottom of
the sea by the weight of the water, is the water’s arching itself, is
very manifest. For if the uppermost orb of the water should descend by
its gravity, it would tend toward the centre of the earth, and place
itself all the way in a less and lesser orb, which is impossible. For
the places of the same body are always equal. But that wheat, sand,
hailshot, or loose stones should make a firm arch, is not credible.
_A._ The author therefore, it seems, quits it, and taketh a second
hypothesis for the true cause, though the former, he saith, be not
useless, but contributes its part to it.
_B._ I see, though he depart from his hypothesis, he looks back upon it
with some kindness. What is his second hypothesis?
_A._ It is, that air and water have an endeavour to motion upward,
downward, directly, obliquely, and every way. For air, he saith, will
come down his chimney, and in at his door, and up his stairs.
_B._ Yes, and mine too; and so would water, if I dwelt under water,
rather than admit of vacuum. But what of that?
_A._ Why then it would follow, that those several tendencies or
endeavours would so abate, impede, and correct one another, as none of
them should gravitate. Which being granted, the fish can take no harm;
wherein I find one difficulty, which is this: the water having an
endeavour to motion every way at once, methinks it should go no way, but
lie at rest; which, he saith, was the opinion of Stevinus, and rejecteth
it, saying, it would crush the fish into pieces.
_B._ I think the water in this case would neither rest nor crush. For
the endeavour being, as he saith, intrinsical, and every way, must needs
drive the water perpetually outward; that is to say, as to this
question, upwards; and seeing the same endeavour in one individual body
cannot be more ways at once than one, it will carry it on perpetually
without limit, beyond the fixed stars; and so we shall never more have
rain.
_A._ As ridiculous as it is, it necessarily follows.
_B._ What are Dr. Wallis’s suppositions?
_A._ He goes upon experiments. And, first, he allegeth this, that water
left to itself without disturbance, does naturally settle itself into a
horizontal plane.
_B._ He does not then, as your author and all other men, take gravity
for that quality whereby a body tendeth to the centre of the earth.
_A._ Yes, he defines gravity to be a natural propension towards the
centre of the earth.
_B._ Then he contradicteth himself. For if all heavy bodies tend
naturally to one centre, they shall never settle in a plane, but in a
spherical superficies. But against this, that such an horizontal plane
is found in water by experience, I say it is impossible. For the
experiment cannot be made in a basin, but in half a mile at sea
experience visibly shows the contrary. According to this, he should
think also that a pair of scales should hang parallel.
_A._ He thinks that too.
_B._ Let us then leave this experiment. What says he farther concerning
gravity?
_A._ He takes for granted, not as an experiment but an axiom, that
nature worketh not by election, but _ad ultimum virium_, with all the
power it can.
_B._ I think he means, (for it is a very obscure passage), that every
inanimate body by nature worketh all it can without election; which may
be true. But it is certain that men, and beasts, work often by election,
and often without election; as when he goes by election, and falls
without it. In this sense I grant him, that nature does all it can. But
what infers he from it?
_A._ That naturally every body has every way, if the ways oppose not one
another, an endeavour to motion; and consequently, that if a vessel have
two holes, one at the side, another at the bottom, the water will run
out at both.
_B._ Does he think the body of water that runs out at the side, and that
which runs out at the bottom, is but one and the same body of water?
_A._ No, sure; he cannot think but that they are two several parts of
the whole water in the vessel.
_B._ What wonder is it then, if two parts of water run two ways at once,
or a thousand parts a thousand ways? Does it follow thence that one body
can go more than one way at once? Why is he still meddling with things
of such difficulty? He will find at last that he has not a genius either
for natural philosophy or for geometry. What other suppositions has he?
_A._ My first author had affirmed, that a lighter body does not
gravitate on a heavier; against this Dr. Wallis thus argueth: Let there
be a siphon, A B C D, filled with quicksilver to the level A D; if then
you pour oil upon A as high as to E, he asketh if the oil in A E, as
being heavy, shall not press down the quicksilver a little at A, and
make it rise a little at D, suppose to F; and answers himself, that
certainly it will; so that it is neither an experiment nor an
hypothesis, but only his opinion.
_B._ Whatsoever it be, it is not true; though the doctor may be
pardoned, because the contrary was never proved.
_A._ Can you prove the contrary?
_B._ Yes; for the endeavour of the quicksilver both from A and D
downward, is stronger than that of the oil downward. If, therefore, the
endeavour of the quicksilver were not resisted by the bottom B C, it
would fall so, by reason of the acceleration of heavy bodies in their
descending, as to leave the oil, so that it should not only not press,
but also not touch the quicksilver. It is true, in a pair of scales
equally charged with quicksilver, that the addition of a little oil to
either scale will make it preponderate. And that was it deceived him.
_A._ It is evident. The last experiment he cites is the weighing of air
in a pair of scales, where it is found manifestly that it has some
little weight. For if you weigh a bladder, and put the weight into one
scale, and then blow the bladder full of air, and put it into the other
scale, the full bladder will outweigh the empty. Must not then the air
gravitate?
_B._ It does not follow. I have seen the experiment just as you describe
it, but it can never be thence demonstrated that air has any weight. For
as much air as is pressed downward by the weight of the blown bladder,
so much will rise from below, and lay itself spherically at the altitude
of the centre of gravity of the bladder so blown. So that all the air
within the bladder above that centre is carried thither imprisoned, and
by violence: and the force that carries it up is equal to that which
presseth it down. There must, therefore, be allowed some little
counterpoise in the other scale to balance it. Therefore, the experiment
proves nothing to his purpose. And whereas they say there be small heavy
bodies in the air, which make it gravitate, do they think the force
which brought them thither cannot hold them there?
_A._ I leave this question of the fish as clearly resolved, because the
water tending every way to one point, which is the centre of the earth,
must of necessity arch itself. And now tell me your own opinion
concerning the cause of gravity, and why all bodies descend or ascend
not all alike. For there can be no more matter in one place than another
if the places be equal.
_B._ I have already showed you in general, that the difference of motion
in the parts of several bodies makes the difference of their natures.
And all the difference of motions consisteth either in swiftness, or in
the way, or in the duration. But to tell you in special why gold is
heaviest, and then quicksilver, and then, perhaps, lead, is more than I
hope to know, or mean to enquire; for I doubt not but that the species
of heavy, hard, opaque, and diaphanous, were all made so at their
creation, and at the same time separated from different species. So that
I cannot guess at any particular motions that should constitute their
natures, farther than I am guided by the experiments made by fire or
mixture.
_A._ You hope not then to make gold by art?
_B._ No, unless I could make one and the same thing heavier than it was.
God hath from the beginning made all the kinds of hard, and heavy, and
diaphanous bodies that are, and of such figure and magnitude as he
thought fit; but how small soever, they may by accretion become greater
in the mine, or perhaps by generation, though we know not how. But that
gold, by the art of man, should be made of not gold, I cannot
understand; nor can they that pretend to show how. For the heaviest of
all bodies, by what mixture soever of other bodies, will be made
lighter, and not to be received for gold.
_A._ Why, when the cause of gravity consisteth in motion, should you
despair of finding it?
_B._ It is certain that when any two bodies meet, as the earth and any
heavy body will, the motion that brings them to or towards one another,
must be upon two contrary ways; and so also it is when two bodies press
each other in order to make them hard; so that one contrariety of motion
might cause both hard and heavy, but it doth not, for the hardest bodies
are not always the heaviest; therefore I find no access that way to
compare the causes of different endeavours of heavy bodies to descend.
_A._ But show me at least how any heavy body that is once above in the
air, can descend to the earth, when there is no visible movent to thrust
or pull it down.
_B._ It is already granted, that the earth hath this compounded motion
supposed by Copernicus, and that thereby it casteth the contiguous air
from itself every way round about. Which air so cast off, must
continually, by its nature, range itself in a spherical orb. Suppose a
stone, for instance, were taken up from the ground, and held up in the
air by a man’s hand, what shall come into the place it filled when it
lay upon the earth?
_A._ So much air as is equal to the stone in magnitude, must descend and
place itself in an orb upon the earth. But then I see that to avoid
vacuum, another orb of air of the same magnitude must descend, and place
itself in that, and so perpetually to the man’s hand; and then so much
air as would fill the place must descend in the same manner, and bring
the stone down with it. For the stone having no endeavour upward, the
least motion of the air, the hand being removed, will thrust it
downward.
_B._ It is just so. And farther, the motion of the stone downward shall
continually be accelerated according to the odd numbers from unity; as
you know hath been demonstrated by Galileo. But we are nothing the
nearer, by this, to the knowledge of why one body should have a greater
endeavour downward than another. You see the cause of gravity is this
compounded motion with exclusion of vacuum.
_A._ It may be it is the figure that makes the difference. For though
figure be not motion, yet it may facilitate motion, as you see commonly
the breadth of a heavy body retardeth the sinking of it. And the cause
of it is, that it makes the air have farther to go laterally, before it
can rise from under it. For suppose a body of quicksilver falling in the
air from a certain height, must it not, going as it does toward the
centre of the earth, as it draws nearer and nearer to the earth, become
more and more slender, in the form of a solid sector? And if it have far
to go, divide itself into drops? This figure of a solid sector is like a
needle with the point downward, and therefore I should think that
facilitating the motion of it does the same that would be done by
increasing the endeavour.
_B._ Do not you see that this way of facilitating is the same in water,
and in all other fluid heavy bodies? Besides, your argument ought to be
applicable to the weighing of bodies in a pair of scales, which it is
not, for there they have no such figure; it should also hold in the
comparison of gravity in hard and fluid bodies.
_A._ I had not sufficiently considered it. But supposing now, as you do,
that both heavy and hard bodies, in their smallest parts, were made so
in the creation; yet, because quicksilver is harder than water, a drop
of water shall in descending be pressed into a more slender sector than
a drop of quicksilver, and consequently the earth shall more easily cast
off any quantity of water than the same quantity of quicksilver.
_B._ This one would think were true; as also that of simple fluid
bodies, those whose smallest parts, naturally, without the force of
fire, do strongliest cohere, are generally the heaviest. But why then
should quicksilver be heavier than stone or steel? Fluidity and hardness
are but degrees between greater fluidity and greater hardness. Therefore
to the knowledge of what it is that causeth the difference, in different
bodies, of their endeavour downward, there are required, if it can be
known at all, a great many more experiments than have been yet made. It
is not difficult to find why water is heavier than ice, or other body
mixed of air and water. But to believe that all bodies are heavier or
lighter according to the quantity of air within them, is very hard.
_A._ I see by this, that the Creator of the world, as by his power he
ordered it, so by his wisdom he provided it should be never disordered.
Therefore leaving this question, I desire to know whether if a heavy
body were as high as a fixed star, it would return to the earth.
_B._ It is hard to try. But if there be this compounded motion in the
great bodies so high, such as is in the earth, it is very likely that
some heavy bodies will be carried to them. But we shall never know it
till we be at the like height.
_A._ What think you is the reason why a drop of water, though heavy,
will stand upon a horizontal plane of dry or unctuous wood, and not
spread itself upon it? For let A B, in the sixth figure, be the dry
plane, D the drop of water, and D C perpendicular to A B. The drop D,
though higher, will not descend and spread itself upon it.
_B._ The reason I think is manifest. For those bodies which are made by
beating of water and air together, show plainly that the parts of water
have a great degree of cohesion. For the skin of the bubble is water,
and yet it can keep the air, though moved, from getting out. Therefore
the whole drop of water at D, hath a good deal of cohesion of parts. And
seeing A B is an horizontal plane, the way from the contact in D either
to A or B is upwards, and consequently there is no endeavour in D either
of those ways, but what proceeds from so much weight of water as is able
to break that cohesion, which so small a drop is too weak to do. But the
cohesion being once broken, as with your finger, the water will follow.
_A._ Seeing the descent of a heavy body increaseth according to the odd
numbers 1, 3, 5, 7, &c. and the aggregates of those numbers, viz. of 1
and 3; and 1 and 3 and 5; and of 1 and 3 and 5 and 7, &c. are square
numbers, namely 4, 9, 16; the whole swiftness of the descent will be, I
think, to the aggregate of so many swiftnesses equal to the first
endeavour, as square numbers are to their sides, 1, 2, 3, 4. Is it so?
_B._ Yes, you know it hath been demonstrated by Galileo.
_A._ Then if, for instance, you put into a pair of scales equal
quantities of quicksilver and water, seeing they are both accelerated in
the same proportion, why should not the weight of quicksilver to the
weight of water be in duplicate proportions to their first endeavours?
_B._ Because they are in a pair of scales. For there the motion of
neither of them is accelerated. And therefore it will be, as the first
endeavour of the quicksilver to the first endeavour of the water, so the
whole weight to the whole weight. By which you may see, that the cause
which takes away the gravitation of liquid bodies from fish or other
lighter bodies within them, can never be derived from the weight.
_A._ I have one question more to ask concerning gravity. If gravity be,
as some define it, an intrinsical quality, whereby a body descendeth
towards the centre of the earth, how is it possible that a piece of iron
that hath this intrinsical quality should rise from the earth, to go to
a loadstone? Hath it also an intrinsical quality to go from the earth?
It cannot be. The cause therefore must be extrinsical. And because when
they are come together in the air, if you leave them to their own
nature, they will fall down together, they must also have some like
extrinsical cause. And so this magnetic virtue will be such another
virtue as makes all heavy bodies to descend, in this our world, to the
earth. If therefore you can from this your hypothesis of compounded
motion, by which you have so probably salved the problem of gravity,
salve also this of the loadstone, I shall acknowledge both your
hypothesis to be true, and your conclusion to be well deduced.
_B._ I think it not impossible. But I will proceed no farther in it now,
than, for the facilitating of the demonstrations, to tell you the
several proprieties of the magnet, whereof I am to show the causes. As
first, that iron, and no other body, at some little distance, though
heavy, will rise to it. Secondly, that if it be laid upon a still water
in a floating vessel, and left to itself, it will turn itself till it
lie in a meridian, that is to say, with one and the same line still
north and south. Thirdly, if you take a long slender piece of iron, and
apply the loadstone to it, and, according to the position of the poles
of the loadstone, draw it over to the end of the iron, the iron will
have the same poles with the magnet, so it be drawn with some pressure;
but the poles will lie in a contrary position; and also this long iron
will draw other iron to it as the magnet doth.
Fourthly, this long iron, if it be so small as that poised upon a pin,
the weight of it have no visible effect, the navigators use it for the
needle of their compass, because it points north and south; saving that
in most places by particular accidents it is diverted; which diversion
is called the variation of the horizontal needle. Fifthly, the same
needle placed in a plane perpendicular to the horizon, hath another
motion called the inclination. Which that you may the better conceive,
draw a fourth figure; wherein let there be a circle to represent the
terrella, that is to say, a spherical magnet.
_A._ Let this be it, whose centre is A, the north pole B, the south pole
C.
_B._ Join B C, and cross it at right angles with the diameter D E.
_A._ It is done.
_B._ Upon the point D set the needle parallel to B C, with the cross of
the south pole, and the barb for the north; and describe a square about
the circle B D C E, and divide the arch D B into four equal parts in
_a_, _b_, _c._
_A._ It is done.
_B._ Then place the middle of the needle on the points _a_, _b_, _c_, so
that they may freely turn; and set the barb which is at D towards the
north, and that which is at C towards the south. You see plainly by
this, that the angles of inclination through the arch D C taken
altogether, are double to a right angle. For when the south point of the
needle, looking north, as at D, comes to look south, as at C, it must
make half a circle.
_A._ That is true. And if you draw the sine of the arch D _a_, which is
_d a_, and the sine of the arch B _a_, which is _a e_, and the sine of
the arch D _b_, which is _b f_, and the sine of the arch B _c_, which is
_c g_, the needle will lie upon _b f_ with the north-point downwards, so
that the needle will be parallel to A D. Then from _a_ draw the line _a
h_, making the angle _e a h_ equal to the angle D A _a_. And then the
needle at _a_ shall lie in the line _a h_ with the south point toward
_h_. Finally, draw the line _c h_, which, with _c g_, will also make a
quarter of a right angle; and therefore if the needle be placed on the
point _c_, it will lie in _c h_ with the south point toward _h_. And
thus you see by what degrees the needle inclines or dips under the
horizon more and more from D till it come to the north pole at B; where
it will lie parallel to the needle in D; but with their barbs looking
contrary ways. And this is certain by experience, and by none
contradicted.
You see then why the degrees of the inclinatory needle, in coming from D
to B, are double to the degrees of a quadrant. It is found also by
experience, that iron both of the mine and of the furnace put into a
vessel so as to float, will lay itself (if some accident in the earth
hinder it not) exactly north and south. And now I am, from this
compounded motion supposed by Copernicus, to derive the causes why a
loadstone draws iron; why it makes iron to do the same; why naturally it
placeth itself in a parallel to the axis of the earth; why by passing it
over the needle it changes its poles; and what is the cause that it
inclines. But it is your part to remember what I told you of motion at
our second meeting; and what I told you of this compounded motion
supposed by Copernicus, at our fourth meeting.
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