The English Works of Thomas Hobbes of Malmesbury, Volume 07 (of 11)

CHAPTER I.. Problems of Gravity.

PROBLEMS OF GRAVITY.

A. What may be the cause, think you, that stones and other bodies thrown upward, or carried up and left to their liberty, fall down again, for aught a man can see, of their own accord? I do not think with the old philosophers, that they have any love to the earth; or are sullen, that they will neither go nor stay. And yet I cannot imagine, what body there is above that should drive them back.

B. For my part, I believe the cause of their descending is not in any natural appetite of the bodies that descend; but rather that the globe of the earth hath some special motion, by which it more easily casteth off the air than it doth other bodies. And then this descent of those we call heavy bodies must of necessity follow, unless there be some empty spaces in the world to receive them. For when the air is thrown off from the earth, somewhat must come into the place of it, in case the world be full: and it must be those things which are hardliest cast off, that is, those things which we say are heavy.

A. But suppose there be no place empty, (for I will defer the question till anon), how can the earth cast off either the air or anything else?

B. I shall show you how, and that by a familiar example. If you lay both your hands upon a basin with water in it, how little soever, and move it circularly, and continue that motion for a while; and you shall see the water rise upon the sides, and fly over. By which you may be assured that there is a kind of circulating motion, which would cast off such bodies as are contiguous to the body so moved.

A. I know very well there is; and it is the same motion which country people use to purge their corn; for the chaff and straws, by casting the grain to the sides of the sieve, will come towards the middle. But I would see the figure.

B. Here it is. There is a circle pricked out, whose centre is A, and three less circles, whose centres are B, C, D. Let every one of them represent the earth, as it goeth from B to C, and from C to D, always touching the uttermost circle and throwing off the air, as is marked at E and F. And if the world were not full, there would follow by this scattering of the air, a great deal of space left empty. But supposing the world full, there must be a perpetual shifting of the air, one part into the place of another.

A. But what makes a stone come down, suppose from G?

B. If the air be thrown up beyond G, it will follow that at the last, if the motion be continued, all the air will be above G, that is, above the stone; which cannot be, till the stone be at the earth.

A. But why comes it down still with increasing swiftness?

B. Because as it descends and is already in motion, it receiveth a new impression from the same cause, which is the air, whereof as part mounteth, part also must descend, supposing as we have done the plentitude of the world. For, as you may observe by the figure, the motion of the earth, according to the diameter of the uttermost circle, is progressive; and so the whole motion is compounded of two motions, one circular and the other progressive; and consequently the air ascends and circulates at once. And because the stone descending receiveth a new pressure in every point of its way, the motion thereof must needs be accelerated.

A. It is true; for it will be accelerated equally in equal times; and the way it makes will increase in a double proportion to the times, as hath heretofore been demonstrated by Galileo. I see the solution now of an experiment, which before did not a little puzzle me. You know that if two plummets hang by two strings of equal length, and you remove them from the perpendicular equally, I mean in equal angles, and then let them go, they will make their turns and returns together and in equal times; and though the arches they describe grow continually less and less, yet the times they spend in the greater arches will still be equal to the time they spend in the lesser.

B. It is true. Do you find any experiment to the contrary?

A. Yes; for if you remove one of the plummets from the perpendicular, so as, for example, to make an angle with the perpendicular of eighty degrees, and the other so as to make an angle of sixty degrees; they will not make their turns and returns in equal times.

B. And what say you is the cause of this?

A. Because the arches are the spaces which these two motions describe, they must be in double proportion to their own times: which cannot be, unless they be let go from equal altitudes, that is, from equal angles.

B. It is right; and the experiment does not cross, but confirm the equality of the times in all the arches they describe, even from ninety degrees to the least part of one degree.

A. But is it not too bold, if not extravagant an assertion, to say the earth is moved as a man shakes a basin or a sieve? Does not the earth move from west to east every day once, upon its own centre; and in the ecliptic circle once a year? And now you give it another odd motion. How can all these consist in one and the same body?

B. Well enough. If you be a shipboard under sail, do not you go with the ship? Cannot you also walk upon the deck? Cannot every drop of blood move at the same time in your veins? How many motions now do you assign to one and the same drop of blood? Nor is it so extravagant a thing to attribute to the earth this kind of motion; but that I believe, if we certainly knew what motion it is that causeth the descent of bodies, we should find it either the same, or more extravagant. But seeing it can be nothing above that worketh this effect, it must be the earth itself that does it; and if the earth, then you can imagine no other motion to do it withal but this. And you will wonder more, when by the same motion I shall give you a probable account of the causes of very many other works of nature.

A. But what part of the heaven do you suppose the poles of your pricked circle point to?

B. I suppose them to be the same with the poles of the ecliptic. For, seeing the axis of the earth in this motion and in the annual motion, keeps parallel to itself, the axis must in both motions be parallel as to sense. For the circle which the earth describes, is not of visible magnitude at the distance it is from the sun.

A. Though I understand well enough how the earth may make a stone descend very swiftly under the ecliptic, or not far from it, where it throws off the air perpendicularly; yet about the poles of the circle methinks, it should cast off the air very weakly. I hope you will not say, that bodies descend faster in places remote from the poles, than nearer to them.

B. No; but I ascribe it to the like motion in the sun and moon. For such motions meeting, must needs cast the stream of the air towards the poles; and then there will be the same necessity for the descent there, that there is in other places, though perhaps a little more slowly. For you may have observed, that when it snows in the south parts, the flakes of snow are not so great as in the north: which is a probable sign they fall in the south from a greater height, and consequently disperse themselves more, as water does that falls down from a high and steep rock.

A. It is not improbable.

B. In natural causes all you are to expect, is but probability; which is better yet, than making gravity the cause, when the cause of gravity is that which you desire to know; and better than saying the earth draws it, when the question is, how it draws.

A. Why does the earth cast off air more easily than it does water, or any other heavy bodies?

B. It is indeed the earth that casteth off that air which is next unto it; but it is that air which casteth off the next air; and so continually, air moveth air; which it can more easily do than any other thing, because like bodies are more susceptible of one another’s motions: as you may see in two lute-strings equally strained, what motion one string being stricken communicates to the air, the same will the other receive from the air; but strained to a differing note, will be less or not at all moved. For there is no body but air, that hath not some internal, though invisible, motion of its parts: and it is that internal motion which distinguisheth all natural bodies one from another.

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