Decameron physiologicum, or, Ten dialogues of natural philosophy by Thomas Hobbes ... ; to which is added The proportion of a straight line to half the arc of a quadrant, by the same author.

CAP. IV. Of the Systeme of the World. (Book 4)

B.

YOu are come in good time; let us * 1.1 therefore sit down. There is Ink, Paper, Ruler, and Compass. Draw a little Circle to represent the Body of the Sun.

A.

'Tis done. The Centre is A, the Cir∣cumference is L M.

B.

Upon the same Centre A, draw a larger Circle to stand for the Ecliptick: For you know the Sun is always in the Plain of the E∣cliptick.

A.

There 'tis. The Diameters of it at right Angles are B Z.

B.

Draw the Diameter of the Aequator.

A.

How?

B.

Through the Centre A (for the Earth is also always in the Plain of the Aequator or of some of its Parallels) so as to be distant from B 23 deg. and a half.

A.

Let it be H I: And let C G be equal to B H; and so G will be one of the Poles of the Ecliptick, suppose the North-Pole; and then H will be East, and I West. And C A produced to the Circumference in E, makes E the South-Pole.

B.

Take C K equal to C G, and the Chord G K will be the Diameter of the Arctick-Cir∣cle, and parallel to H I, the Diameter of the Aequator. Lastly, upon the point B, draw a little Circle wherein I suppose to be the Globe of the Earth.

A.

'Tis drawn, and marked with l m. And B D and K G joyned will be parallel; and as H and I are East and West, and so are B and D, and G and K.

B.

True; but producing Z B to the Cir∣cumference l m in b, the Line B b will be in the Diameter of the Ecliptick of the Earth, and B m in the Diameter of the Aequator of the Earth. In like manner, if you produce K G cutting the Circle, whose Centre is G, in d and e, and make an Angle n G d equal to b B m, the Line n G will be in the Ecliptick of the Earth, because G d is in the Aequator of the Earth. So that in the Annual Motion of the Earth through the Ecliptick, every streight Line drawn in the Earth, is perpetually kept parallel to the place from whence it is removed.

A.

'Tis true; and 'tis the Doctrine of Co∣pernicus. But I cannot yet conceive by what one Motion this Circle can be described other∣wise than we are taught by Euclid. And then I am sure that all the Diameters shall cross one another in the Centre, which in this Figure is A.

B.

I do not say that the Diameters of a Sphere or Circle can be parallel; but that if a Circle of a lesser Sphere be moved upon the Circum∣ference of a great Circle of a greater Sphere, that the streight Lines that are in the lesser Sphere may be kept parallel perpetually to the places they proceed from.

A.

How? And by what Motion?

B.

Take into your hand any streight Line, (as in this Figure) the Line L A M, which we sup∣pose to be the diameter of the Suns Body; and moving it parallelly, with the ends in the Cir∣cumference, so as that the end M may withal de∣scribe a small Circle, as M a. It is manifest that all the other points of the same Line L M will by the same Motion, at the same time, describe equal Circles to it. Likewise if you take in your hand any two Diameters fastened toge∣ther, the same Parallel-motion of the line L M, shall cause all the points of the other Diame∣ter to make equal Circles to the same M a.

A.

'Tis evident; as also that every point of the Suns body shall do the like. And not onely so, but also if one end describe any other Figure, all the other points of the Body shall describe like and equal Figures to it.

B.

You see by this, that this Parallel-motion is compounded of two Motions, one Circular upon the Superficies of a Sphere, the other a streight Motion from the Centre to every point of the same Superficies, and beyond it.

A.

I see it.

B.

It follows hence, that the Sun by this Motion must every way repel the Air; and since there is no empty place for retiring, the Air must turn about in a Circular stream; but slower or swifter according as it is more or less remote from the Sun, and that according to the nature of Fluids, the Particles of the Air must continually change place with one another; and also that the stream of the Air shall be the contrary way to that of the Motion, for else the Air cannot be repelled.

A.

All this is certain.

B.

Well. Then if you suppose the Globe of the Earth to be in this stream which is made by the Motion of the Suns Body from East to West, the stream of Air wherein is the Earths Annual Motion will be from West to East.

A.

'Tis certain.

B.

Well. Then if you suppose the Globe of the Earth, whose Circle is moved Annually, to be l m, the stream of the Air without the Ecliptick falling upon the Superficies of the Earth l m without the Ecliptick, being slow∣er, and the stream that falleth within swifter, the Earth shall be turned upon its own Centre proportionally to the greatness of the Circles; and consequently their Diameters shall be pa∣rallel; as also are other streight Lines corre∣spondent.

A.

I deny not but the streams are as you

say; and confess that the proportion of the swiftness without, is to the swiftness within, as the Suns Ecliptick to the Ecliptick of the Earth; that is to say, as the Angle HAB to the Angle m B b. And I like your Argument the better, because it is drawn from Copernicus his foundation. I mean the compounded Mo∣tion of Streight and Circular.

B.

I think I shall not offer you many de∣monstrations of Physical conclusions that are not derived from the Motions supposed or pro∣ved by Copernicus. For those Conclusions in Natural Philosophy I most suspect of falshood, which require most variety of Suppositions for their demonstrations.

A.

The next thing I would know, is how great or little you suppose that Circle a M.

B.

I suppose it less than you can make it: For there appears in the Sun no such Motion sensible. 'Tis the first Endeavour of the Suns Motion. But for all that, as small as the Circle is, the Motion may be as swift, and of as great strength as 'tis possible to be named. 'Tis but a kinde of trembling that necessarily happen∣eth in those Bodies, which with great resistance press upon one another.

A.

I understand now from what Cause pro∣ceedeth the Annual Motion: Is the Sun the Cause also of the Diurnal Motion?

B.

Not the immediate Cause. For the Di∣urnal Motion of the Earth is upon its own

Centre, and therefore the Suns Motion cannot describe it. But it proceedeth as a necessary con∣sequence from the Annual Motion. For which I have both experience and demonstration. The Experiment is this: Into a large Hemisphere of Wood, spherically Concave, put in a Globe of Lead, and with your hands hold it fast by the brim, moving your hands circularly, but in a very small compass, you shall see the Globe circulate about the Concave Vessel, just in the same manner as the Earth doth every year in the Air; and you shall see withal, that as it goes, it turns perpetually upon its own Centre, and very swiftly.

A.

I have seen it: And 'tis used in some great Kitchins to grinde Mustard.

B.

Is it so? Therefore take a Hemisphere of Gold (if you have it) the greater the better, and a Bullet of Gold, (and without Mustard) you shall see the same Effect.

A.

I doubt it not. But the of it Cause is evi∣dent. For any Spherical Body being in Motion upon the sides of a Concave and hard Sphere, is all the way turned upon its own Centre by the resistance of the hard Wood or Metal. But the Earth is a Bullet without weight, and meeteth onely with Air, without any harder body in the way to resist it.

B.

Do you think the Air makes no resi∣stance, especially to so swift a Motion as is the Annual Motion of the Earth? If it do

make any resistance, you cannot doubt but that it shall turn the Earth circularly, and in a contrary way to its Annual Motion; that is to say, from East to West, because the Annual Motion is from West to East.

A.

I confess it. But what deduce you from these Motions of the Sun?

B.

I deduce (first) that the Air must of necessity be moved both circularly about the Body of the Sun according to the Ecliptick, and also every way directly from it. For the Motion of the Suns Body is compounded of this Circular Motion upon the Sphere L M, and of the streight motion of its Semi-diame∣ters from the Centre A to the Superficies of the Suns Body, which is LM. And therefore the Air must needs be repelled every way, and also continually change place to fill up the places forsaken by other parts of the Air, which else would be empty, there being no Vacuum to retire unto. So that there would be a perpetual stream of Air, and in a contrary way to the Motion of the Suns Body, such as is the Motion of Water by the sides of a Ship under sail.

A.

But this Motion of the Earth from West to East, is onely Circular, such as is described by a Compass about a Centre; and cannot therefore repel the Air as the Sun does. And the Disciples of Copernicus will have it to be the Cause of the Moons monthly motion about the Earth.

B.

And I think Copernicus himself would have said the same, if his purpose had been to have shewn the Natural Causes of the Moti∣ons of the Stars. But that was no part of his designe; which was onely from his own ob∣servations, and those of former Astronomers, to compute the times of their Motions; part∣ly to foretel the Conjunctions, Oppositions, and other Aspects of the Planets; and partly to regulate the times of the Churches Festi∣vals. But his followers, Kepler and Galileo, make the Earths Motion to be the Efficient Cause of the Monthly motion of the Moon a∣bout the Earth; which without the like Mo∣tion to that of the Sun in LM, is impossible. Let us therefore for the present take it in as a necessary Hypothesis; which from some Ex∣periment that I shall produce in our following discourses, may prove to be a certain truth.

A.

But seeing A is the Centre both of the Suns Body and of the Annual Motion of the Earth, How can it be (as all Astronomers say it is) that the Orb of the Annual Motion of the Earth should be Excentrique to the Suns Body? For you know that from the Vernal Aequinox to the Autumnal, there be 187 days; but from the Autumnal Aequinox to the Ver∣nal, there be but 178 days. What Natural Cause can you assigne for this Excentricity?

B.

Kepler ascribes it to a Magnetique vertue, viz. that one part of the Earths Superficies has

a greater kindness for the Sun than the other part.

A.

I am not satisfied with that. It is Ma∣gical rather than Natural, and unworthy of Kepler. Tell me your own opinion of it.

B.

I think that the Magnetical vertue he speaks of, consisteth in this: that the Southern Hemisphere of the Earth is for the greatest part Sea, and that the greatest part of the Northern Hemisphere is dry Land. But how it is possible that from thence should proceed the Excentricity (the Sun being neerest to the Earth, when he is in the Winter-Solstice) I shall shew you when we come to speak of the Motions of Air and Water.

A.

That's time enough: For I intend it for our next meeting. In the mean time I pray you tell me what you think to be the Cause why the Equinoctial (and consequently the Solstitial) points are not always in one and the same point of the Ecliptique of the Fixt Stars. I know they are not, because the Sun does not rise and set in points diametrally op∣posite: For if it did, there would be no dif∣ference of the Seasons of the year.

B.

The cause of that can be no other, than that the Earth (which is l m) hath the like Motion to that which I suppose the Sun to have in L M, compounded of streight and circular from West to East in a day, as the An∣nual Motion hath in a year; so that (not

reckoning the Excentricity) it will be moved through the Ecliptiques one Revolution (as Copernicus proveth) about one degree. Sup∣pose then the whole Earth moved from H to I, (which is half the year) circularly, but fal∣ling from I to i in the same time about 30 minutes, and as much in the other Hemi∣sphere from H to k; then draw the line i k, which will be equal and parallel to H I, and be the Diameter of the Aequator for the next year. But it shall not cut the Diameter of the Ecliptick B Z in A, which was the Equinoctial of the former year, but in o 36 seconds from the first degree of Aries. Suppose the same done in the Hemisphere under the Plain of the paper, and so you have the double of 36 seconds, that is 72 seconds, or very neer, for the progress of the Vernal Equinox in a year. The cause why I suppose the Arch I i to be half a degree in the Ecliptique of the Earth, is, that Coperni∣cus and other Astronomers, and Experience, a∣gree in this, that the Aequinoctial points pro∣ceed according to the order of the Signes, A∣ries, Taurus, Gemini, &c. from West to East every 100 year one degree or very neer.

A.

In what time do they make the whole Revolution through the Ecliptique of the Sky?

B.

That you may reckon. For we know by Experience that it hath proceeded about one degree, that is 60 minutes constantly a

long time in a hundred years. But as 100 years to one degree, so is 36000 years to 360 de∣grees. Also as 100 years to one degree, so is one year to the hundred part of one degree or 60 minutes; which is 60/100, or 36 seconds for the progress of one year; which must be some∣what more than a degree according to Coper∣nicus, who, lib. 3. cap. 2. saith, That for 400 years before Ptolomie it was one degree almost constantly. Which is well enough as to the Natural Cause of the Precession of the Aequi∣noctial points, which is the often-said com∣pounded Motion, though not an exact Astro∣nomical Calculation.

A.

And 'tis a great signe that his Supposi∣tion is true. But what is the Cause that the Obliquity of the Ecliptique, that is, the distance between the Aequinoctial and the Solstice, is not always the same?

B.

The necessity of the Obliquity of the E∣cliptique is but a consequence to the Precessi∣on of the Aequinoctial points. And therefore if from C the North-Pole you make a little Cir∣cle C u equal to 15 minutes of a degree upon the Earth, and another u s equal to the same, which will appear like this Figure 8, that is (as Copernicus calls it) a Circle twined, the Pole C will be moved half the time of the Ae∣quinoctial points, in the arc C u, and as much in the alternate arc u s descending to s. But in the arc s u, and its alternate rising to C. The

cause of the twining is the Earths Annual Mo∣tion the same way in the Ecliptique, and makes the four quarters of it; and makes also their revolution twice as slow as that of the Aequinoctial points. And therefore the Mo∣tion of it is the same compounded Motion which Copernicus takes for his Supposition, and is the cause of the Precession of the Ae∣quinoctial points, and consequently of the variation of the Obliquity, adding to it or ta∣king from it somewhere more, somewhere less; so as that one with another the addition is not much more, nor the substraction much less than 30 minutes. But as for the Natural Ef∣ficient cause of this compounded Motion, ei∣ther in the Sun, or the Earth, or any other Na∣tural Body, it can be none but the immediate hand of the Creator.

A.

By this it seems that the Poles of the Earth are always the same, but make this 8 in the Sphere of the fixt Stars neer that which is called Cynosura.

B.

No: 'Tis described on the Earth, but the Annual Motion describes a Circle in the Sphere of the fixed Stars. Though I think it improper to say a Sphere of the fixt Stars, when 'tis so unlikely that all the fixt Stars should be in the Superficies of one and the same Globe.

A.

I do not believe they are.

B.

Nor I, since they may seem less one than another, as well by their different distances, as

by their different magnitudes. Nor is it likely that the Sun (which is a fixt Star) is the Ef∣ficient Cause of the Motion of those remoter Planets, Mars, Jupiter, and Saturn; seeing the whole Sphere, whose Diameter is the distance between the Sun and the Earth, is but a point in respect of the distance between the Sun and any other fixed Star. Which I say onely to excite those that value the knowledge of the Cause of Comets, to look for it in the Domi∣nion of some other Sun than that which mo∣veth the Earth. For why may not there be some other fixed Star, neerer to some Planet than is the Sun, and cause such a light in it as we call a Comet?

A.

As how?

B.

You have seen how in high and thin Clouds above the Earth, the Sun-beams pier∣cing them have appeared like a Beard; and why might not such a Beard have appeared to you like a Comet, if you had lookt upon it from as high as some of the fixed Stars?

A.

But because it is a thing impossible for me to know, I will proceed in my own way of enquiry. And seeing you ascribe this com∣pounded Motion to the Sun and Earth, I would grant you that the Earth (whose Annual Mo∣tion is from West to East) shall give the Moon her Monthly-motion from East to West. But then I ask you whether the Moon have also that compounded Motion of the Earth, and

with it a Motion upon its own Centre, as hath the Earth? For seeing the Moon has no other Planet to carry about her, she needs it not.

B.

I see reason enough, and some necessity, that the Moon should have both those Motions. For you cannot think that the Creator of the Stars, when he gave them their Circular Mo∣tion, did first take a Centre, and then describe a Circle with a Chain or Compass, as men do? No; he moved all the parts of a Star together and equally in the Creation: And that's the reason I give you. The necessity of it, comes from this Phaenomenon, that the Moon doth turn one and the same face towards the Earth: Which cannot be by being moved about the Earth parallelly, unless also it turn about its own Centre. Besides, we know by experi∣ence, that the Motion of the Moon doth adde not a little to the Motion of the Sea: Which were impossible if it did not adde to the Stream of the Air, and by consequence to that of the Water.

A.

If you could get a piece of the true and intimate Substance of the Earth, of the bigness of a Musquet-bullet, do you believe that the Bullet would have the like compounded Mo∣tion to that which you attribute to the Sun, Earth, and Moon?

B.

Yes truly; but with less strength, ac∣cording to its magnitude; saving that by its

Gravity falling to the Earth, the activity of it would be unperceived.

A.

I will trouble you no more with the Na∣ture of Celestial appearances. But I pray you tell me by what art a man may finde what part of a Circle the Diameter of the Suns Body doth subtend in the Ecliptique Circle.

B.

Kepler says it subtends 30 minutes, which is half a degree. His way to finde it is by let∣ting in the Sun-beams into a close room through a small hole, and receiving the image of it upon a plain perpendicularly. For by this means he hath a Triangle, whose sides and angles he can know by measure; and the ver∣tical Angle he seeks for, and the substance of the arc of the Suns Body.

A.

But I think it impossible to distinguish where the part illuminate toucheth the part not illuminate.

B.

Another way is this: Upon the Aequi∣noctial-day, with a Watch that shews the mi∣nutes standing by you, observe when the lower brim of the Suns setting first comes to the Ho∣rizon, and set the Index to some minute of the Watch; and observe again the upper brim when it comes to the Horizon: then count the minutes, and you have what you look for. O∣ther way I know none.

Notes

• * 1.1

  Fig. 1.