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. V. Of the Motions of Water and Air. (Book 5)

A.

I Have considered, as you bad me, this compounded Motion with great admira∣tion. First, it is that which makes the diffe∣rence between Continuum and Contiguum, which till now I never could distinguish. For Bodies that are but Contiguous, with any lit∣tle force are parted; but by this compounded Motion (because every point of the body makes an equal Line in equal time, and every Line crosses all the rest) one part cannot be separated from another, without disturbing the Motion of all the other parts at once. And is not that the Cause, think you, that some Bo∣dies when they are prest or bent, as soon as the force is removed, return again of themselves to their former figure?

B.

Yes sure; saving that it is not of them∣selves that they return, (for we were agreed that nothing can move it self) but it is the Motion of the parts which are not prest, that delivers those that are. And this restitution the Learned now call the Spring of a Body. The Greeks called it Antitypia.

A.

When I considered this Motion in the Sun

and the Earth and Planets, I fancied them as so many Bodies of the Army of the Almighty in an immense field of Air, marching swiftly, and commanded (under God) by his glori∣ous Officer the Sun, or rather forced so to keep their order in every part of every of those Bo∣dies, as never to go out from the distance in which he had set them.

B.

But the parts of the Air and other Fluids keep not their places so.

A.

No. You told me that this Motion is not natural in the Air, but received from the Sun.

B.

True. But since we seek the Natural Causes of Sublunary Effects, where shall we begin?

A.

I would fain know what makes the Sea to ebbe and flow at certain Periods, and what causeth such variety in the Tides.

B.

Remember that the Earth turneth every day upon its own Axis from West to East; and all the while it so turneth, every point thereof by its compounded Motion makes other Cir∣clings, but not on the same Centre, which is (you know) a rising in one part of the day, and a falling in the other part. What think you must happen to the Sea, which resteth on it, and is a Fluid Body?

A.

I think it must make the Sea rise and fall. And the same happeneth also to the Air, from the Motion of the Sun.

B.

Remember also, in what manner the Sea is situated in respect of the Dry Land.

A.

Is not there a great Sea that reacheth from the Straight of Magellan Eastward to the Indies, and thence to the same Straight again? And is not there a great Sea called the Atlan∣tick Sea that runneth Northward to us? and does not the great South-Sea run also up into the Northern Seas? But I think the Indian and the South-Sea of themselves to be greater than all the rest of the Surface of the Globe.

B.

How lieth the water in those two Seas?

A.

East and West, and rises and falls a little, as it is forc'd to do by this compounded Mo∣tion, which is a kind of succussion of the Earth, and fills both the Atlantick and Nor∣thern Seas.

B.

All this would not make a visible diffe∣rence between High and Low water, because this Motion being so regular, the unevenness would not be great enough to be seen. For though in a Bason the water would be thrown into the Air, yet the Earth cannot throw the Sea into the Air.

A.

Yes. The Bason, if gently moved, will make the water so move, that you shall hardly see it rise.

B.

It may be so. But you should never see it rise as it doth, if it were not checkt. For at the Straight of Magellan, the great South-Sea is checkt by the shore of the Continent of Peru

and Chily, and forced to rise to a great height, and made to run up into the Northern Seas on that side by the coast of China; and at the re∣turn is checkt again and forced through the Atlantick into the British and German Seas. And this is done every day. For we have supposed that the Earths Motion in the Ecli∣ptique caused by the Sun is Annual; and that its Motion in the Aequinoctial, is Diurnal. It followeth therefore from this compounded Motion of the Earth, the Sea must Ebb and Flow twice in the space of Twenty four hours, or thereabout.

A.

Has the Moon nothing to do in this bu∣siness?

B.

Yes. For she hath also the like Motion. And is, though less swift, yet much neerer to the Earth. And therefore when the Sun and Moon are in Conjunction or Opposition, the Earth, as from two Agents at once, must needs have a greater Succussion. And if it chance at the same time the Moon also be in the Ecli∣ptique, it will be yet greater, because the Moon then worketh on the Earth less obliquely.

A.

But when the Full or New Moon hap∣pen to be then when the Earth is in the Aequi∣noctial points, the Tides are greater than or∣dinary. Why is that?

B.

Because then the force by which they move the Sea, is at that time, to the force by which they move the same at other times,

as the Aequinoctial Circle to one of its Paral∣lels, which is a lesser Circle.

A.

'Tis evident. And 'tis pleasant to see the Concord of so many and various motions, when they proceed from one and the same Hypothe∣sis. But what say you to the stupendious Tides which happen on the Coasts of Lincoln∣shire on the East, and in the River of Severn on the West?

B.

The cause of that, is their proper Scitua∣tion. For the Current of the Ocean through the Atlantick Sea, and the Current of the South-Sea through the Northern Seas meeting together, raise the water in the Irish and Bri∣tish Seas a great deal higher than ordinary. Therefore the mouth of the Severn being di∣rectly opposite to the Current from the Atlan∣tick Sea, and those Sands on the Coast of Lincolnshire directly opposite to the Current of the German Sea, those Tides must needs fall furiously into them, by this Succussion of the water.

A.

Does, when the Tide runs up into a Ri∣ver, the water all rise together, and fall toge∣ther when it goes out?

No: One part riseth and another falleth at the same time; because the Motion of the Earth rising and falling, is that which makes the Tide.

A.

Have you any Experiment that shews it?

B.

Yes. You know that in the Thames, it is high water at Greenwich before it is high water at London-bridge. The water there∣fore falls at Greenwich whilst it riseth all the way to London. But except the top of the water went up, and the lower part downward, it were impossible.

A.

'Tis certain. It is strange that this one Motion should salve so many apparences, and so easily. But I will produce one Experiment of water, not in the Sea, but in a Glass. If you can shew me that the cause of it is this compounded Motion, I shall go neer to think it the Cause of all other Effects of Nature hi∣therto disputed of. The Experiment is com∣mon, and described by the Lord Chancellour Bacon, in the third page of his Natural Histo∣ry. Take (saith he) a Glass of water, and draw your finger round about the lip of the Glass, pressing it somewhat hard; after you have done so a few times, it will make the wa∣ter frisk up into a fine Dew. After I had read this, I tried the same with all diligence my self, and found true not onely the frisking of the water to above an inch high, but also the whole Superficies to circulate, and withal to make a pleasant sound. The Cause of the fri∣sking he attributes to a tumult of the inward parts of the Substance of the Glass striving to free it self from the pressure.

B.

I have tried and found both the Sound

and Motion; and do not doubt but the pres∣sure of the parts of the Glass was part of the Cause. But the Motion of my finger about the Glass was always parallel; and when it chanced to be otherwise, both Sound and Motion ceased.

A.

I found the same. And being satisfied, I proceed to other questions. How is the water (being a heavie Body) made to ascend in small particles into the Air, and be there for a time sustained in form of a Cloud, and then fall down again in Rain?

B.

I have shewn already, that this compoun∣ded Motion of the Sun, in one part of its Cir∣cumlation, drives the Air one way, and in the other part, the contrary way; and that it can∣not draw it back again, no more than he that sets a stone a flying can pull it back. The Air therefore, which is contiguous to the wa∣ter, being thus distracted, must either leave a Vacuum, or else some part of the water must rise and fill the spaces continually forsaken by the Air. But, that there is no Vacuum, you have granted. Therefore the water riseth in∣to the Air, and maketh the Clouds; and see∣ing they are very small and invisible parts of the water, are (though naturally heavie) ea∣sily carried up and down with the Wind, till, meeting with some Mountain or other Clouds, they be prest together into greater drops, and fall by their weight. So also it is forced up in

moist ground, and with it many small Atomes of the Earth, which are either twisted with the rising water into Plants, or are carried up and down in the Air incertainly. But the greatest Quantity of Water is forced up from the great South and Indian Seas, that lie under the Tropique of Capricorn. And this Climate is that which makes the Suns Perigaeum to be always on the Winter-Solstice. And that is the part of the Terrestrial Globe which Keple∣rus says is kinde to the Sun; whereas the o∣ther part of the Globe (which is almost all dry land) has an Antipathy to the Sun. And so you see where this Magnetical vertue of the Earth lies. For the Globe of the Earth having no Natural Appetite to any place, may be drawn by this Motion of the Sun a little neerer to it, together with the water which it raiseth.

A.

Can you guess what may be the Cause of Wind?

B.

I think it manifest that the unconstant Winds proceed from the uncertain Motion of the Clouds ascending and descending, or meeting with one another. For the Winds after they are generated in any place by the descent of a Cloud, they drive other Clouds this way and that way before them, the Air seeking to free it self from being pent up in a straight. For when a Cloud descendeth, it makes no wind sensible directly under it self.

But the Air between it and the Earth is prest and forced to move violently outward. For it is a certain Experiment of Mariners, that if the Sea go high when they are becalmed, they say they shall have more Wind than they would; and take in their Sails all but what is necessary for steering. They know (it seems) that the Sea is moved by the descent of Clouds at some distance off: Which presseth the wa∣ter, and makes it come to them in great Waves. For a Horizontal Wind does but curl the Water.

A.

From whence come the Rivers?

B.

From the Rain, or from the falling of Snow on the higher ground. But when it de∣scendeth under ground, the place where it a∣gain ariseth is called the Spring.

A.

How then can there be a Spring upon the top of a Hill?

B.

There is no Spring upon the very top of a Hill, unless some Natural Pipe bring it thi∣ther from a higher Hill.

A.

Julius Scaliger says, there is a River, and in it a Lake, upon the top of Mount Cenis in Savoy; and will therefore have the Springs to be ingendred in the Caverns of the Earth by Condensation of the Air.

B.

I wonder he should say that. I have pass'd over that Hill twice since the time I read that in Scaliger, and found that River as I pass'd, and went by the side of it in plain

ground almost two miles. Where I saw the water from two great Hills, one on one side, the other on the other, in a thousand small Rillets of melting Snow fall down into it. Which has made me never to use any Expe∣riment the which I have not my self seen. As for the conversion of Air into Water by Con∣densation, and of Water into Air by Rarefa∣ction (though it be the doctrine of the Peri∣patetiques) it is a thing incogitable, and the words are insignificant. For by Densum is signified onely frequencie and closeness of parts; and by Rarum the contrary. As when we say a Town is thick with houses, or a Wood with trees, we mean not that one house or tree is thicker than another, but that the spaces between are not so great. But (since there is no Vacuum) the spaces between the parts of Air are no larger than between the parts of Water, or of any thing else.

A.

What think you of those things which Mariners that have sailed through the Atlan∣tick Sea called Spouts, which pour down wa∣ter enough at once to drown a great Ship?

B.

'Tis a thing I have not seen. And there∣fore can say nothing to it; though I doubt not but when two very large and heavie Clouds shall be driven together by two great and con∣trary winds, the thing is possible.

A.

I think your reason good. And now will propound to you another Experiment.

I have seen an exceeding small Tube of Glass with both ends open set upright in a Vessel of Water, and that the Superficies of the Water within the Tube was higher a good deal than of that in the Vessel; but I see no reason for it.

B.

Was not part of the Glass under Water? Must not then the Water in the Vessel rise? Must not the Air that lay upon it rise with it? Whither should this rising Air go, since there is no place empty to receive it? It is therefore no wonder if the Water, press'd by the Sub∣stance of the Glass which is dipt into it, do ra∣ther rise into a very small Pipe, than come a∣bout a longer way into the open Air.

A.

'Tis very probable. I observed also that the top of the inclosed Water was a concave Superficies; which I never saw in other Flu∣ids.

B.

The Water hath some degree of tenaci∣ty, though not so great but that it will yield a little to the Motion of the Air; as is mani∣fest in the Bubbles of water, where the conca∣vity is always towards the Air. And this I think the cause why the Air and Water mee∣ting in the Tube make the Superficies towards the Air concave, which it cannot do to a Flu∣id of greater tenacity.

A.

If you put into a Bason of Water a long rag of Cloth, first drenched in water, and let the longer part of it hang out, it is known by

Experience, that the Water will drop out as long as there is any part of the other end un∣der Water.

B.

The cause of it is, that water (as I told you) hath a degree of tenacity. And there∣fore being continued in the rag till it be lower without than within, the weight will make it continue dropping, though not onely because it is heavie (for if the rag lay higher without than within, and were made heavier by the breadth, it would not descend) but 'tis be∣cause all heavie Bodies Naturally descend with proportion of swiftness duplicate to that of the time; whereof I shall say more when we talk of Gravity.

A.

You see how despicable Experiments I trouble you with. But I hope you will pardon me.

B.

As for mean and common Experiments, I think them a great deal better Witnesses of Nature, than those that are forced by fire, and known but to very few.