Seven philosophical problems and two propositions of geometry by Thomas Hobbes of Malmesbury ; with an apology for himself and his writings.

CHAP. VI. Problems of Rain, Wind, and other WEATHER. (Book 6)

A.

WHat is the original cause of Rain? and how is it gene∣rated?

B.

The motion of the Air (such as I have described to you already) tending to the dis-union of the parts of the Air, must needs cause a continual endeavour (there being no possibility of Vacuum) of whatsoever fluid parts there are upon the face of the Earth and Sea, to supply the place which would else be empty.

This makes the water, and also very small and loose parts of the Earth and Sea

to rise, and mingle themselves with the Air, and to become mist and Clouds. Of which the greatest quantity arise there, where there is most water, namely, from the large parts of the Ocean; which are the South Sea, the Indian Sea, and the Sea that divideth Europe and Africa from America; over which the Sun, for the greatest part of the year is perpendicular, and consequently raiseth a greater quan∣tity of water. Which afterwards gathe∣red into Clouds, falls down in Rain.

A.

If the Sun can thus draw up the wa∣ter; though but in small drops, why can it not as easily hold it up?

B.

It is likely it would also hold them up, if they did not grow greater by meeting together, nor were carried away by the Air towards the Poles.

A.

What makes them gather together?

B.

It is not improbable that they are carried against Hills, and there stopt till more overtake them. And when they are carried towards the North or South where the force of the Sun is more obli∣que and thereby weaker, they descend gently by their own weight. And be∣cause they tend all to the center of the Earth, they must needs be united in their way for want of room, and so grow big∣ger. And then it Rains.

A.

What is the reason it Rains so seldom, but Snows so often upon very high Moun∣tains?

B.

Because perhaps when the water is drawn up higher then the highest Mountains, where the course of the Air between the Aequator and the Poles is free from stopping, the Stream of the Air Freezeth it into Snow. And 'tis in those places only where the Hills shelter it from that Stream, that it falls in Rain.

A.

Why is there so little Rain in Egypt, and yet so much in other parts nearer the Aequinoctial, as to make the Nile over∣flow the Countrey?

B.

The cause of the falling of Rain, I told you was the the stopping, and con∣sequently the collection of Clouds about great Mountains, especially when the Sun is near the Aequinoctial, and there∣by draws up the water more potently, and from greater Seas. If you consider therefore that the Mountains in which are the springs of Nile, lye near the Aequinoctial and are exceeding great, and near the Indian Sea, you will not think it strange there should be great store of Snow. This as it melts makes the Rain of Nile to rise, which in April and May going on toward Egypt arri∣ved there about the time of the Solstice, and overflow the Countrey.

A.

Why should not the Nile then over∣flow that Countrey twice a year? For it comes twice a year to the Aequinoctial.

B.

From the Autumnal Aequinox, the Sun goeth on toward the Southern Tro∣pique. And therefore cannot dissolve, the Snow on that side of the Hills that look towards Egypt.

A.

But then there ought to be such ano∣ther Innundation Southward.

B.

No doubt but there is a greater descent of water there in their Summer then at other times; as there must be wheresoever there is much Snow melted. But what should that innundate, unless it should overflow the Sea that comes close to the foot of those Mountains? And for the cause why it seldom Rains in Egypt, it may be this, That there are no very high Hills near it to collect the Clouds. The Mountains whence Nile riseth being near 2000 Miles off. The nearest on one side are the Mountains of Nubia, and on the other side Sina, and the Mountains of Arrabia.

A.

Whence think you proceed the Winds?

B.

From the Motion (I think) espe∣cially of the Clouds, partly also from whatsoever is moved in the Air.

A.

It is manifest, that the Clouds are

moved by the Winds; so that there were Winds before any clouds could be moved. Therefore I think you make the Effect before the Cause.

B.

If nothing could move a Cloud but Wind, your objection were good. But you allow a Cloud to descend by it's own weight. But when it so descends, it must needs move the Air before it, even to the Earth, and the Earth again repel it, and so make lateral Winds every way. Which will carry forwards other Clouds if there be any in their way, but not the Cloud that made them The Vapour of the water rising into Clouds, must needs also as they rise, raise a Wind?

A.

I grant it. But how can the slow motion of a Cloud make so swift a Wind as it does?

B.

It is not one or two little Clouds, but many and great ones that do it. Be∣sides, when the Air is driven into places already covered; it cannot but be much the swifter for the narrowness of the passage

A.

Why does the South Wind more often then any other bring Rain with it?

B.

Where the Sun hath most power, and where the Seas are greatest, that is in the South, there is most water in the Air; which a South wind can only bring

to us. But I have seen great showers of Rain sometimes also when the wind hath been North, but it was in Summer, and came first, I think, from the South or West, and was but brought back from the North.

A.

I have seen at Sea very great Waves when there was no Wind at all. What was it then that troubled the Water?

B.

But had you not Wind enough pre∣sently after?

A.

We had a Storm within a little more then a quarter of an hour after.

B.

That Storm was then coming and had moved the Water before it. But the Wind you could not perceive, for it came downwards with the descending of the Clouds, and pressing the Water bounded above your Sail till it came very near. And that was it that made you think there was no Wind at all.

A.

How comes it to pass that a Ship should go against the Wind which moves it, even almost point blank, as if it were not driven but drawn?

B.

You are to know first, that what Body soever is carryed against another Body, whether perpendicularly, or ob∣liquely, it drives it in a perpendicular to the superficies it lighteth on. As for Ex∣ample, a Bullet shot against a flat wall,

maketh the Stone (or other matter it hits) to retire in a perpendicular to that flat; or, if the Wall be round, towards the center, that is to say, per∣pendicularly. For if the way of the motion be oblique to the Wall, the mo∣tion is compounded of two motions, one parrallel to the Wall, and the other perpendicular. By the former whereof the Bullet is carried along the Wall side, by the other it approacheth to it. Now the former of these motions can have no effect upon it; all the battery is from the motion perpendicular, in which it approacheth. And therefore the part it hits must also retire perpendicularly. If it were not so, a Bullet with the same swiftness would execute as much oblique∣ly shot, as perpendicularly; which you know it does not.

A.

How do you apply this to a Ship?

B.

Let A. B. be the Ship, the head of it A. If the Wind blow just from A. to∣wards B. 'tis true, the Ship cannot go forward howsoever the sail be set. Let C. D. be perpendicular to the Ship, and let the Sail E. C. be never so little oblique to it, and F. C. perpendicular to E. C. and then you see the Ship will gain the space D. F. to the headward.

A.

It will so, but when it is very near

to the Wind it will go forward very slowly, and make more way with her side to the Lee∣ward.

A.

It will indeed go slower in the pro∣portion of the Line A. E. to the Line C. E. But the Ship will not go so fast as you think sideward: One is the force of that Wind which lights on the side of the Ship it self; the other is, the bellying of the Sail; for the former, it is not much because the Ship does not easily put from her the Water with her side; and belly∣ing of the Sail, gives some little hold for the Wind to drive the Ship a stern.

A.

For the motion sideward I agree with you; but I had thought the bellying of the Sail, had made the Ship go faster.

B.

But it does not; only in a fore-wind it hinders least.

A.

By this reason a broad thin Board should make the best Sail.

B.

You may easily foresee the great in∣commodities of such a Sail. But I have seen tryed in little what such a Wind can do in such a case. For I have seen a Board set upon four truckles, with a staff set up in the midst of it for a Mast, and another very thin and broad Board fastned to that staff in the stead of a Sail; and so set as to receive the Wind very obliquely, I mean so as to be within a point of the

Compass directly opposite to it; and so placed upon a reasonable smooth pave∣ment where the Wind blew somewhat strongly. The event was first, that it stood doubting whether it should stir at all or no, but that was not long; and then it ran a head extream swiftly, till it was overthrown by a Rub.

A.

Before you leave the Ship tell me how it comes about that so small a thing as a Rudder, can so easily turn the greatest Ship?

B.

'Tis not the Rudder only, there must also be a stream to do it; you shall never turn a Ship with a Rudder in a standing pooll, nor in a natural current.

You must make a stream from head to stern, either with Oares or with Sails: when you have made such a stream, the turning of the Rudder obliquely holds the Water from passing freely; and the Ship or Boat cannot go on directly, but as the Rudder inclines to the stern, so will the Ship turn. But this is too well known to insist upon: you have observed, that the Rudders of the greatest Ships are not very broad, but go deep into the Water, whereas Western Barges, though but small Vessels, have their Rudders much broader, which argues that the holding of Water from passing is the

office of a Rudder: and therefore to a Ship that draws much Water the Rud∣der is made deep accordingly, and in Barges that draw little Water, the Rud∣ders as less deep, must so much the more be extended in breadth.

A.

What makes Snow?

B.

The same cause which (speaking of Hardness) I supposed for the cause of Ice. For the Stream of Air proceeding from. That both the Earth and the Sun cast off the Air, and consequently maketh a stream of Air from the Aequinoctial to∣wards the Poles, passing amongst the Clouds, shaving those small drops of Water whereof the Clouds consist, and congeals them as they do the Water of the Sea, or of a River. And these small frozen drops are that which we call Snow.

A.

But then how are great drops frozen in∣to Hailstones, and that especially (as we see they are) in Summer?

B.

It is especially in Summer; and hot weather, that the drops of Water which make the Clouds, are great enough; but it is then also that Clouds are sooner and more plentifully carryed up. And there∣fore the current of the Air strengthned between the Earth and the Clouds, be∣comes more swift; and thereby freezeth

the drops of Water, not in the Cloud it self, but as they are falling. Nor does it freeze them throughly, the time of their falling not permitting it, but gives them only a thin coat of Ice; as is mani∣fest by their suddain dissolving.

A.

Why are not somteimes also whole Clouds when pregnant and ready to drop, frozen into one piece of Ice?

B.

I belive they are so whensoever it Thunders.

A.

But upon what ground do you believe it?

B.

From the manner or kind of noise they make, namely a crack; which I see not how it can possibly be made by Wa∣ter or any other soft Bodies whatsoever.

A.

Yes, the Powder they call Aurum Fulminans, when throughly warm, gives just such another crack as Thunder.

B.

But why may not every small grain of that Aurum Fulminans by it self be heard, though a heap of them together be soft, as is any heap of Sand. Salts of all sorts are of the nature of Ice. But Gold is dissolved into Aurum Fulminans by Nitre and other Salts. And the least grain of it gives a little crack in the fire by it self. And therefore when they are so warmed by degrees, the crack can∣not chuse but be very great.

A.

But before it be Aurum Fulminans they use to wash away the Salt (which they call dulcifying it,) and then they dry it gently by degrees.

B.

That is, they exhale the pure Wa∣ter that is left in the Powder, and leave the Salt behind to Harden with drying. Other Powder made of Salts without a∣ny Gold in them will give a crack as great as Aurum Fulminans. A very great Chymist of our times hath written, that Salt of Tarter, Salt-peter, and a little Brimstone ground together into a Pow∣der, and dryed, a few grains of that Powder will be made by the fire to give as great a Clap as a Musquet.

A.

Me thinks it were worth your tryal to see what effect a Quart or a Pint of Au∣rum Fulminans would produce, being put into a great Gun made strong enough on purpose, and the Breech of the Gun set in hot Cinders, so as to heat by degrees, till the Powder fly.

B.

I pray you try it your self; I can∣not spare so much Money.

A.

What is it that breaketh the Clouds when they are frozen?

B.

In very hot weather the Sun raiseth from the Sea and all moist places abun∣dance of Water, and to a great height. And whilst this Water hangs over us in

Clouds, or is again descending, it raiseth other Clouds, and it hapens very often that they press the Air between them, and squeeze it through the Clouds themselves very violently; which as it passes shaves and hardens them in the manner decla∣red.

A.

That has already been granted, my question is what breaks them?

B.

I must here take in one supposition more.

A.

Then your Basen (it seems) holds not all you have need of.

B.

It may for all this, for the suppositi∣on I add is no more but this; that what internal motion I ascribe to the Earth, and other the Concrete parts of the World, is to be supposed also in every of their parts how small soever; for what reason is there to think, in case the whole Earth have in truth the motion I have ascribed to it, that one part of it taken away, the remaining part should love that moti∣on. If you break a Load-stone both parts will retain their vertue, though weakened according to the diminution of their quantity; I suppose therefore in every small part of the Earth, the same kind of motion, which I have supposed in the whole: and so I recede not yet from my Basen.

A.

Let it be supposed, and withall, that abundance of Earth (which I see you aim at,) be drawn up together with the Water. What then?

B.

Then if many pregnant Clouds, some ascending and some descending meet together, and make concavities between, and by the pressing out of the Air, as I have said before, become Ice; those Atomes (as I may call them) of Earth will be by the straining of the Air through the water of the Clouds, be left behind, and remain in the Cavities of the Clouds, and be more in number then for the proportion of the Air therein. There∣fore for want of liberty they must needs justle one another, and become (as they are more and more streightened of room) more and more swift, and consequently at last break the Ice suddenly and violent∣ly, now in one place, and by and by in another; and make thereby so many claps of Thunder, and so many Flashes of Lightning. For the Air Recoiling up∣on our Eyes, is that which maketh those Flashes to our Fancy.

A.

But I have seen Lightning in a very clear Evening, when there has been neither Thunder nor Clouds.

B.

Yes in a clear evening; because the Clouds and the Rain were below the

Horison, perhaps 40 or 50 Miles off; so that you could not see the Clouds nor hear the Thunder.

A.

If the Clouds be indeed Frozen into Ice, I shall not wonder if they be sometimes also so scituated, as (like Looking-Glasses) to make us see sometimes three or more Suns by Refraction and Reflection.