Part 9

He says, No. 2. "Let a given body or mass of matter be called _a_, and let any given celerity be called c. That _celerity_ doubled, tripled, &c. or halved, thirded, &c. will be 2 _c_, 3 _c_, &c. or ½ _c_, ⅓ _c_, &c. respectively: also the _body_ doubled, tripled, or halved, thirded, will be 2 _a_, 3 _a_, or ½ _a_, ⅓ _a_, respectively." Thus far is clear.--But he adds, "Now to move the body _a_ with the celerity _c_, requires a certain force to be impressed upon it; and to move it with a celerity as 2 _c_, requires _twice that force_ to be impressed upon it, &c." Here I suspect some mistake creeps in by the author's not distinguishing between a great force applied at once, or a small one continually applied, to a mass of matter, in order to move it. I think it is generally allowed by the philosophers, and, for aught we know, is certainly true, that there is no mass of matter, how great soever, but may be moved by any force how small soever (taking friction out of the question) and this small force continued, will in time bring the mass to move with any velocity whatsoever.--Our author himself seems to allow this towards the end of the same No. 2. when he is subdividing his celerities and forces: for as in continuing the division to eternity by his method of ½ _c_, ⅓ _c_, ¼ _c_, ⅕ _c_, &c. you can never come to a fraction of velocity that is equal to 0 _c_, or no celerity at all; so dividing the force in the same manner, you can never come to a fraction of force that will not produce an equal fraction of celerity.--Where then is the mighty vis inertiæ, and what is its strength; when the greatest assignable mass of matter will give way to, or be moved by the _least_ assignable force? Suppose two globes, equal to the sun and to one another, exactly equipoised in Jove's balance; suppose no friction in the centre of motion, in the beam or elsewhere: if a musketo then were to light on one of them, would he not give motion to them both, causing one to descend and the other to rise? If it is objected, that the force of gravity helps one globe to descend, I answer, the same force opposes the other's rising: here is an equality that leaves the whole motion to be produced by the musketo, without whom those globes would not be moved at all.--What then does vis inertiæ do in this case? and what other effect could we expect _if there were no such thing_? Surely if it were any thing more than a phantom, there might be enough of it in such _vast_ bodies to annihilate, by its opposition to motion, so trifling a force?

Our author would have reasoned more clearly, I think, if, as he has used the letter _a_ for a certain quantity of matter, and _c_ for a certain quantity of celerity, he had employed one letter more, and put _f_ perhaps, for a certain quantity of force. This let us suppose to be done; and then as it is a maxim that the force of bodies in motion is equal to the quantity of matter multiplied by the celerity, (or _f_ = _c_ X _a_); and as the force received by and subsisting in matter, when it is put in motion, can never exceed the force given; so if, _f_ moves _a_ with _c_, there must needs be required 2 _f_ to move _a_ with 2 _c_; for _a_ moving with 2 _c_ would have a force equal to 2 _f_, which it could not receive from 1 _f_; and this, not because there is such a thing as vis inertiæ, for the case would be the same _if that had no existence_; but because nothing _can_ give more than it has, if 1 _f_ can to 1 _a_ give 1 _c_, which is the same thing as giving it 1 _f_; (i. e. if force applied to matter at rest, can put it in motion, and give it _equal_ force) where then is vis inertiæ? If it existed at all in matter, should we not find the quantity of its resistance subtracted from the force given?

In No. 4. our author goes on and says, "the body _a_ requires a certain force to be impressed on it to be moved with a celerity as _c_, or such a force is necessary; and therefore makes a certain resistance, &c. A body as 2 _a_ requires _twice_ that force to be moved with the _same celerity_, or it makes twice that resistance; and so on."--This I think is not true; but that the body 2 _a_ moved by the force 1 _f_ (though the eye may judge otherwise of it) does really move with the same celerity as it did when impelled by the same force; for 2 _a_ is compounded of 1 _a_ + 1 _a_: and if each of the 1 _a_'s or each part of the compound were made to move with 1 _c_ (as they might be by 2 _f_) then the whole would move with 2 _c_, and not with 1 _c_, as our author supposes. But 1 _f_ applied to 2 _a_, makes each _a_ move with ½ _c_; and so the whole moves with 1 _c_; exactly the same as 1 _a_ was made to do by 1 _f_ before. What is equal celerity but a _measuring the same space by moving bodies in the same time_?--Now if 1 _a_ impelled by 1 _f_ measures 100 yards in a minute; and in 2 _a_ impelled by 1 _f_, each _a_ measures 50 yards in a minute, which added make 100; are not the celerities as the forces equal? and since force and celerity in the same quantity of matter are always in _proportion_ to each other, why should we, when the quantity of matter is doubled, allow the force to continue unimpaired, and yet suppose one half of the celerity to be lost?--I wonder the more at our author's mistake in this point, since in the same number I find him observing: "We may easily conceive that a body as 3 _a_, 4 _a_, &c. would make 3 or 4 bodies equal to once _a_, each of which would require once the first force to be moved with the celerity _c_." If then in 3 _a_, each _a_ requires once the first force _f_ to be moved with the celerity _c_, would not each move with the force _f_ and celerity _c_; and consequently the whole be 3 _a_ moving with 3 _f_ and 3 _c?_ After so distinct an observation, how could he miss of the consequence, and imagine that 1 _c_ and 3 _c_ were the same? Thus as our author's abatement of celerity in the case of 2 _a_ moved by 1 _f_ is imaginary, so must be his additional resistance.--And here again, I am at a loss to discover any effect of the vis inertiæ.

In No. 6, he tells us, "that all this is likewise certain when taken the contrary way, viz. _from motion to rest_; for the body _a_ moving with a certain velocity, as _c_, requires a certain degree of force or resistance to stop that motion, &c. &c." that is, in other words, equal force is necessary to destroy force. It may be so. But how does that discover a vis inertiæ? would not the effect be the same _if there were no such thing_? A force 1 _f_ strikes a body 1 _a_, and moves it with the celerity 1 _c_, i. e. with the force 1 _f_: It requires, even according to our author, only an opposing 1 _f_ to stop it. But ought it not (if there were a vis inertiæ) to have not only the force 1 _f_, but an additional force equal to the force of vis inertiæ, that _obstinate power by which a body endeavours with_ all its might _to continue in its present state, whether of motion or rest_? I say, ought there not to be an opposing force equal to the sum of these?--The truth however is, that there is no body, how large soever, moving with any velocity, how great soever, but may be stopped by any opposing force, how small soever, continually applied. At least all our modern philosophers agree to tell us so.

Let me turn the thing in what light I please, I cannot discover the vis inertiæ, nor any effect of it. It is allowed by all, that a body 1 _a_ moving with a velocity 1 _c_, and a force 1 _f_ _striking another_ body 1 _a_ at rest, they will afterwards _move on together_, each with ½ _c_ and ½ _f_; which, as I said before, is equal in the whole to 1 _c_ and 1 _f_. If vis inertiæ, as in this case, neither abates the force nor the velocity of bodies, what does it, or how does it discover itself?

I imagine I may venture to conclude my observations on this piece, almost in the words of the author; that if the doctrines of the immateriality of the soul and the existence of God and of divine providence are demonstrable from no plainer principles, the _deist_ [i.e. _theist_] has a desperate cause in hand. I oppose _my theist_ to his atheist, because I think they are diametrically opposite; and not near of kin, as Mr. Whitfield seems to suppose; where (in his journal) he tells us, "_M. B. was a deist, I had almost said an atheist_;" that is, _chalk_, I had almost said _charcoal_.

The din of the market[19] increases upon me; and that, with frequent interruptions, has, I find, made me say some things twice over; and, I suppose, forget some others I intended to say. It has, however, one good effect, as it obliges me to come to the relief of your patience with

Your humble servant,

B. FRANKLIN.

FOOTNOTES:

[18] Baxter's Inquiry into the Nature of the Human Soul. B. V.

[19] Philadelphia market, in which Dr. Franklin lived. B. V.

TO JOHN PRINGLE, M. D. AND F. R. S.

_On the different Strata of the Earth._

_Craven-Street, Jan. 6, 1758._

SIR,

I return you Mr. Mitchell's paper on the strata of the earth[20] with thanks. The reading of it, and perusal of the draft that accompanies it, have reconciled me to those convulsions which all naturalists agree this globe has suffered. Had the different strata of clay, gravel, marble, coals, lime-stone, sand, minerals, &c. continued to lie level, one under the other, as they may be supposed to have done before those convulsions, we should have had the use only of a few of the uppermost of the strata, the others lying too deep and too difficult to be come at; but the shell of the earth being broke, and the fragments thrown into this oblique position, the disjointed ends of a great number of strata of different kinds are brought up to day, and a great variety of useful materials put into our power, which would otherwise have remained eternally concealed from us. So that what has been usually looked upon as a _ruin_ suffered by this part of the universe, was, in reality, only a preparation, or means of rendering the earth more fit for use, more capable of being to mankind a convenient and comfortable habitation.

I am, Sir, with great esteem, yours, &c.

B. FRANKLIN.

FOOTNOTE:

[20] See this paper afterwards printed in the _Philosophical Transactions_.

TO THE ABBE SOULAVIE.

Occasioned by his sending me some notes he had taken of what I had said to him in conversation on the Theory of the Earth. I wrote it to set him right in some points wherein he had mistaken my meaning.[21]

_Passy, September 22, 1782._

SIR,

I return the papers with some corrections. I did not find coal mines under the calcareous rock in Derbyshire. I only remarked, that at the lowest part of that rocky mountain which was in sight, there were oyster shells mixed in the stone; and part of the high county of Derby being probably as much above the level of the sea, as the coal mines of Whitehaven were below it, seemed a proof, that there had been a great _bouleversement_ in the surface of that island, some part of it having been depressed under the sea, and other parts, which had been under it, being raised above it. Such changes in the superficial parts of the globe, seemed to me unlikely to happen, if the earth were solid to the centre. I therefore imagined, that the internal parts might be a fluid more dense, and of greater specific gravity than any of the solids we are acquainted with, which therefore might swim in or upon that fluid. Thus the surface of the globe would be a shell, capable of being broken and disordered by the violent movements of the fluid on which it rested. And as air has been compressed by art so as to be twice as dense as water, in which case, if such air and water could be contained in a strong glass vessel, the air would be seen to take the lowest place, and the water to float above and upon it; and as we know not yet the degree of density to which air may be compressed, and M. Amontons calculated, that its density increasing as it approached the centre, in the same proportion as above the surface, it would at the depth of [___] leagues, be heavier than gold, possibly the dense fluid occupying the internal parts of the globe might be air compressed. And as the force of expansion in dense air when heated is in proportion to its density, this central air might afford another agent to move the surface, as well as be of use in keeping alive the subterraneous fires; though, as you observe, the sudden rarefaction of water coming into contact without those fires, may also be an agent sufficiently strong for that purpose, when acting between the incumbent earth and the fluid on which it rests.

If one might indulge imagination in supposing how such a globe was formed, I should conceive, that all the elements in separate particles being originally mixed in confusion, and occupying a great space, they would (as soon as the almighty fiat ordained gravity, or the mutual attraction of certain parts, and the mutual repulsion of others, to exist) all move to their common centre: that the air being a fluid whose parts repel each other, though drawn to the common centre by their gravity, would be densest towards the centre, and rarer as more remote; consequently all matters lighter than the central parts of that air, and immersed in it, would recede from the centre, and rise till they arrived at that region of the air which was of the same specific gravity with themselves, where they would rest; while other matter, mixed with the lighter air, would descend, and the two meeting would form the shell of the first earth, leaving the upper atmosphere nearly clear. The original movement of the parts towards their common centre would naturally form a whirl there; which would continue upon the turning of the new-formed globe upon its axis, and the greatest diameter of the shell would be in its equator. If by any accident afterwards the axis should be changed, the dense internal fluid, by altering its form, must burst the shell, and throw all its substance into the confusion in which we find it. I will not trouble you at present with my fancies concerning the manner of forming the rest of our system. Superior beings smile at our theories, and at our presumption in making them. I will just mention, that your observation of the ferruginous nature of the lava which is thrown out from the depths of our volcanoes, gave me great pleasure. It has long been a supposition of mine, that the iron contained in the surface of the globe has made it capable of becoming, as it is, a great magnet; that the fluid of magnetism perhaps exists in all space; so that there is a magnetical north and south of the universe, as well as of this globe, and that if it were possible for a man to fly from star to star, he might govern his course by the compass; that it was by the power of this general magnetism this globe became a particular magnet. In soft or hot iron the fluid of magnetism is naturally diffused equally; when within the influence of the magnet it is drawn to one end of the iron, made denser there and rarer at the other. While the iron continues soft and hot, it is only a temporary magnet; if it cools or grows hard in that situation, it becomes a permanent one, the magnetic fluid not easily resuming its equilibrium. Perhaps it may be owing to the permanent magnetism of this globe, which it had not at first, that its axis is at present kept parallel to itself, and not liable to the changes it formerly suffered, which occasioned the rupture of its shell, the submersions and emersions of its lands and the confusion of its seasons. The present polar and equatorial diameters differing from each other near ten leagues, it is easy to conceive, in case some power should shift the axis gradually, and place it in the present equator, and make the new equator pass through the present poles, what a sinking of the waters would happen in the present equatorial regions, and what a rising in the present polar regions; so that vast tracts would be discovered, that now are under water, and others covered, that are now dry, the water rising and sinking in the different extremes near five leagues. Such an operation as this possibly occasioned much of Europe, and among the rest this Mountain of Passy on which I live, and which is composed of limestone, rock and sea-shells, to be abandoned by the sea, and to change its ancient climate, which seems to have been a hot one. The globe being now become a perfect magnet, we are, perhaps, safe from any change of its axis. But we are still subject to the accidents on the surface, which are occasioned by a wave in the internal ponderous fluid; and such a wave is producible by the sudden violent explosion you mention, happening from the junction of water and fire under the earth, which not only lifts the incumbent earth that is over the explosion, but impressing with the same force the fluid under it, creates a wave, that may run a thousand leagues, lifting, and thereby shaking, successively, all the countries under which it passes. I know not, whether I have expressed myself so clearly, as not to get out of your sight in these reveries. It they occasion any new enquiries, and produce a better hypothesis, they will not be quite useless. You see I have given a loose to imagination; but I approve much more your method of philosophising, which proceeds upon actual observation, makes a collection of facts, and concludes no farther than those facts will warrant. In my present circumstances, that mode of studying the nature of the globe is out of my power, and therefore I have permitted myself to wander a little in the wilds of fancy. With great esteem,

I have the honour to be, Sir, &c.

BENJ. FRANKLIN.

_P. S._ I have heard, that chymists can by their art decompose stone and wood, extracting a considerable quantity of water from the one, and air from the other. It seems natural to conclude from this, that water and air were ingredients in their original composition: for men cannot make new matter of any kind. In the same manner may we not suppose, that when we consume combustibles of all kinds, and produce heat or light, we do not create that heat or light; but only decompose a substance, which received it originally as a part of its composition? Heat may be thus considered as originally in a fluid state; but, attracted by organized bodies in their growth, becomes a part of the solid. Besides this, I can conceive, that in the first assemblage of the particles of which this earth is composed, each brought its portion of the loose heat that had been connected with it, and the whole, when pressed together, produced the internal fire that still subsists.

FOOTNOTE:

[21] In an American periodical publication, this paper is said to have been so endorsed in Dr. Franklin's hand. We extract the paper itself, from the Transactions of the American Philosophical Society, where it was read Nov. 21, 1788. The two papers that follow it are from the same work, and were read in the Society the preceding day, and the other Jan. 15, 1790. _Editor._

TO DAVID RITTENHOUSE, ESQ.

_New and curious Theory of Light and Heat._

[No date.]

Universal space, as far as we know of it, seems to be filled with a subtle fluid, whose motion, or vibration, is called light.

This fluid may possibly be the same with that, which, being attracted by, and entering into other more solid matter, dilates the substance by separating the constituent particles, and so rendering some solids fluid, and maintaining the fluidity of others; of which fluid, when our bodies are totally deprived, they are said to be frozen; when they have a proper quantity, they are in health, and fit to perform all their functions; it is then called natural heat; when too much, it is called fever; and when forced into the body in too great a quantity from without, it gives pain, by separating and destroying the flesh, and is then called burning, and the fluid so entering and acting is called fire.

While organised bodies, animal or vegetable, are augmenting in growth, or are supplying their continual waste, is not this done by attracting and consolidating this fluid called fire, so as to form of it a part of their substance? And is it not a separation of the parts of such substance, which, dissolving its solid state, sets that subtle fluid at liberty, when it again makes its appearance as fire?

For the power of man relative to matter, seems limited to the separating or mixing the various kinds of it, or changing its form and appearance by different compositions of it; but does not extend to the making or creating new matter, or annihilating the old. Thus, if fire be an original element or kind of matter, its quantity is fixed and permanent in the universe. We cannot destroy any part of it, or make addition to it; we can only separate it from that which confines it, and so set it at liberty; as when we put wood in a situation to be burnt, or transfer it from one solid to another, as when we make lime by burning stone, a part of the fire dislodged in the fuel being left in the stone. May not this fluid, when at liberty, be capable of penetrating and entering into all bodies, organised or not, quitting easily in totality those not organised, and quitting easily in part those which are; the part assumed and fixed remaining till the body is dissolved?

Is it not this fluid which keeps asunder the particles of air, permitting them to approach, or separating them more, in proportion as its quantity is diminished or augmented?

Is it not the greater gravity of the particles of air, which forces the particles of this fluid to mount with the matters to which it is attached, as smoke or vapour?

Does it not seem to have a greater affinity with water, since it will quit a solid to unite with that fluid, and go off with it in vapour, leaving the solid cold to the touch, and the degree measurable by the thermometer?

The vapour rises attached to this fluid, but at a certain height they separate, and the vapour descends in rain, retaining but little of it, in snow or hail less. What becomes of that fluid? Does it rise above our atmosphere, and mix with the universal mass of the same kind?

Or does a spherical stratum of it, denser, as less mixed with air, attracted by this globe, and repelled or pushed up only to a certain height from its surface, by the greater weight of air, remain there surrounding the globe, and proceeding with it round the sun?

In such case, as there may be a continuity or communication of this fluid through the air quite down to the earth, is it not by the vibrations given to it, by the sun, that light appears to us? And may it not be, that every one of the infinitely small vibrations, striking common matter with a certain force, enters its substance, is held there by attraction, and augmented by succeeding vibrations, till the matter has received as much as their force can drive into it?

Is it not thus, that the surface of this globe is continually heated by such repeated vibrations in the day, and cooled by the escape of the heat when those vibrations are discontinued in the night, or intercepted and reflected by clouds?

Is it not thus, that fire is amassed and makes the greatest part of the substance of combustible bodies?

Perhaps, when this globe was first formed, and its original particles took their place at certain distances from the centre, in proportion to their greater or less gravity, the fluid fire, attracted towards that centre, might in great part be obliged, as lightest, to take place above the rest, and thus form the sphere of fire above supposed, which would afterwards be continually diminishing by the substance it afforded to organised bodies, and the quantity restored to it again, by the burning or other separating of the parts of those bodies.

Is not the natural heat of animals thus produced, by separating in digestion the parts of food, and setting their fire at liberty?