ON THE TEMPERATURE OF THE PLANETS.
MAN newly created, and even the ignorant man at this day beholds the extent and nature of the universe only by the simple organ of light: to him the earth is but a solid body, whose volume is unbounded, and whose extent is without limits, of which he can only survey small superficial spaces: while the sun and planets seem to be luminous points, of which the sun and moon appear to be the only objects worthy regard in the immensity of the heavens. To this false idea on the extent of nature and the proportions of the universe is joined the still more disproportionate sentiment of superiority. Man, by comparing himself with other terrestrial beings, feels that he ranks the first, and hence he presumes that all was made for him; that the earth was created only to serve for his habitation, and the heavens for a spectacle; and in short the whole universe ought to yield to his necessities, and even his pleasures. But in proportion as he makes use of that divine light, which alone ennobles hisbeing; in proportion as he obtains instruction, he is forced to abate his pretensions; he finds himself lessened in proportion as the universe increases in his ideas, and it becomes demonstrable to him, that the earth, which forms all his domain, and on which unfortunately he cannot subsist without trouble and sorrow, is as small with respect to the universe, as he is with respect to the Creator. In short, from study and application, he finds that there does not remain a possible doubt, that this earth, large and extensive as it may seem to him, is but a moderate sized planet, a small mass of matter, which, with others, has a regular course round the sun: for as it appears our globe is at the distance of at least 33 millions of leagues, and the planet Saturn at 313 millions, the natural conclusion is, that the extent of the sun’s empire is a sphere, whose diameter is 627 millions of leagues, and that the earth, relative to this space, is not more than a grain of sand to the volume of the globe.
However, the planet Saturn, although the furthest from the sun, is not by any means near the confines of his empire: his limits extend much further, since comets pass over spaces beyond that distance, as may be estimated by the time of their revolutions: a comet whichlike that of the year 1680 revolves round the sun in 575 years must be 15 times more remote from him than Saturn; for the great axis of its orbit is 138 times greater than the distance from the earth to the sun. Hence we must still augment the extent of the solar power 15 times the distance from the sun to Saturn, so that all the space in which the planets are included is only a small province of his domain, whose bounds should be placed at least 138 times his distance from the earth.
What immensity of space! What quantity of matter! For independently of the planets, there is a probability of the existence of 400 or 500 comets, perhaps larger than the earth, which run over the different regions of this vast sphere of which the terrestrial globe only constituting a part, a unity on 191,201,612,985,514,272,000, a quantity represented by numbers, which imagination cannot attain or comprehend.
Nevertheless, this enormous extent, this vast sphere, is yet only a very small space in the immensity of the heavens; each fixed star is a sun, a center of a sphere equally as extensive; and as we reckon more than 2000 of these fixed stars perceived by the naked eye, and as with telescopes we can discover so much the greater number as these instruments are more powerful;the extent of the universe appears to be without bounds and the solar system forms only a province of the universal empire of the Creator; an infinite empire like himself.
Sirius, the most brilliant fixed star, and which for that reason may be regarded as the nearest sun to our’s, affords to our sight only a second of annual parrallax on the whole diameter of the earth’s orbit, and is therefore at the distance of 6,771,770, millions of leagues distant from us, that is, 6,767,216 millions of leagues from the limits of the solar system, such as we have assigned it after the depth to which the comets immerse. Supposing then, there is an equal space from Sirius to that which belongs to our sun, we shall perceive that we must extend the limits of our solar system 742 times more than it is at present, as far as the aphelion of the comet, whose enormous distance from the sun is nevertheless only a unit on 742 of the total diameter of the solar system.
We can form another idea of our immense distance from Sirius, by recollecting that the sun’s disk forms to our sight an angle of 32 minutes, whereas that of Sirius forms only that of a second; and Sirius being a sun like ours, which we shall suppose of equal magnitude, since there is no reason to conceive it larger or smaller, it would appear to us as large as the sun, if it were but a like distance. Taking therefore two numbers proportional to the square of 32 minutes, and to the square of a second, we shall have 3,686,400 for the distance of the earth to Sirius, and one for its distance to the sun; and as this unit is equal to 33 millions of leagues, we see how many millions of leagues Sirius is distant from us, since we must multiply these 33 millions by 3,686,400; and if we divide the space between these two neighbouring suns, although at so great a distance, we shall see that the comets might be removed to a distance 1,800,000 timesgreater than that of the earth to the sun without quitting the limits of the solar universe, and without being subjected to other laws than that of our sun, and hence it may be concluded that the solar system for its diameter has an extent, which, although prodigious, nevertheless, forms only a very small portion of the heavens; and we must infer a truth therefrom but little known, namely, that from the sun, the earth and all the other planets, the sky must appear the same.
When in a serene and clear night we contemplate all those stars with which the celestial vault is illuminated, it might be imagined that by being conveyed into another planet more remote from the sun, we should see these glittering stars larger, and emitting a brighter light, since we should be so much nearer to them. Nevertheless, the calculation we have just made demonstrates that if we were placed in Saturn, which is 300 millions of leagues nearer Sirius, it would appear only an 194,021st part bigger, an augmentation absolutely insensible; from which it must be concluded, that the heaven, with respect to all the planets, has the same aspect as it has to the earth. Therefore if even there should exist comets whose periods of revolution might be double, ortreble the period of 575 years, the longest known to us; if even the comets in consequence thereof, immerse at a depth ten times greater, there would still be a space 74 or 75 times deeper, to reach the last confines, as well of the solar system, as of the sirian; so that by allowing Sirius as much magnitude as our sun has, and supposing in his system as many or more cometary bodies than there are comets existing in the solar, Sirius will govern them as the sun governs his, and there will remain an immense interval between the confines of the two empires; an interval which appears to be no more than a desart in the vast space, and which must give a suspicion that cometary bodies do exist, whose periods are longer, and which are to a much greater distance than we can determine by our actual knowledge. Sirius may also be a sun much larger and more powerful than ours; and if that is the case, it must throw the borders of his domain so much the further back by approaching them to us, and at the same time retrench the circumference of the sun.
I cannot avoid presuming, that in this great number of fixed stars, which are all so many suns, there are some greater and others smaller than ours; others more or less luminous, somenearer, which are represented to us by those stars called by astronomers,stars of the first magnitude, and many others more remote, which for that reason appear to us smaller. The stars callednebulousseem to want light and fire, and to be only half lighted; those which appear and disappear alternately are, perhaps, of a form flattened by the violence of the centrifugal force in their motion of rotation, and are perceiveable only when they are in the full, disappearing when they are sideways. In this grand order of things, and in the nature of the stars, there are the same varieties, and the same differences, in number, size, space, motion, form, and duration; the same relation, the same degrees, and the same connection, as are found in all the other orders of the creation.
Each of the suns being endowed like ours, and like all matter, with an attractive power, which extends to an indefinite distance, and decreases, as the space increases, analogy leads us to imagine that within each of their spheres there exists a great number of opaque bodies, planets, or comets, which circulate round them, but which being much smaller than the suns which serve them for heat, they are beyond the reach of our sight.
It might be imagined that comets pass from one system to the other, and that if they happened to approach the confines of the two empires they would be attracted by the preponderating power, and forced to obey the laws of a new master. But, by the immensity of space which is beyond the aphelion of our comets, it appears that the Sovereign Ruler has separated each system by immense desarts, a thousand and a thousand times larger than all the extent of known spaces. These desarts, which numbers cannot fathom the depth of, are external and invincible barriers, that all the powers of created nature cannot surmount. To form a communication from one system to the other, and for the subjects of one to pass into the other, it would be requisite that the centre was not immoveable, for the sun, the head of the system, changing place, would draw with it in its course all the bodies which depend thereon, and hence might approach and invade another demesne. If its route were directed towards a weaker star, it would commence by carrying off the subjects of its most distant provinces, afterwards those more interior, and would oblige them all to increase its train by revolving round it; and its neighbour thus deprived of its subjects, no longer having planets nor comets, would lose both its lightand fire, which their motion alone can excite and support; hence this detached star, being no longer maintained in its place by the equilibrium of its forces, would be obliged to change nutrition, by changing nature, and becoming an obscure body, would, like the rest, obey the power of the conqueror, whose fire would increase in proportion to the number of its conquests.
For what can be said on the nature of the sun but that it is a body of prodigious volume, an enormous mass of matter penetrated by fire, which appears to subsist without aliment, and which resembles a metal or a solid body in incandescence? And from whence can this constant state of incandescence, this continually renewed production of fire proceed, whose consumption does not appear to be supported by any aliment, and whose deperdition is at least insensible, although constant for such a great number of years? Is there, or can there be, any other cause of the production of this permanent fire, but the rapid motion from the strong pressure of all bodies, which revolve round this common heat, and which heats and sets fire to it, like a wheel rapidly turned round its axis? The pressure, which they exercise by virtue of their weight is equivalent to the friction, and even more powerful, because thispressure is a penetrating power, which not only rubs the external surface but all the internal parts of the mass: the rapidity of their motion is so great that the friction acquires a force almost infinite, and consequently sets the whole mass of the axis in a state of incandescence, of light, of heat, and of fire, which hence has no need of aliment to be supported, and which, in spite of the deperdition each day made by the emission of light, may remain for ever without any sensible alteration, other suns rendering as much light to ours as it sends to them, and no part of the smallest atom of fire, or any other matter, being lost in a system where all is attracted.
If from this sketch of the great table of the heavens, and in which I have only attempted to represent to myself the proportion of the spaces, and that of the motion of bodies which travel over them; if from this point of view, to which I only raised myself to see how greatly nature must be multiplied in the different regions of the universe, we descend to that proportion of space which we are better acquainted with, and in which the sun exercises its power, we shall discover, that although it governs all bodies therein, it, nevertheless, has not thepower of vivifying them, nor even that of supporting life and vegetation.
Mercury, which is the nearest to the sun, nevertheless receives only a heat 400 times stronger than that of the earth, and this heat, so far from being burning, as it has always been supposed, would not be strong enough of itself to support animated nature, for the actual heat of the sun on the earth being only1/50part of the heat of the terrestrial globe, that of the sun on Mercury consequently is only1/8part of the actual heat of the earth. Now if7/8parts were subtracted from the heat which is at present the temperature of the earth, it is certain animated nature would be checked, if not entirely extinguished. Since the sun alone cannot maintain organised nature in the nearest planet, how much more aid must it require to animate those at a greater distance? To Venus it only sends a heat2/50times stronger than that it sends to the earth, which instead of being strong enough to support animated nature, would not certainly suffice to maintain the liquidity of water, nor perhaps even the fluidity of air, since our actual temperature would be refrigerated to2/49, which is very near the term1/25we have given as the external limit of theslightest heat, relative to living nature. And with respect to Mars, Jupiter, Saturn, and all their satellites, the quantity of heat which the sun sends to them, in comparison with that which is necessary for the support of nature, which may be looked upon as of little effect, especially in the two larger planets, which, nevertheless, appear to be the essential objects of the solar system.
All the planets, therefore, have always been volumes (as large as useless) of matter more than dead, profoundly frozen, and consequently places uninhabited and uninhabitable for ever, if they do not include within themselves treasures of heat much superior to what they receive from the sun. The heat which our globe possesses of itself, and which is 50 times greater than that which comes to it from the sun, is, in fact, the treasure of nature, the true fund which animates us as well as every being: it is this internal heat of the earth which causes all things to germinate and to develope; it is that which constitutes the element of fire, properly called an element, which alone gives motion to other elements, and which if it was reduced to1/20could not conquer their resistance, but would itself fall into an inertia. Now this element, this sole active power, whichmay render the air fluid, the water liquid, and the earth penetrable, might it not have been given to the terrestrial globe alone? Does analogy permit us to doubt that the other planets do not likewise contain a quantity of heat, which belongs to them alone, and which must render them capable of receiving and supporting living nature? Is it not greater and more worthy the idea we ought to have of the Creator, to suppose that there every where exists beings who acknowledge his power and celebrate his glory, than to depopulate all the universe, excepting the earth, and to despoil it of all beings, by reducing it to a profound solitude, in which we should only find a desart space, and frightful masses of inanimate matter.
Since the heat of the sun is so small on the earth, and other planets, it is necessary that they should possess a heat belonging solely to themselves, and our enquiry must be to see whence this heat proceeds which alone can constitute in them this element of fire. Now where shall we be able to discover this great quantity of heat if it be not in the source itself, in the sun alone? for the matter of which the planets have been formed and projected by a like impulsion will have preserved their motion in the same direction, and their heat inproportion to their magnitude and density. Whoever weighs these analogies, and conceives the power of their relations, will not doubt that the planets have issued from the sun by the stroke of a comet, because in the solar system comets only could have power and sufficient motion, to communicate a similar impulsion to the masses of matter which compose the planets. If to all these circumstances we unite that of the innate heat of the earth, and of the insufficiency of the sun to support nature, we must rest persuaded, that in the time of their formation the planets and earth were in a state of liquefaction, afterwards in a state of incandescence, and at last in a successive state of heat, always decreasing from incandescence to actual temperature, for there is no other mode of conceiving the origin and duration of this heat peculiar to the earth. It is difficult to imagine that the fire, termed central, can subsist at the bottom of the globe without air (that is, without its first aliment, and from whence this fire should proceed, which is supposed to be shut up in the centre of the globe), because what origin, what source shall we then find for it? Descartes has imagined the earth and planets were only small incrusted suns; in other words, suns entirelyextinguished. Leibnitz has not hesitated to pronounce that the terrestrial globe owes its source, and the consistence of its matters, to the element of fire; yet these two great philosophers had not the assistance of these numerous circumstances and observations which have been acquired and collected in our days, and which are so well established that it appears more than probable that the earth, as well as the planets, were projected out of the sun, and being consequently of a like matter, which was at first in a state of liquefaction they obeyed the centrifugal power, at the same time that it collected itself together by that of attraction, which has given a round form to all the planets under the equator, and flattened under the poles, on account of the variety of their rotation; that afterwards this fire being gradually dissipated, the benign temperature, suitable to organized nature, succeeded in different planets according to the difference of their thickness or density. If there should be other particular causes of heat assigned for the earth and planets, which might combine with those whose effects we have calculated, our results are not less curious, nor less useful to the advancement of science; and we shall here only observe, that those particular causes may prolong the time of the refrigeration of the globe, and theduration of living nature, beyond the terms we have indicated.
But I may be asked is this Theory equally as well founded in every point which serves for its basis; is it certain, according to your experiments, that a globe, as large as the earth, and composed of the same matters, cannot refrigerate from incandescence to actual temperature in less than 74,000 years, and that in order to become heated to the point of incandescence a 15th of this time, that is 5000 years, would be required: and also that it should be surrounded all that time by the most violent fire; if so, there are as you say strong presumptions that this great heat of the earth could not have been communicated to it from a distance, and that consequently the terrestrial matter formerly made a part of the mass of the sun; but it does not appear equally proved that the heat of this body on the earth is at present but1/50part of the heat of the globe. The testimony of our senses seems to refute this opinion, which you lay down as a certain truth, for although we cannot doubt that the earth has an innate heat, which is demonstrated by its always equal temperature, in all deep places where the coldness of the air cannot communicate; yet does it result that this heat, whichappears of moderate temperature, is greater than that of the sun which seems to burn us?
To all these objections I can give full satisfaction, but let us first reflect on the nature of our sensations. A very slight, and often imperceptible, difference in the causes which affect us, produces considerable ones in their effects. Is there any thing which comes nearer to extreme pleasure than grief? and who can assign the distance between the lively irritation by which we are moved with delight, and the friction which gives us pain? between the fire which warms and that which burns? between the light which is agreeable to our sight and that which blinds us? between the savour which pleases our taste and that which is disagreeable? between the smell of which a small quantity will at first be agreeable and yet soon after create nausea? We must therefore cease from being astonished that a small augmentation of heat, such as1/50should appear so striking.
I do not pretend positively to assert that the innate heat of the earth is really 49 times greater than that which comes to it from the sun: for as the heat of the globe belongs to all terrestrial matter, we have no means of separating it, nor consequently any sensibleand real limits to which we might relate it. But even if the solar heat be greater or smaller than we have supposed, relative to the terrestrial heat, our theory would only alter the proportion of the results.
For example, if we include the whole extent of our sensations of the greatest heat to the greatest cold, within the limits given by the observations of M. Amontons, that is, between seven and eight, and at the same time suppose that the heat of the sun can alone produce this difference of our sensations, we shall from thence have the proportion of 8 to 1 of the innate heat of the terrestrial globe to that which proceeds from the sun; and consequently the compensation which this heat of the sun actually makes on the earth, would be1/8and the compensation which it made in the time of incandescence will have been1/260: adding together these two terms, we have26/900, which multiplied by 121/2, the half of the sum of all the terms of the diminution of heat, gives325/400or5/8for the total compensation made by the sun’s heat during the the period of 74047 years of the refrigeration of the earth to actual temperature. And as the total loss of the innate heat is to the total compensation in the same ratio as the time of the period of refrigeration,we shall have 25 : 15/8:; 74047 : 48131/25, so that the refrigeration of the globe of the earth instead of having been prolonged only 770 years, would have been 48131/25years; which joined to the longest prolongation, the heat of the moon would also produce in this supposition, would give more than 5000 years.
If we adopt the limits laid down by M. de Marian, which are from 31 to 32, and suppose that the solar heat is no more than1/32of that of the earth, we shall have only1/4of this prolongation, about 1250 years, instead of 770, which gives the supposition of1/50which we have adopted.
But if we suppose that the sun’s heat is only1/250of that of the earth, as appears to result from the observations made at Paris, we should have for the compensation of the incandescence1/6250and1/250for the compensation to the end of the period of 7407 years of the refrigeration of the terrestrial globe to actual temperature, and we should find17/250for the total compensation made by the heat of the sun during this period, which would give only 154 years, or the 5th part of 770 years for the time of the prolongation of refrigeration. And likewise, if in the place of1/50we suppose that the solar heat was1/50of the terrestrial, we should findthat the time of prolongation would be five times longer, that is 3850 years; so that the more we endeavour to increase the heat which comes to us from the sun relative to that which emanates from the earth, the more we shall extend the duration of nature, and date the antiquity of the earth further back; for by supposing the heat of the sun was equal to the innate of the globe, we should find that the time of prolongation would be 38504 years, which consequently gives the earth a greater antiquity of 38 or 39000 years.
If we cast our eye on the table which M. de Mairan has calculated with great exactness, and in which he gives the proportion of the heat which comes to us from the sun, to that which emanates from the earth in all climates, we shall discover a well attested fact, which is, that in all climates where observations have been made, the summers are equal, whereas the winters are prodigiously unequal; this learned naturalist, attributes this constant equality of the intensity of heat in summer in all climates to the reciprocal compensation of the solar heat, and from the heat of the emanations of the central fire.
All naturalists who have employed themselveson this subject agree with me that the terrestrial globe possesses of itself a heat independently of that which comes from the sun. Is it not evident that this innate heat should be equal at every place on the surface of the globe, and that there is no other difference in this respect than that which results from the swelling of the earth at the equator, and of its flatness under the poles? A difference, which being in the same ratio nearly as the two diameters, does not exceed1/230, so that the innate heat of the terrestrial spheriod must be1/230times greater under the equator than under the poles. The deperdition which is made, and the time of refrigeration must, therefore, have been quicker, or more sudden, in the northern climates, where the thickness of the globe is not so great as in the southern climates, but this difference of1/230cannot produce that of the inequality of the central emanations, whose relation to the heat of the sun in winter being equal 50 to 1 in the adjacent climates to the equator, is found double to the 27th degree, triple to the 35th, quadruple to the 40th, tenfold to the 49th, and 35 times greater to the 60th degree of latitude. This cause, which presents itself, contributes to the cold of thenorthern climates, but it is insufficient for the effect of the inequality of the winters, since this effect would be 35 times greater than its cause to the 60th degree, and even excessive in climates nearer the poles; at the same time it would in no part be proportional to this same cause.
On the other hand there is not any foundation for supposing that in a globe which has received, or which possesses a certain degree of heat, there might be some parts of it much colder than others. We are sufficiently acquainted with the progress of heat and the phenomena of its communication, to be convinced that it is every where distributed alike, since by placing a cold body on one that is hot, the latter will communicate to the other sufficient heat to render heat of the same degree of temperature in a short time. It must not, therefore, be supposed that towards the poles there are strata of colder matters less permeable to the heat than in other climates, for of whatever nature they may be supposed to be, experience has demonstrated that in a very short time they would become as hot as the rest.
It is evident that great cold in the north does not proceed from these pretended obstacles which might oppose themselves to the issue of heat, nor from the slight difference which thatof the diameters of the terrestrial spheroid must produce; but it appears to me, after much reflection upon it, that we ought to attribute the equality of the summers, and the great inequality of the winters to a much more simple cause, but which, notwithstanding, has escaped the notice of all naturalists.
It is certain that as the native heat of the earth is much greater than that which comes to it from the sun, the summers ought to appear nearly equal every where, because this same heat from the sun makes only a small augmentation to the stock of real heat which the earth possesses; and consequently if this heat issuing from the sun, be only 1/[T.N.] of the native heat of the globe, the greater or less stay of it on the horizon, its greater or less obliquity on the climate, and even its total absence, would only produce one-fiftieth difference on the temperature of the climate, and hence the summers must appear, and are, in fact, nearly equal in all the climates of the earth. But what makes the winters so very unequal is the emanations of this internal heat of the globe being in a great measure suppressed as soon as the cold and frost bind and consolidate the surface of the earth and waters.
[T.N.: denominator missing from printed version.]
This heat which issues from the globe, decreases in the air in proportion, and in thesame ratio as the space increases, and the sole condensation of the air by this cause is sufficient to produce cold winds, which acting against the surface of the earth, bind and freeze it. As long as this confinement of the external strata of the earth remains, the emanations of the internal heat are retained, and the cold appears to be, nay in fact is, very considerably increased by this suppression of a part of this heat; but as soon as the air becomes milder, and the superficial strata of the globe loses its rigidity, the heat, retained all the time of the frost, issues out in greater abundance than in climates where it does not freeze, so that the sum of the emanations of the heat becomes equal and every where alike; and this is the reason that plants vegetate quicker, and the harvest is reaped in much less time in northern countries; and for the same reason it is, that often at the beginning of summer we feel such considerable heats.
If there were any doubt of the suppression of the emanations of the internal heat by the effect of frost, we might easily be convinced of the fact; for it is a circumstance universally known, that after a frost, we may perceive snow to thaw in pits, acqueducts, cisterns, quarries, subterraneous vaults or mines, when even these depths, pits or cisterns, contain nowater; the emanations of the earth having their free issue through these kinds of vents, the ground which covers this top is never frozen so strong as the open land; to the emanations, it permits their general course, and their heat is sufficient to melt the snow, especially in hollow places, at the same time that it remains on all the rest of the surface where the earth is not excavated.
This suppression of the emanations of the native heat of the earth is not only made by the frost, but likewise by the simple binding of the earth, often occasioned by a less degree of cold than that which is necessary to freeze the surface; there are very few countries where it freezes in the plains beyond the 35th degree latitude, particularly in the northern hemisphere. It appears, therefore, that from the equator, as far as the 35th degree, the emanations of the terrestrial heat having always their free issue, there ought to be in that part little or no difference between winter and summer, since this difference proceeds only from two causes, both too slight to produce any sensible effect. The first cause is the difference of the solar action, but as this action is itself much smaller than that of the terrestrial heat, its difference is too inconsiderable to be regarded as any thing. The second cause is the thickness of the globe,which towards the 35th degree, is near1/590th part less than at the equator, but even this difference can only produce a very slight effect, since at 35 degrees the relation of the emanations of the terrestrial to the solar heat is in summer from 33 to 1, and in winter from 153 to 1, which gives 186 to 2 or 93 to 1. From hence it can only be owing to the consolidation of the earth occasioned by the cold, or even to the cold produced by the durable rains which fall in these climates, that we can attribute this difference between winter and summer; the binding of the earth by cold suppresses a part of the emanations of the internal heat, and the cold, always renewed by the fall of rain, diminishes its intensity; these two causes, therefore, together produce the difference between winter and summer.
After having proved that the heat which comes to us from the sun is greatly inferior to the native heat of our globe; after having explained that, by supposing it only1/50part, the refrigeration of the globe to actual temperature cannot be made but in 75,832 years; after having demonstrated that the time of this refrigeration would still be longer, if the heat sent from the sun to the earth were in a greater relation, namely, of1/25or1/10instead of1/50, wecannot be blamed for having adopted that proportion which appears the most plausible from physical reasonings, and at the same time the most probable, as it does not extend too far back the time of the commencement of nature, which we have fixed at 37 or 38,000 years, dating it from the first day.
I nevertheless acknowledge that this time, all considerable as it is, does not appear sufficiently long for certain changes, certain successive alterations, which Natural History demonstrates to have taken place, and which seem to have required a still longer course of centuries; and from which I should be inclined to imagine, that, in reality, this time would be increased perhaps double if every phenomena were completely investigated; but I have confined myself to the least terms, and restrained, the limits of time as much as possible, without contradicting facts and experiments.
This theory, perhaps, may be attacked by another objection, which it is right to guard against. It may be told me that I have supposed, after Newton, the heat of boiling water to be three times greater than that of the sun in summer, and iron heated red-hot eight times greater than boiling water, that is, 24 or 25 times greater than that of the actual temperatureof the earth, and that there is something hypothetical in this supposition, on which I have founded the second basis of my calculations, whose results would be, without doubt, very different if this red heat of iron, or glass in incandescence, instead of being, in fact, 25 times greater than the actual heat of the globe, were, for example only 5 or 6 times as great.
The better to feel the force of this objection, let us make a calculation of the refrigeration of the earth, upon the supposition that in the time of incandescence it was only five times hotter than it is according to our calculations; this solar heat, instead of a compensation of1/50would have only made the compensation of1/250in the time of incandescence, these two terms added together gives6/250, which multiplied by 21/2, the half of the sum of all the terms of the diminution of heat, gives15/250for the total compensation which the heat of the sun has made during the whole period of the deperdition of the innate heat of the globe, which is 74047 years: therefore we shall have :15/250:: 74047 : 88814/25from which we see that the prolongation of refrigeration, which for a heat 28 times greater than actual temperature, has been only 770 years, should havebeen 88814/25in the supposition that this first heat should have been only five times greater than this actual temperature. This alone shews us that if we even suppose this primitive heat below 25, there would only be a longer prolongation of the refrigeration of the globe, and that alone appears to me sufficient to satisfy the objection.
It may likewise be said, "you have calculated the duration of the refrigeration of the planets, not only by the inverted ratio of their diameters, but also by the inverted ratio of their density; this might be well founded if we could imagine that in fact there exists matter whose density is as different from that of our globe: but does it exist? What, for example, will be the matter of which Saturn is composed, since its density is more than five times less than that of the earth?
To this I answer, that it would be very easy to find, in the vegetable class, matters five or six times less dense than a mass of iron, marble, hard calcareous stone, &c. of which we know that the earth is principally composed; but without quiting the mineral kingdom, and considering the density of these five matters, we have 2110/72for iron, 825/72for white marble, for gres 724/72, for common marble and calcareous stone 720/72; taking the mean term of thedensities of these five matters, of which the terrestrial globe is principally composed, we find its density to be 105/18. It is therefore required to find a matter whose density is in the relation of 189 to 1000 density, which is the same as that between Saturn and the Earth. Now this matter might be a kind of pumice stone, somewhat less dense than common pumice stone, whose relative density is here 166/70; whence it appears that Saturn is principally composed of a light matter similar to pumice stone.
So likewise the density of the Earth being to that of Jupiter as 1000 to 292, we must suppose that Jupiter is composed of a more dense matter than pumice stone, but much less dense than chalk.
The density of the Earth being to that of the Moon as 1000 to 702, this secondary planet appears composed of a matter whose density is not quite so great as that of hard calcareous stone, but more so than soft.
The density of the Earth being to that of Mars as 1000 to 730, this planet must be composed of a matter somewhat more dense than that of gres, and not so great as that of white marble.
But the density of the Earth being to that of Venus as 1000 to 12700, it may be supposedthat this planet is chiefly composed of a more dense matter than emery, and less dense than zinc.
Finally, the density of the Earth being to that of Mercury :: 1000 : 2040, or :: 105/18: 20,(9662/3)/1000, it must be thought that this planet is composed of a matter less dense than iron but more so than tin.
To the question, how can animated nature, which you suppose every where established, exist in planets of iron, emery, or pumice stone? I shall answer, by the same causes, and by the same means as it exists on the terrestrial globe, although composed of stone, gres, marble, iron, and glass. There are other planets like our globe, whose principal is one of these matters; but the external causes will soon have altered its superficial strata, and according to the different degrees of heat or cold, dryness or humidity, they will have converted this matter into a fertile earth proper to receive the seeds of organized nature, which only needs heat and moisture to develope itself.
Having answered the most obvious objections, it is necessary now to explain the facts, and observations, by which we are assured that the sun is only an accessory to the real heat, which continually emanates from the globe ofthe earth; and it will be just, at the same time, to see how comparable thermometers have taught us in a certain manner that the heat in summer is equal in all the climates of the earth, excepting Senegal, and some other parts of Africa, where the heat is greater than elsewhere.
It may be incontestibly demonstrated, that the light, and consequently the heat of the sun, emitted on the earth in the summer, is very great, comparatively with that emitted by the same body in winter; and yet, by very exact and reiterated observations, the difference of the real heat of the sun in summer is very small. This alone would be sufficient to prove that the heat of the sun makes only a small part of that of the terrestrial globe; but in addition to this M. Amontons, by receiving the rays of the sun on the same thermometer in summer and winter, observed that the greatest heat in summer in our climate differs from the cold in winter, when the water congeals, as only 7 differs from 6; whereas it can be demonstrated that the action of the sun in summer is about 66 times greater than that of the sun in winter; it therefore cannot be doubted, that there is a fund of very great heat in the terrestrial globe, on which, as a basis, the degreesof heat arise, and that at the surface it does not give a greater quantity of heat than that which comes from the sun.
If it be asked, how we can then assert that the heat in summer is 66 times grater than that in winter in our climate? I cannot give a better answer than by referring to the memoirs given by the late M. de Mairan in 1719, 1722, and 1765, and inserted in those of the Academy, where he examines, with a scrupulous attention, the vicissitudes of summers in different climates; the various causes for which may be reduced to four principal ones: 1. The inclination under which the light of the sun falls according to the different height of the sun on the horizon; 2dly. The greater or less intensity of light in proportion as its passage in the atmosphere is more or less oblique; 3dly. The different distance of the earth to the sun in summer and winter; and 4thly. The inequalities of the length of days in different climates. By the principle that heat is proportional to the action of light it will be easily demonstrated, that these four united causes, combined and compared, diminish with respect to our climate, this action of the sun’s heat in a ratio of about 66 to 1 between the summer and the wintersolstice; and this theoretical truth may be regarded as certain, as the second truth from experience, and which demonstrates, by the observations of the thermometer, immediately exposed to the sun’s rays in winter and summer, that the difference of real heat in these two is, nevertheless, at most only from 7 to 6; I say at most, for this determination given by M. Amontons is not nearly so exact as that which has been made by M. de Mairan, who, after a great number of final observations, proves that this relation is only as 32 to 31. What, therefore, must indicate this prodigious inequality between these two relations of the action of thesolar heat, in summer and winter, which is from 66 to 1; and of that of thereal heat, which is only from 32 to 31? Is it not evident that the innate heat of the globe of the earth is considerably greater than that which comes to us from the sun? It appears, in fact, that in the climate of Paris this heat of the earth is 29 times greater in summer, and 491 times greater in winter than that of the sun, as M. de Mairan has determined it. But I have already said that we must not conclude, from these two combined relations, the real one of the heat of the globe of the earth to that which comes from the sun, and I have given reasons which have determined me to supposethat we may estimate this heat of the sun 49 times less than the heat which emanates from the earth.
From the year 1701 to 1756 inclusive, a variety of observations were made with thermometers, and the following were the results. The greatest degree of heat, and of cold, which was experienced at Paris in each year was collected; a total of these was made, and it was found that the mean estimate, in all the thermometers, reduced to Rheaumur’s division, was 1026, for the greatest heat in summer, that is 26 degrees above the freezing point; and that the mean degree of cold in winter, during those 56 years, was 994, or 6 degrees below the freezing point of water, whence we concluded that the greatest heat in our summers at Paris differs from the greatest cold of our winters only1/32, since 994 : 1026 :: 31 : 32; and it was on this foundation that we stated the latter to be the relation of the greatest heat to the greatest cold. But it may be objected against the precision of this valuation, the defect of the construction of the thermometer, and Rheaumur’s division (to which we have here reduced the scale of all the rest); and this defect is extending only 1000 degrees below that of ice, as if 1000 degrees were in fact, that of absolute cold, whereas absolute colddoes not exist in nature; and that of the smallest heat should be supposed 10,000 instead of 1000, which would alter the thermometer’s gradation. It may likewise be said that it is possible all our sensations between the greatest heat and the greatest cold are comprised in as small an interval as that of a unit on 32 of heat, but that the voice of judgment seems to be raised against this opinion, and tells us this limit is too confined, and that it is much easier to reduce this interval than to give it an eighth, or a seventh instead of a thirty-second.
But be this valuation as it may, there can be no doubt of the truth of these facts which we have drawn from our observations, for in the same manner as we found, from the comparison of 56 successive years, the heat of summer at Paris 1026, or 26 degrees above the freezing point, we also found, with the same thermometers, that the heat in summer was 1026 in every climate of the earth, from the equator to the polar circle;[K]at Madagascar, in the islands of France and Bourbon, Roderigo, Siam, and the East-Indies; at Algiers, Malta, Cadiz, Montpelier, Lyons, Amsterdam, Upsal, Petersburgh, and as far as Lapland, near thepolar circle. At Cayenne, Peru, Martinico, Carthagena in America; at Panama; in short, in all the climates of the two hemispheres and continents where observations could be made, it has been constantly found that the liquor of the thermometer rose equally to 25, 26, or 27 degrees in the hottest days in summer; and hence ensues the incontestible fact of the equality of heat in summer in all climates of the earth. There are indeed some exceptions, for at Senegal, and some few other places, the thermometer rises 5 or 6 degrees higher, to 31 or 32 degrees; but that arises from accidental and local causes, which do not alter the truth of the observations, nor the certainty of the general fact, which alone might demonstrate to us, that there really exists a very great heat in the terrestrial globe, that the effect, or the emanations, of which are nearly equal in all the points of its surface, and that the sun, very far from being the only sphere of heat which animates nature, is at best only the regulator. This important fact, which we consign to posterity, will enable it to discover the real progression of the diminution of the heat of the terrestrial globe, which we have been only able to determine in a hypothetical manner. In a few centuries, I am confident it will be foundthat the greatest heat of summer, instead of raising the liquor of the thermometer to 26, will not raise it to more than 25, or 24; and from this effect, which is the result of all the combined causes, a judgment may be formed of the value of each of the particular causes, which produce the total effect of heat on the surface of the globe; for the heat which belongs to the earth, and which it has possessed from the time of incadescence, has very considerably diminished, and will continue to diminish with the course of time: this heat is independent of that which comes from the sun; the latter may be looked upon as constant, and consequently in futurity will make a greater compensation than at present. To the loss of this innate heat of the globe there are two other particular causes, which may add a considerable quantity of heat to the effect of the two first, the only ones we have as yet taken notice of.