FRANÇOIS JEAN DOMINIQUE ARAGO

(1786-1853)

RAGO, one of the founders of nineteenth-century science, was born near Perpignan, France, February 26th, 1786. His discoveries in magnetism and optics combined with his work as an astronomer to make him one of the most famous men of his day, and he increased his celebrity by his uncompromising republicanism. As a member of the French Chamber of Deputies in 1830, he was one of the leaders of the most advanced section of the Republican party. In 1848, after the fall of Louis Philippe, he became Minister of War and Marine, using his influence in the ministry to secure the abolition of slavery in the French colonies and of flogging in the army and navy. After the coup d'état he refused allegiance to Louis Napoleon, and offered his resignation from a scientific board of which he was a member. With a good taste he could show on occasion, Louis refused to accept it, and when Arago died, October 2d, 1853, decreed him a funeral with the highest possible honors. Besides his numerous scientific works, Arago wrote a volume of studies of the lives of scientific men, which has become a part of popular literature in English as well as in French.

THE CENTRAL FIRES OF THE EARTH

N ALL places of the earth, as soon as we descend to a certain depth, the thermometer no longer experiences either diurnal or annual variation. It marks the same degree, and the same fraction of a degree, from day to day, and from year to year. Such is the fact: what says theory?

Let us suppose, for a moment, that the earth has constantly received all its heat from the sun. Descend into its mass to a sufficient depth, and you will find, with Fourier, by the aid of calculation, a constant temperature for each day of the year. You will recognize further, that this solar temperature of the inferior strata varies from one climate to another; that in each country, finally, it ought to be always the same, so long as we do not descend to depths which are too great relatively to the earth's radius.

Well, the phenomena of nature stand in manifest contradiction to this result. The observations made in a multitude of mines, observations of the temperature of hot springs coming from different depths, have all given an increase of one degree of the centigrade for every twenty or thirty metres of depth. Thus, there was some inaccuracy in the hypothesis which we were discussing upon the footsteps of our colleague. It is not true that the temperature of the terrestrial strata may be attributed solely to the action of the solar rays.

This being established, the increase of heat which is observed in all climates when we penetrate into the interior of the globe is the manifest indication of an intrinsic heat. The earth, as Descartes and Leibnitz maintained it to be, but without being able to support their assertions by any demonstrative reasoning,

thanks to a combination of the observations of physical inquirers with the analytical calculations of Fourier,-is an incrusted. sun, the high temperature of which may be boldly invoked every time that the explanation of ancient geological phenomena will require it.

After having established that there is in our earth an inherent heat, a heat the source of which is not the sun, and which, if we may judge of it by the rapid increase which observation indicates, ought to be already. sufficiently intense at the depth of only seven or eight leagues to hold in fusion all known substances, there arises the question, What is its precise value at the surface of the earth; what weight are we to attach to it in the determination of terrestrial temperatures; what part does it play in the phenomena of life?

According to Mairan, Buffon, and Bailly, this part is immense. For France, they estimate the heat which escapes from the interior of the earth, at twenty-nine times in summer, and four hundred times in winter, the heat which comes to us from the sun. Thus, contrary to general opinion, the heat of the body which illuminates us would form only a very small part of that whose propitious influence we feel.

This idea was developed with ability and great eloquence in the "Memoirs of the Academy," in "Les Époques de la Nature » of Buffon, in the letters from Bailly to Voltaire upon the "Origin of the Sciences" and upon the "Atlantide." But the ingenious romance to which it has served as a base has vanished like a shadow before the torch of mathematical science.

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Fourier, having discovered that the excess of the aggregate temperature of the earth's surface above that which would result from the sole action of the solar rays has a determinate relation to the increase of temperature at different depths, succeeded in deducing from the experimental value of this increase a numerical determination of the excess in question. This excess is the thermometric effect which the solar heat produces at the surface; now, instead of the large numbers adopted by Mairan, Bailly, and Buffon, what has our colleague found? A thirtieth of a degree, not more.

The surface of the earth, which originally was perhaps incandescent, has cooled then, in the course of ages, so as hardly to preserve any sensible trace of its primitive heat. However, at great depths, the original heat is still enormous. Time will alter sensibly the internal temperature; but at the surface (and the phenomena of the surface can alone modify or compromise the existence of living beings), all the changes are almost accomplished. The frightful freezing of the earth, the epoch of which Buffon fixed at the instant when the central heat would be totally dissipated, is then a pure dream. At the surface the earth is no longer impregnated except by the solar heat. So long as the sun shall continue to preserve the same brightness, mankind will find, from pole to pole, under each latitude, the climates which have permitted them to live and to establish their residence. These are great results. While recording them in the annals of science, historians will not neglect to draw attention to this singular peculiarity: that the geometer, to whom we owe the first certain demonstration of the existence of a heat independent of a solar influence in the interior of the earth, has annihilated the immense part which this primitive heat was made to play in the explanation of the phenomena of terrestrial temperature.

Besides divesting the theory of climates of an error which occupied a prominent place in science, supported as it was by the imposing authority of Mairan, of Bailly, and of Buffon, Fourier is entitled to the merit of a still more striking achievement: he has introduced into this theory a consideration which hitherto had been totally neglected; he has pointed out the influence exercised by the temperature of the celestial regions, amid which the hearth describes its immense orbit around the sun.

When we perceive, even under the equator, certain mountains covered with eternal snow, upon observing the rapid dimi

nution of temperature which the strata of the atmosphere undergo during ascents in balloons, meteorologists have supposed that in the regions wherein the extreme rarity of the air will always exclude the presence of mankind, and that especially beyond the limits of the atmosphere, there ought to prevail a prodigious intensity of cold. It was not merely by hundreds, it was by thousands of degrees, that they had arbitrarily measured it. But, as usual, the imagination (cette folie de la maison) had exceeded all reasonable limits. The hundreds, the tens of thousands of degrees, have dwindled down, after the rigorous researches of Fourier, to fifty or sixty degrees only. Fifty or sixty degrees beneath zero, such is the temperature which the radiation of heat from the stars has established in the regions furrowed indefinitely by the planets of our system.

You recollect, gentlemen, with what delight Fourier used to converse on this subject. You know well that he thought himself sure of having assigned the temperature of space within eight or ten degrees. By what fatality has it happened that the memoir, wherein no doubt our colleague had recorded all the elements of that important determination, is not to be found? May that irreparable loss prove at least to so many observers, that instead of pursuing obstinately an ideal perfection, which it is not allotted to man to attain, they will act wisely in taking the public, as soon as possible, into the confidence of their labors.

From the essay on "Fourier," read before

the French Academy of Sciences.