The flocks and herds improve the rural found, ART. II. H. Hiftoire de l'Academie Royale de Sciences, &c. Année 1765. The Hiftory of the Royal Academy of Sciences at Paris; together with the Mathematical and Phyfical Memoirs, for the Year 1765. 4to. Paris, 1768. GENERAL PHYSICS. MEMOIR I. An enquiry into the general caufe of the heat in fummer, and of the cold in winter. By M. de Mairan. AROM the beginning of the world down to our own times, F it has, we believe, been an opinion held in common both by the learned and the ignorant, that the fun is the principal, if not almost the sole fountain of the heat which we enjoy here on earth. The common fenfe of mankind appears, at first fight, to be grofsly infulted by an attempt to prove that we are obliged to that glorious luminary, only for a very fmall part of the heat which we experience; and that the earth poffeffes, in its own right, a quantity of fire not only independent of, but greatly fuperior to, that which we receive from his rays.-Such however is the aftonifhing conclufion drawn by the Author of this memoir; without much apparent violence, and with as few gratuitous affumptions as are ufually employed in enquiries of this nature. In the year 1719, M. de Mairan first started this opinion; and now, after near fifty years further confideration of the subject, he finds fresh reasons to be fatisfied with the truth of it. The proofs which he adduces are founded on a great number of phyfical obfervations and mathematical calculations, which it would be difficult to prefent to our Readers within a moderate compafs, with any tolerab'e perfpicuity; but we fhall endeavour to gratify, in fome meafure, the curiofity which we may already have excited, by exhibiting a general sketch of his fyftem, fyftem, and of the principal mathematical and physical data, on which it is founded. According to this hypothefis, the fun, though it preferves life and motion throughout the universe, and by the obliquity of its apparent courfe between the two tropics, is the caufe of the viciffitude of seasons which we enjoy, produces these effects only by the fuperaddition of a very fmall quantity of heat, to that already refiding in the earth. In proof of this opinion, M. de Mairan, in the first place, calculates what would be the relative degrees of heat, produced by the fole action of the fun, on the days of the fummer and winter folftices, without the intervention of any other caufe. If the ratio between these two different degrees of heat, deduced from mathematical calculations founded on the greater or leffer obliquity of direction of the folar rays, and on the longer or fhorter continuance of that luminary above the horizon, at these two seasons, should turn out to be exactly, or even nearly proportional to the difference between the degrees of heat which we actually experience; the fun may then be juftly confidered as the only cause producing them but the heats actually obferved at these two feafons of the year can by no means, he affirms, be reconciled with fuch a fuppofition. In calculating the different power of the folar rays, in fummer and winter, in the production of heat, the Author takes into confideration every caufe, which can be supposed to have any poffible influence on the effect. He firft enquires in what manner the power of the fun's rays to heat any particular climate is to be measured; and confirms the opinion of Newton, that it is proportional to their quantity, or to the density of the folar light, which falls on any given space: and as it has been objected that the fun's rays, on being crowded into a less space, may, by their mutual action on each other, produce effects fuperior to thofe deduced from their mere number, he relates fome experiments made with a view to determine this question; in which the fun's light was reflected from 1, 2, and 3 mirrors, fucceffively, on the bulb of a thermometer; where it produced a dilatation of the included liquor exactly proportionable to the number of mirrors employed: from whence the Author experimentally infers, perhaps on too fmall a number of trials, that the heat produced by the folar rays is exactly proportionable to their number. He likewife demonftrates, with Halley, that the intensity of the folar heat is in proportion to the fines of the fun's altitude, and not to the fquares of these fines; as had been fuppofed by Fatio. He next confiders the different intenfity of the folar light, (or heat,) according as it is more or lefs affected by paffing through a greater or leffer portion of the earth's atmo fphere: and, after taking notice of fome other elements which may 4 may affect the conclufion, he at laft determines that the heat produced by the fole action of the fun's rays in fummer, of more particularly, on the day of the fummer folftice, in the latitude of Paris, is to that produced by the fame caufe, on the day of the winter folftice, as 16ths to 1, or nearly as 17 to 1. Let us now see what is the ratio of the real fummer and winter heats, as they are deduced from actual obfervation, by means of the thermometer. We cannot pretend to follow the Author, ftep by step, through the very minute and complicated difcuffions into which he enters, relative to the numerous thermometrical observations, which he has here collected. It will be fufficient to obferve that the greatest heat in fummer, taken on a medium of obfervations made at Paris, during 56 years, is to the greatest degree of cold in winter, only as 32 to 31; or, more accurate ly, in the ratio of 1026 to 994.-A refult very different from the foregoing. For the information of fuch of our Readers as may be unacquainted with the conftruction of Reaumur's thermometer, we fhall stop to obferve, that the two laft-mentioned numbers exprefs the spaces actually occupied by the liquor in that inftrument, when it is faid to ftand refpectively at 26 degrees above o, and at 6 degrees below o. When water begins to freeze," the liquor contained in this thermometer occupies 1000 spaces, or portions of the ball and part of the tube, and is then popu-" larly faid to ftand at 6. In the greatest heat of summer, indicated above, it is dilatedth parts of its whole bulk above this freezing point; and in the greateft cold, or, to fpeak more properly, the loweft heat, of winter, it is contracted 6 fuch parts below it. The Author accordingly affumes the numbers. 1026 and 994, as denoting the actual ratio of these two degrees of heat to each other *. This is perhaps the most vulnerable part of the Author's fystem. Reaumur's degrees, though they actually and accurately meafure the dilatation of the liquor included in the thermometer, are hypothetical with regard to the abfolute, and even to the relative quantity of heat, fuppofed to be denoted by them. We are ignorant of the primum frigidum, or of the actual quantity of fire which any body contains; and in what ratio the thermometer rifes or falls, on the addition or fubtraction of certain quantities of heat, in the different parts of its fcale. Nevertheless, though this uncertainty may affect the Author's numbers, there are perhaps data to be had from another quarter, fufficient to render probable this general propofition, that the earth poffeffes a degree of heat greatly fuperior to that which it receives immediately from the fun. The Author, however, does not prove that this very terreftrial heat may not be the accumulated produce of the action of the fun upon the earth, through a long series of ages. APP. Rev. vol. lxi. LI If If these calculations and obfervations be juftly founded, it will follow, on a comparison of them with each other, that, if the fun were the fole fountain of heat, that which we feel in winter ought to be 17 times less than the heat which we experience in fummer: whereas from actual obfervations we find that the degree of heat in winter, at Paris, is only a 32d part lefs than the fummer heat: and from hence, and from fome other confiderations which we shall not ftop to particularize, it likewife follows that, in fummer, the heat received from the fun alone, taken at a medium from the equator to the pole, is only the 29th part of the heat then actually exifting; and, in winter, is not much more than the 500th part. The conclufion to be drawn from hence, and which the Author accord ingly draws, is, that there exifts in the body of the earth itfelf, a permanent principle of heat, intirely independent of, and greatly fuperior to, that which it receives from the fun. having thus established its existence, and afcertained its quantity, he does not undertake to affign its caufe, or particular Stuation; but naturally enough confiders it as proceeding from the center of the earth, and exerting its energy ia emanations tending from that point towards the furface. After The Author elsewhere fhews, that, fuppofing this central fire to be annihilated, and that the fun were to illuminate even two thirds of the globe at once, and to communicate a heat equal to that produced by him under the equator; his fingle action would produce only a heat expreffed by 20 fuch degrees, of which, as we have already obferved, 1000 are required to keep water from freezing: fo that without this central fire, the earth, notwithstanding the action of the fun, would be only a bard, inanimate and barren block of ice. It may appear incredible that fo fmall a difference, as that which the Author affigns, between the actual degrees of heat and cold, in the fummer and winter feafons, fhould produce fo confiderable a difference in our fenfations. This difficulty, we conceive, will in fome measure be diminished, if we confider what great alterations are produced on the mind and its fenfations, by the fmalleft changes in the bodies which produce them. We perceive a different colour or a different tone, in confequence of the flighteft alteration in the motions or vibrations of the media of light and found. By the flightest change in the action of the nervous power, pleasure is turned into pain, and reason is converted into madness; and (to apply this analogy to the inanimate part of nature) the memorable experiments made at Petersburgh in 176c, on the artificial production of cold, afford probable grounds to fufpect that ice, in the fmallest degree of cold which will fuffer it to remain in that Bate, may contain 500, or 1000, or even a greater number of degress degrees of heat; and yet it will be converted into water, on the addition of 1 fuch degree.-Allow us, only for a moment, to confider this ice as poffeffed of fenfibility. What an enormous change muft it feel in its whole texture and conftitution, on this fmall addition! It finds every particle of its rigid, fixed, and coherent frame, melting into foftnefs and mobility: and all this by the addition of I degree of heat, to 500 or 1000 which it poffeffed before. A metal, on the point of fufion, affords a ftill ftronger illuftration. We are obliged to pals over many curious particulars contained in this memoir; and, among others, thofe by which the Author proves, from actual obfervations made in different parts of the globe, that the intensity of the greatest fummer heat is nearly equal, (allowing for particular and local circumftances) in every part of the earth, trom the line even to the polar circle: a circumftance for which he endeavours to account by means of these central emanations; with regard to which we fhall only obferve that M. de Mairan, with preceding thearifts, fuppofes the carth, in its primitive ftate, to have been a fluid or foft mafs; and that its prefent furface is only a cruft gradually formed by the action of the folar heat. This cortical part he fuppofes to have become thicker and harder in thofe parts of the earth where the fun's power was greateft. He af firms too that this exterior bak must oppose the transmission of the central emanations, in proportion to its thickness, &c. while thofe parts of the earth, which have been acted upon by the fun's heat in a lefs degree, continue more permeable to, and enjoy in a greater degree the benefit of, thefe central emanations. We fhall add one obfervation more, deduced from this memoir. By this hypothefis, the Author puts us out of pain for the well-being of the fuppofed inhabitants of Saturn and Mercury, who, according to the old fyftem, feem not to be placed in the moft agreeable fituation, with regard to heat and cold. Former theorifts, and particularly Newton, who have looked upon the fun as the fole fountain of heat, have juftly, on that fuppofition, calculated that, in the former of thefe planets (whofe diftance from the fun is ten times greater, and confequently the folar heat there 100 times less than that of the earth) our water would conftantly remain in a frozen ftate; and that in the latter, which is 2 or 3 times nearer to the fun than our earth, it would be inftantly converted into vapour. These circumftances are fufficient, according to our Author, to overturn the fyftem of a plurality of worlds, which in every other point feems to be fo well established.-For our parts, we have been accustomed to get over this difficulty, by refting on our ignorance of the temperaments or bodily conftitutions of these fame Saturnians LI 2 |