“La révolution qui pourrait y causer les plus grands changements thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir alternativement et, par suite, la planète à passer, aux époques de périhélie, plus ou moins près du soleil, embrasse une période de plus de cent mille années terrestres et demeure comprise dans de si étroites limites que les habitants doivent être à peine avertis que la chaleur décroît, par cette raison, depuis une haute antiquité et décroîtra encore pendant des siècles en variant en même temps dans sa répartition selon les diverses époques de l’année.... Enfin, le tournoiement de l’axe du globe s’empreint également d’une manière particulière sur l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux hémisphères, deviennent graduellement, durant une période d’environ vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal que règne l’uniformité, et quoique les étés et les hivers y tendent, dès à présent, à se trancher de plus en plus, il ne paraît pas douteux que la modération des saisons n’y produise, pendant longtemps encore, des effets appréciables. En résumé, de tous ces changements il n’en est donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des valeurs considérables; ils se règlent tous sur un mode de développement presque insensible, et il s’ensuit que les années de la terre, malgré leur complexité virtuelle, se distinguent par le constance de leurs caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes principes, dans les autres systèmes planétaires de l’univers, mais même de ce qui s’observe dans plusieurs des mondes qui composent le nôtre.”—Philosophie Religieuse: Terre et Ciel.
M. ADHÉMAR.
Adhémar does not consider the effects which ought to result from a change in the eccentricity of the earth’s orbit; he only concerns himself with those which, in his opinion, arise from the present amount of such eccentricity. He admits, of course, that both hemispheres receive from the sun equal quantities of heat per annum; but, as the southern hemisphere has a winter longer by 168 hours than the corresponding season in the northern hemisphere, an accumulation of heat necessarily takes place in the latter, and an accumulation of cold in the former. Adhémar also measures the loss of heat sustained by the southern hemisphere in a year by the number of hours by which the southern exceeds the northern winter. “The south pole,” he says, “loses in one year more heat than it receives, because the total duration of its nights surpasses that of the days by 168 hours; and the contrary takes place for the north pole. If, for example, we take for unity the mean quantity of heat which the sun sends off in one hour, the heat accumulated at the end of the year at the north pole will be expressed by 168, while the heat lost by the south pole will be equal to 168 times what the radiation lessens it by in one hour; so that at the end of the year the difference in the heat of the two hemispheres will be represented by 336 times what the earth receives from the sun or loses in an hour by radiation,”[322] and at the end of 100 years the difference will be 33,600 times, and at the end of 1,000 years 336,000 times, or equal to what the earth receives from the sun in 38½ years, and so on during the 10,000 years that the southern winter exceeds in length the northern. This, in his opinion, is all that is required to melt the ice off the arctic regions, and cover the antarctic regions with an enormous ice-cap. He further supposes that in about 10,000 years, when our northern winter will occur in aphelion and the southern in perihelion, the climatic conditions of the two hemispheres will be reversed; that is to say, the ice will melt at the south pole, and the northern hemisphere will become enveloped in one continuous mass of ice, leagues in thickness, extending down to temperate regions.
This theory, as shown in [Chapter V.], is based upon a misconception regarding the laws of radiant heat. The loss of heat sustained by the southern hemisphere from radiation, resulting from the greater length of the southern winter, is vastly over-estimated by M. Adhémar, and could not possibly produce the effects which he supposes. But I need not enter into this subject here, as the reader will find the whole question discussed at length in the chapter above referred to. By far the most important part of Adhemar’s theory, however, is his conception of the submergence of the land by means of a polar ice-cap. He appears to have been the first to put forth the idea that a mass of ice placed on the globe, say, for example, at the south pole, will shift the earth’s centre of gravity a little to the south of its former position, and thus, as a physical consequence, cause the sea to sink at the north pole and to rise at the south. According to Adhémar, as the one hemisphere cools and the other grows warmer, the ice at the pole of the former will increase in thickness and that at the pole of the latter diminish.
The sea, as a consequence, will sink on the warm hemisphere where the ice is decreasing and rise on the cold hemisphere where the ice is increasing. And, again, in 10,000 years, when the climatic conditions of the two hemispheres are reversed, the sea will sink on the hemisphere where it formerly rose, and rise on the hemisphere where it formerly sank, and so on in like manner through indefinite ages.
Adhémar, however, acknowledges to have derived the grand conception of a submergence of the land from the shifting of the earth’s centre of gravity from the following wild speculation of one Bertrand, of Hamburgh:—
“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a pour titre: Renouvellement périodique des Continents, avait déjà émis cette idée, que la masse des eaux pouvait être alternativement entraînée d’un hémisphère à l’autre par le déplacement du centre de gravité du globe. Or, pour expliquer ce déplacement, il supposait que la terre était creuse et qu’il y avait dans son intérieur un gros noyau d’aimant auquel les comètes par leur attraction communiquaient un mouvement de va-et-vient analogue à celui du pendule.”—Révolutions de la Mer, p. 41.
The somewhat extravagant notions which Adhémar has advanced in connection with his theory of submergence have very much retarded its acceptance. Amongst other remarkable views he supposes the polar ice-cap to rest on the bottom of the ocean, and to rise out of the water to the enormous height of twenty leagues. Again, he holds that on the winter approaching perihelion and the hemisphere becoming warm the ice waxes soft and rotten from the accumulated heat, and the sea now beginning to eat into the base of the cap, this is so undermined as, at last, to be left standing upon a kind of gigantic pedestal. This disintegrating process goes on till the fatal moment at length arrives, when the whole mass tumbles down into the sea in huge fragments which become floating icebergs. The attraction of the opposite ice-cap, which has by this time nearly reached its maximum thickness, becomes now predominant. The earth’s centre of gravity suddenly crosses the plain of the equator, dragging the ocean along with it, and carrying death and destruction to everything on the surface of the globe. And these catastrophes, he asserts, occur alternately on the two hemispheres every 10,000 years.—Révolutions de la Mer, pp. 316−328.
Adhémar’s theory has been advocated by M. Le Hon, of Brussels, in a work entitled Périodicité des Grands Déluges. Bruxelles et Leipzig, 1858.