Page:Popular Science Monthly Volume 40.djvu/828

806 and the quantity of heat received from the sun by the earth in different parts of its orbit is supposed to be correspondingly modified. These differences are greatest when the eccentricity is greatest. If with this is combined such a position of the equinoxes that summer in one hemisphere shall correspond with the perihelion and winter with the aphelion, the contrast of the seasons in that hemisphere will be most marked, and we shall have the conditions, according to one theory, for a glacial period.

Such, according to M. Jean Reynaud, was the case in the northern hemisphere about 9500 when, he thinks, our last glacial period was at its height. From that time the differences were gradually reduced till about 1250 when they became least, and the northern seasons were mildest and most equable. The differences then began to enlarge again, and we are now advanced a little more than six hundred years toward another glacial period. According to this theory, the seasons were growing milder all through human history till 1250 and have been tending to become more severe since then.

A question of fact is here presented, evidence respecting which is sought, in the absence of exact observations, in such records as may happen to exist of the character of seasons in the past. M. Arago several years ago collected a considerable list of mentions in the literature and documents of former times of periods of unusual cold, of long or cold winters, unusually hard freezing of rivers, and remarkable heat, drought, or rain, which constitutes our principal source of information on the subject. Parts of this list have been used by M. Jules Peroche and M. Amadée Guillemin to establish opposite conclusions as to the validity of M. Reynaud's hypothesis.

Latin poets furnish some of these data, as when Ovid complains of the inclemency of his place of exile on the Black Sea, in what is now pleasant southern Russia; or Horace and his compeers describe terrible storms in Rome; or Juvenal tells of a Roman lady having to break the ice of the Tiber to wash her face. Cicero and some of the historians speak of the severe climates of Gaul and other outlying provinces, evidently contrasting them with the pleasures of life in Italy. The discomforts experienced by Hannibal in crossing the Alps were what an army from the south would suffer in any age in crossing those mountains in winter, if they were roadless and inhabited by barbarians. To a candid critic, these representations mean nothing on one side or the other, and such is the conclusion which M. Angot has reached after carefully examining the subject.

Of fifty-six instances of extreme winter severity cited by M. Peroche from M. Arago's list, fourteen occurred before the supposed "Great Summer" year, 1250. There seem to be more of