Page:Popular Science Monthly Volume 40.djvu/832

810 the greatest distance from the earth it is possible for it to attain. The winter would be short and warm. The present difference in the length of the seasons is seven days, and the position of the perihelion is such that it is near its maximum for the present eccentricity. The directions in which the precession of the equinoxes and the variation of the obliquity of the ecliptic are tending are for reduction of the inequality, and ice ages are not to be expected from vicissitudes such as are now possible.

It is not denied that climates have been, and are, changing; but the changes are believed to be special, local, temporary, and oscillatory, and most largely determined by causes that may be found on the surface of the earth. M. Arago thought they might all be attributed to agricultural works, to the clearing of woods from plains and mountains, to the drying up of marshes; and he doubted if it could be proved that the climate had become warmer or colder in any place the physical aspect of which had not been perceptibly changed during a series of ages.

The present drift of the opinion of many careful students of the subject seems to be that exaggerated ideas have been held of the extent of climatic variations, both in the present and the past. M. Woeikoif, whose opportunities for studying climatological phenomena over a large extent of territory have not been surpassed, believes that this is so, even when the application is made to the Glacial period; that not intense cold, but those conditions of temperature and moisture most conducive to the precipitation and accumulation of snow, formed the chief factors of its characteristic phenomena. Chief among these were proximity of the sea and a temperature of the surface-water rather below than above the freezing-point. The effect on glacial accumulation of the conditions commonly supposed to correspond with the combination of high eccentricity and an aphelion winter would, in his opinion, be the opposite to what is attributed to it; for the greater cold assumed to prevail in winter would not be conducive to the precipitation of snow, while the more intense heat of midsummer would probably melt the snow at heights where the present temperature rises but little above the melting-point. Hence the conditions in the interior and eastern part of a continent like Asia would be less favorable than they are now to marked glaciation. The western parts of continents and islands would be more fully under the influence of the sea; and as there is no reason to suppose that its surface temperature would be lower than now, it follows that there would not, all other things being equal, be more snow than now in countries where rain is the rule, even in winter. The effect of the combination would be in any case but slight, and not by far, in M. Woeikoff's opinion, to be compared