been an unbroken contin

time.

Great depth of the sea u land connected with them.. conclude this chapter by period the climate of the globe than it is now, it would have a tend or lower temperature for a succession off That tendency would usually be in favour because these would be consistent with geography; but, if once abnormal condition prevailed, they would be persistent for unde ages. The slowness of climatal change would arise from the great depth of the sea the height of the land, and the consequent le quired to alter the position of continents and get basins.

replace any one ential to bear in al geography of ause of the pertion of heat or to the doctrine t even one of the both of them do nces against the rmal quantity of

pressions of 1,000 feet would submerge large areas of the existing land, but fifteen times as much movement would be required to convert such land into an ocean of average depth,

MAPS showing the position of LAND and SEA which might produce the Extremes of HEAT and COLD in the Climates of the GLOBE.

OBSERVATIONS.-These maps are intended to show that continents and islands having the same shape and relative dimensions as those now existing, might be placed so as to occupy either the equatorial or polar regions.

In fig. 14, scarcely any of the land extends from the Equator towards the poles beyond the 30th parallel of latitude; and in fig. 15, a very small proportion of it extends from the poles towards the Equator beyond the 40th parallel of latitude.

or to cause an ocean three miles deep to replace any one of the existing continents. It is quite essential to bear in mind this remarkable feature in the physical geography of the earth, when we are speculating on the cause of the permanence of a particular climate, or distribution of heat or cold during a series of epochs. According to the doctrine of chances, it would not often happen that even one of the polar regions would contain so much land as both of them do at present, but great indeed would be the chances against the simultaneous preponderance of such an abnormal quantity of land, both in arctic and antarctic latitudes.

The annexed maps will enable the reader to understand the manner in which land, having the same proportion to the sea as it now has, might be collected together in equatorial or polar regions. Such extremes may never have occurred, but we may safely conclude that there must sometimes have been an approximation to them in the course of those ages to which our geological records refer. A glance at these maps will make it evident that in the present state of the globe we are much nearer to the winter than to the summer of the 'Annus magnus,' or great cycle of terrestrial climate.

268

CHAPTER XIII.

VICISSITUDES IN CLIMATE HOW FAR INFLUENCED BY
ASTRONOMICAL CHANGES.

THE PRECESSION OF THE EQUINOXES, AND VARIATIONS IN THE EXCENTRICITY OF THE EARTH'S ORBIT CONSIDERED AS AFFECTING CLIMATE.-UNDER WHAT CONDITIONS EXTREME EXCENTRICITY MAY EXAGGERATE COLD.-MEASUREMENT OF HEAT.-TEMPERATURE OF SPACE. CLIMATES OF SUCCESSIVE PHASES OF PRECESSION.-VARIATION IN THE OBLIQUITY OF THE ECLIPTIC.-RADIATION OF HEAT IMPEDED BY A COVERING OF SNOW.-QUANTITY OF POLAR ICE AND ITS INFLUENCE IN ALTERING THE LEVEL OF THE OCEAN.-MIGRATIONS OF THE GREENLAND WHALE. —LIQUEFACTION AND EVAPORATION OF SNOW.—HOW FAR THE DATES OF FORMER GLACIAL PERIODS MAY BE FIXED BY COMPUTING THE ERAS OF MAXIMUM EXCENTRICITY.-DATES OF THE NEOLITHIC AND PALEOLITHIC ERAS. OF THE INTENSITY OF GLACIAL COLD.-DURATION OF THE GLACIAL PERIOD AS COMPARED TO SUCCESSIVE TERTIARY, SECONDARY, AND PRIMARY EPOCHS.-SUPPOSED VARIATIONS IN THE TEMPERATURE OF SPACE.--SOLAR MAGNETIC PERIODS AND VARIABLE SPLENDOUR OF THE STARS.—SUPPOSED GRADUAL DIMINUTION OF THE EARTH'S PRIMITIVE HEAT.-SUPPOSED CHANGE IN THE POSITION OF THE AXIS OF THE EARTH'S CRust.

The precession of the equinoxes and variations in the excentricity of the earth's orbit considered as affecting climate.-IN the last chapter we were chiefly occupied in considering how far changes in physical geography or in the position of land and sea may account for those variations of climate to which geology bears testimony. I endeavoured to show that this class of causes must always have exerted a dominant influence; and we may now consider how far those variations in the relative position of our planet to the other heavenly bodies which astronomy reveals to us, may have affected climate. In other words, to what extent may the precession. of the equinoxes, the revolution of the apsides, and the excentricity of the earth's orbit, have co-operated with geographical conditions in bringing about fluctuations of temperature in the habitable parts of the globe in former ages.

Sir John Herschel, in 1832,* entertained the question whether there are any astronomical causes which might offer a possible explanation of the difference between the actual temperature of the earth's surface and the climates which appear formerly to have prevailed. Geometers,' he observed, 'had demonstrated the absolute invariability of the earth's mean distance from the sun, whence it would seem to follow that the mean annual supply of light and heat would be alike invariable. This, however, is not exactly true: the total quantity of heat received in one revolution is inversely proportional to the minor axis;' still, as the extreme amount of difference in the quantity of heat annually received, owing to such change in the minor axis, can never by possibility exceed the whole supply in a ratio of more than 1,003 to 1,000, it may, he says, be neglected in our geological speculations.

But there is another way in which changes in the excentricity of the orbit affect climate. Climate depends not merely on the absolute amount of heat, but on the manner in which it is distributed through different parts of the year, especially in the polar and circumpolar zones of the earth.

At present the earth's orbit is becoming every year more circular, but only at a very slow and somewhat irregular rate, and it will become in 23,980 years after A.D. 1800 nearly as circular as it can ever be, or will approach a minimum excentricity, after which it will again increase at the same slow rate. These perturbations are caused by the attraction of the nearest and largest planets, Jupiter and Saturn playing the principal part, and the other planets, especially Venus and Mars, also exerting a sensible influence. It has long been known that the deviation of the orbit from a circle could never exceed certain limits, and these limits were very nearly defined by Lagrange towards the end of the last century, and more exactly by Leverrier in 1839. The extreme range of excentricity as expressed by the difference in distance of the earth from the sun in aphelion and peri-. helion amounts in round numbers to a little more than fourteen millions of miles, the minimum but slightly exceeding half a million. In other words, recent observation having

* Trans. Geol. Soc., 2nd series, vol. iii.