SOME PHYSICOGEOGRAPHIC FACTS Our third-order regularity in the evolution of the Earth was formulated on a purely theoretical basis. Does the long-term development of the Earth's drainage system really obey this regularity? In order to answer this question, let us consider what different authors have said about the evolutionary history of certain rivers and water bodies during the last 4500 years or so. For the time being let us look only at this time interval, for the following reasons: first, this is a long enough time to verify the correctness of the corresponding generalizations; second, we have a more or less reliable history of climatic fluctuations for this period.
"A direct analysis of numerous physicogeographic phenomena has made it possible to establish the absolute chronology of the last three periods of changing wetness of the continents of the Northern Hemisphere ... It turns out that the period of greatest wetness of the territories and worsening of the climatic conditions (according to the 1850-year cycle) is followed, after a lag of about 100 years, by a phase of general advance ment of mountain glaciers, in the wake of their previous general large scale retreat" (Shnitnikov, 1953). Moreover, studies in Antarctica have shown that during the present period of warm climate the Antarctic glacial cover is retreating synchronously with the retreat of the great majority of glaciers in the world. Apparently, during other epochs as well, climatic warming and cooling took place simultaneously in the two hemispheres.
During the last 4500 years three periods of quite cool climate alternated with three periods of quite warm climate: t h e fir s t c o 01 period had a maximum at about the middle of the third millenium B.C. (end of Neolithic and beginning of Eneolithic), and corresponded to mountain glaciation in the Daunian stage; the second cool period had a maximum about the middle of the first millenium B.C. (antiquity), and corresponded to mountain glaciation in the Egesen stage; t he third c o o l p e r i o d had a maximum about the middle of the present millenium (the late Middle Ages), and corresponded to mountain glaciation in the Fernau stage; the first warm period had a maximum about the middle of the second millenium B.C. (Bronze Age), and corresponded to a subboreal
xerothermic period of optimum climate; the sec on d warm period had a maximum at the beginning of the second half of the first millenium A.D. (the early Middle Ages), and corresponded to a minor climatic optimum (as this period is called by certain authors); the third warm p e r i o d is now in progress, and the maximum has apparently not yet occurred.
It was pointed out above that the water mass of the Earth becomes redistributed (to some degree) relative to the polar and equatorial regions, as a result of climatic fluctuation. The vertical-horizontal ratio between dry land and water in the present geologic epoch is such that an increase in the water mass takes place during periods of climatic cooling, because of the solid water formed, predominantly in high-latitude continental areas; this leads to an increase in the diurnal velocity of the Earth. Climatic warming, on the other hand, will result in an increased mass of water in the low latitudes (since solid water is converted into liquid water), and the rotational velocity of the Earth will decrease.
Thus, to sum up, during the cold periods of the last 4500 years (maximum cooling took place about half way through the third and first millenia B. C. and the second millenium A.D.), the diurnal-rotation velocity of the Earth was relatively high. During the warm periods, accordingly (maximum warming took place at about the middle of the second millenium B. C. and at the beginning of the second half of the first millenium A.D.), this velocity was relatively low. The maximum warming of the present warm period has apparently not yet been attained. If the 1850-year period of climatic fluctuations established by Shnitnikov actually exists (that is, if the periodic time intervals are equal), then this maximum should occur around the middle of the third millenium A.D., closer to the beginning of the millenium. Consequently, until approximately this time, the general, spasmodic, progressive slowing down of the Earth's diurnal rotation should continue.