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14 COSMICAL ASPECTS OF GEOLOGY. [Book 

from the equator passes into regions having a less velocity due to 
rotation than it possesses itself, and hence it travels on in advance _ 
and appears to be gradually deflected eastward. The aerial currents _ 
blowing steadily across the surface of the ocean produce oceanic — 
currents which have a westward tendency indirectly communicated 
to them from the effects of rotation. 
It has been maintained by Von Baer,’ and the statement has been 
accepted as a general law by some writers, that a certain deflection is 
experienced by rivers that flow in a meridional direction, like the 
Volga. Those travelling polewards are asserted to press upon their 
eastern rather than their western banks, while those which run in 
the opposite direction are stated to be thrown more against the 
western than the eastern. When, however, we consider the com- 
paratively small volume, slow motion, and continually meandering 
course of rivers, it may reasonably be doubted whether any effects of 
this vera causa have yet been observed.? 
§ 2. Revolution.—Besides turning on its axis, the globe performs 
a movement round the sun, termed revolution. ‘This movement, 
accomplished in rather more than 365 days, determines for us the 
length of our year, which is, in fact, merely the time required for one 
complete revolution. The path or orbit followed by the earth round 
the sun is nota perfect circle but an ellipse, with the sun in one 
of the foci, the mean distance of the earth from the sun being 
92,400,000 miles. By slow secular variations the form of the orbit 
alternately approaches to and recedes from that of a circle. At the 
nearest possible approach between the two bodies, owing to change 
in the ellipticity of the orbit, the earth is 14,868,200 miles nearer 
the sun than when at its greatest possible distance. These maxima 
and minima of distance occur at vast intervals of time. The last 
considerable eccentricity took place about 200,000 years ago, and 
the previous one more than half a million years earlier. Since 
the amount of heat received by the earth from the sun is inversely 
as the square of the distance, eccentricity may have had in past 
time much effect upon the climate of the earth, as will be pointed 
out further on (§ 8). 
»  § 3. Precession of the Equinoxes.—lIf the axis of the earth 
were perpendicular to the plane of its orbit, there would be equal day 
and night all the year round. But it is really inclined to that plane 
at an angle of 234°. Hence our hemisphere is alternately presented 
to and turned away from the sun, and in this way brings the familiar 
alternation of the seasons. Again, were the earth a perfect sphere of 
uniform density throughout, the position of its axis of rotation would — 
not be changed by attractions of external bodies. But owing to the 
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1 “Ueber ein allgemeines Gesetz in der Gestaltung der Flussbetten.” Bull. Acad. 
St. Petersburg, i. (1860). See also Ferrel on the motions of fluids and solids relatively 
to the eartl’s surface, Camb. (Mass.) Math. Monthly, vols. i. and ii. (1859-60). Dulk. 
Z. Deutsch. Geol. Ges. xxxi. (1879) p. 224. 
* Sce K. Dunker, Zeitsch. fiir die gesammten Naturwissenchaften, 1875, p. 463. 

