Seor. Il.] MOVEMENTS OF THE EARTH. i3 
the conception, first outlined by Kant, that not only in our own 
solar system, but throughout the regions of space, there has been a 
common plan of evolution, and that the matter diffused through 
space in nebule, stars, and planets is substantially the same as that 
with which we are familiar. Hence the study of the structure and 
probable history of the sun and the other heavenly bodies comes to 
possess an evident geological interest, seeing that it may yet enable 
us to carry back the story of our planet far beyond the domain of 
ordinary geological evidence, and upon data not less trustworthy than 
those furnished by the rocks of the earth’s crust. 
Ill. T'sz Movements oF THE EARTH IN THEIR GEOLOGICAL | 
RELATIONS. 
Weare here concerned only with those aspects of the earth’s motions 
which materially influence the progress of geological phenomena. 
§ 1. Rotation.—In consequence of its angular momentum at its 
original separation, the earth rotates on its axis. The rate of rotation 
has once been much more rapid than it now is (p. 20). At present 
a complete rotation is performed in about twenty-four hours, and to 
it is due the succession of day and night. So far as observation has 
yet gone, this movement is uniform, though recent calculations of 
the influence of the tides in retarding rotation tend to show that a 
very slow diminution of the angular velocity is in progress. If this 
be so, the length of the day and night will slowly increase until 
finally the duration of the day and that of the year will be equal. 
The earth will then have reached the condition into which the moon 
has passed relatively to the earth, one half being in continual day, 
the other in perpetual night. 
_ The linear velocity due to rotation varies in different places, 
according to their position on the surface of the planet. At 
each pole there can be no velocity, but from these two points 
‘towards the equator there is a continually increasing rapidity of 
motion, till at the equator it is equal to a rate of 507 yards in a 
second. 
Lo the rotation of the earth are due certain remarkable influences 
upon currents of air circulating either towards the equator or towards 
the poles. Currents which move from polar latitudes travel from 
parts of the earth’s surface where the velocity due to rotation is small 
to others where it is great. Hence they lag behind, and their course 
is bent more and more westward. An air current quitting the north 
polar or north temperate regions as a north wind is deflected out of 
its course and becomes a north-east wind. On the opposite side of 
the equator a similar current setting out straight for the equator is 
changed into a south-east wind. This is the reason why the well- 
known ‘Trade-winds have their characteristic westward deflection. 
On the other hand, a current setting out northwards or southwards 
