MIRACLES OF SCIENCE 



absolute space shifts moment by moment. Six hours 

 from now it will be at right angles to its present po- 

 sition, as it will have gone a quarter way round our 

 globular earth in the meantime. But suppose, now, 

 that the pencil represents the axis of a heavy gyro- 

 scope that is free to oscillate but is held in horizontal 

 plane by force of gravity. Recall, then, what happens 

 when you attempt to tip the axis of a gyroscope as the 

 movement of the earth will tip this one. The axis 

 does not move in the direction in which you attempt 

 to tip it, but at right angles to that direction. And 

 that means, in this case, that the axis must swing 

 about in the direction of the north and south line, 

 just as if you turn the pencil about on the table with- 

 out lifting it. 



Suppose you turn your pencil until it lies straight 

 north and south on the table, and then as before con- 

 sider its relation to the rotating earth. A moment's 

 reflection will show that the pencil now occupies the 

 one only position in which it will remain parallel to 

 its present position hour after hour. Six hours from 

 now, or twelve or twenty-four hours from now, it is 

 still aimed in the same direction. And of course the 

 same thing holds true for the axis of our gyroscope. 

 When it finally reaches the north and south position, 

 it ceases to feel the disturbing influence of the earth's 

 motion. Now it is in stable equilibrium ; it will hold 

 its position rigidly unless some new force disturbs 

 it. And if forcibly deflected, it will instantly come 

 again under the torsional (precessional) strain of the 

 earth's motion, and hence will be promptly swung 

 back into meridianal position. 



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