GYROSCOPIC COMPASS METHODS OF SURVEYING 209 



end a down pull acts. The force of gravity thus gives 

 rise to the precession motion of the gyroscope whereby 

 the east extremity of the axis moves toward the north. 

 When the axis comes into the plane of the meridian the 

 effect of gravitation on both ends of the axis is similar and 

 balances. The gyroscope remains in this position because 

 the meridians at the equator run parallel, and thus it is 

 independent of the earth's rotation. The meridian is 

 the position of equilibrium which the gyroscope tends to 

 attain in consequence of the earth's rotation. On the equa- 

 tor the turning axes of the earth and the gyroscope are 



parallel and their rotational senses are the same. In 

 general all rotating bodies tend so to place themselves 

 that in similar turning senses their turning axes are similarly 

 directed. If now we hang the rotating wheel not on the 

 equator but on any other chosen spot on the earth's surface 

 the gyroscope will still tend to set its axis in the same direc- 

 tion as that of the earth. This can not occur completely 

 owing to the line of gravitation being here no longer normal 

 to the earth's axis as at the equator, but being inclined 

 to it. The gyroscope will now, as far as is possible, set its 

 axis in the line of the earth's axis, and it attains its greatest 

 proximity to this when its axis lies in the meridian. If 

 at any place of latitude </> the axis of the gyroscope makes 



