THE SPERRY GYRO-COMPASS IN SERVICE. 289 



At the same time the phantom ring stands in practically constant relation to 

 the gyro wheel and the moving element of the compass proper. The bail or 

 pendulum R serves as a yielding connection between the gyro casing and the 

 phantom ring, providing a means of suppressing freedom about the horizontal 

 axis when the compass is off the meridian and the rotation of the earth has 

 caused that axis to incline out of the horizontal. The bail being attached 

 eccentrically to the vertical axis serves to partly suppress freedom about that 

 axis, thus damping the oscillations of the gyro to bring it to the meridian by 

 the shortest path. Further, since the phantom ring G stands in constant 

 relation to the casing B, but is none the less independent from it, this ring 

 forms the base from which movements may be made to produce a positive 

 orientation of the wheel. At any position on the earth's surface except at 

 the equator, the earth's rotation has a vertical component, its action being 

 such as to tend to leave the north end of the axis of the gyro on the east side 

 of the meridian. When the axis is left behind the meridian a portion of the 

 tangential component of the earth's rotation causes a tilt which introduces 

 the gravity couple of the pendulum about the horizontal axis. Simultane- 

 ously with this the couple about the vertical axis is also introduced, tending 

 to destroy the couple about the horizontal axis. For every latitude there 

 is some position of lag behind the meridian at which the couple about the 

 vertical axis just maintains the axis of rotation at a tilt such that the couple 

 about the horizontal axis can cause the necessary precession to follow the 

 vertical component of the earth's rotation. A northerly or southerly com- 

 ponent of the ship's speed acts to change the apparent direction of the earth's 

 rotation, so that the resting position of the axis of the gyro varies with change 

 in speed and course of the ship on which it is mounted. The variation of 

 lag behind the meridian for variation of latitude and variation in course 

 and speed of the vessel make it necessary to correct the axis of the gyro in 

 accordance with the equation — 



7-, a K cos H , , r 



D = ^ b tan L, 



cos L 



in which a and b are constants, K is the speed, H the course and L the latitude. 

 This correction is automatically solved and applied to the compass by means 

 of a mechanical device, shown in Fig. 2 , Plate 1 01 , which when set for approxi- 

 mate speed and latitude constantly corrects the lubber's point of the compass 

 card and the repeaters in such manner that the headings shown are always 

 referred to the true geographical north. 



Whenever the gyro is off the meridian its axis will incline by reason of 

 the earth's rotation at a rate proportional to its angular distance from the 



