SECT. 3] 



APPLICATIONS OF THE GYROPENDTJLUM 



327 



However promising this suggestion seems, a physical system has yet to be 

 developed for practical use. 



One that has been developed employs a different principle ; that of slaving 

 to a north-seeking gyrocompass rotor a second horizontal rotor having its 

 spin axis oriented east-west. When suitably damped by torques controlled by 

 signals from electrolytic levels or linear horizontal accelerometers this com- 

 bination of rotors will precess to seek both gravity vertical and the geographic 

 north point and thus to define the local meridian plane. Meridian gyro systems 

 tend to drift in azimuth and to a far lesser extent depart from gravity vertical 

 when exposed to sustained horizontal motions or accelerations of the supporting 

 vehicle. Still, by these means the direction of gravity can be sensed at sea 

 with an accuracy of about 10 -4 rad. 



GRAVITATIONAL TORQUE 

 OF GREAT CIRCLE MOTION 



GRAVITATIONAL TORQUE 

 OF VERTICAL MISALIGN- 

 MENT ON A 



SPIN VECTOR 

 GYRO A 



GRAVITATIONAL TORQUE 

 OF VERTICAL MISALIGN- 

 MENT ON B 



SPIN VECTOR 

 GYRO B 



COMMON AXIS 

 OF PRECESSIONAL 

 CONES 



GREAT CIRCLE PATH 



Fig. 1. Principle of the Schuler double pendulum. 



Having a means by which the direction of gravity can be determined both 

 closely and reliably from surface ships a number of oceanographic applications 

 come within range of useful consideration : 



A. Recently tried at sea is the possibility that celestial navigation can be 

 practiced at any time that clear skies exist. Given the local vertical and north 

 point to define the local meridian plane, Greenwich Mean Time and the celestial 

 co-ordinates of a body, it is possible to fix the momentary latitude and longitude 

 of a moving ship from a single observation, and to track its change of position 

 with time by taking serial observations on a single celestial body. 



B. The same navigational apparatus also suggests uses for a gravity vertical 

 as part of a system for direct measurements of the shape of the earth at sea. 



C. It is conceivable that gyro verticals may one day be used to determine 



