328 VON AKX [OHAP. 16 



the steric and dynamic departures of the ocean surface from horizontal and thus 

 to permit absolute measurements of the horizontal gradients of pressure. 



A. Celestial Navigation 



For celestial navigation an indication of local vertical to an accuracy of one 

 minute of arc, or better, makes it unnecessary to employ the horizon as a 

 fiducial mark. By the same token it is unnecessary to transform the celestial co- 

 ordinates of a body to the horizon system, or to make separate determinations 

 of latitude and longitude if the plane of the local meridian can be known con- 

 tinuously from shipboard (Fig. 2). Except for observations in very high latitudes, 

 this can be done both day and night with the help of a meridian gyrocompass. 



Fig. 2. Given gravity vertical and the local meridian by gyro, the astronomical triangle 

 can be solved from a single sight on a single celestial object at a known time. 



Recent experiments with a Sperry Mark 19 Mod 3 meridian gyrocompass 

 mounted on the Research Vessel Chain of the Woods Hole Oceanographic 

 Institution show that it is within the range of present technical achievement to 

 maintain a vertical reference on shipboard that can be relied upon to a small 

 fraction of a minute of arc and north reference to a small fraction of a degree 

 (Fig. 3). Ideally, to reduce the confusion of horizontal accelerations caused by 

 waves and steering, the meridian gyrocompass should be mounted near the 

 metacenter but preferably between the turning center and the intersection of 

 the pitch and roll axes of the ship as shown in Fig. 4. The point from which 

 celestial observations are made would, in consequence, have to be remote from 

 the master gyropendulum. In recent practice, it seemed simplest to accept the 



