178 



BULLARD AND MASON 



[chap. 10 



measurement of the field strength; (2) the estabHshment of the direction of the 

 magnetic field; (3) the establishment of the true vertical; and (4) the estab- 

 lishment of the direction of the true north. 



Until about 1945 all magnetic measurements at sea were made with adapta- 

 tions of the instruments used on land. The instruments were mounted on 

 gimbals whose j)eriod was short compared to that of the ship's motion ; they 

 must, therefore, have maintained a fixed attitude relative to the apparent 

 vertical. It is to be supposed that the effect of the difference between the true 

 and apparent verticals was eliminated or reduced by averaging by eye while 



100* 120* ido* ISO* lac 160' i«o* i20* 100* bo* so* 



40*j^ 



60* 80* 



80- 



iOO* IJC laO* 160* leO' 160* 140" IJO* 100* 80* 60* 40* 20' 



20* 40* 60* 80* 



Fig. 2. Tracks of Project Magnet. (After Vestine, 1960, fig. 4.) 



taking a reading. The published accounts are curiously reticent about this 

 important point ; even the period of the Carnegie's gimbals is not stated. 

 Azimuth was then, as now, determined by astronomical observation ; this in- 

 volves not only the observation of the direction of the sun or of a star, but also 

 the determination of the vertical. It is not necessary to describe here the 

 compasses, dip circles, earth inductors, deflectors and other instruments used, 

 since they are now only of historical interest. A very detailed description of 

 their final forms has been given by Bauer et al. (1917). 



In recent years two basic improvements have been made. First, gyro- 

 stabilized platforms have become available and, secondly, electronic techniques 



