SECT. 1] THE MAGNETIC FIELD OVER THE OCEANS 179 



have been devised for measuring the field, A description of a stabilized platform 

 used in an aircraft has been given by Serson, Mack and Whitham (1957). This 

 provides a vertical reference direction which is within a few minutes of arc of 

 the true vertical and an azimuth reference direction which is stable to a similar 

 accuracy for periods of up to an hour ; the azimuth of this reference is checked 

 at intervals of about 15 min by astronomical observations. This device was 

 made some years ago and probably does not represent the limit of what would 

 now be possible ; in particular, a gyro-stabilized sextant would greatly improve 

 the astronomical azimuth determinations. Such stabihzed platforms are easier 

 to use in an aircraft than in a ship, since the rolling and pitching of an aircraft 

 is usually much less than that of a ship. So far as the writers know, no magnetic 

 measurements have been made in a ship using a stabilized platform. This is 

 probably because the magnetic disturbances in a steel ship make accurate 

 measurements impossible and no ocean-going non-magnetic ship was in 

 service between the loss of the Carnegie and the recent commissioning of the 

 Zarya. An airborne instrument without a gyro-stabilized platform, which 

 measures both the strength and the direction of the field, has been described by 

 Schonstedt and Irons (1955). 



In deep water the magnetic field may have a gradient of over 100 y/km 

 (1 Y= 10"5 gauss) and on the continental shelf it may, exceptionally, reach ten 

 times this value. To obtain meaningful contours of features with such gradients 

 and a total amplitude of the order of 1000 y requires positions to be known to a 

 few hundred metres. Within the range of precise radio-navigational aids, such 

 as Decca, there is no difficulty, but in the open ocean the accuracy required is 

 beyond that which can ordinarily be attained by astronomical observations 

 interpolated by dead reckoning. The usual j)rocedure in surveying features far 

 from shore is to establish a system of buoys moored on light wires and carrying 

 radar reflectors, and to conduct the survey relative to these. A ship has the 

 great advantage over an aircraft that it is possible to observe the field and the 

 depth simultaneously, so that the magnetic survey is correctly related to 

 the topography, even if the whole survey is displaced and distorted by errors 

 in fixing the position of the ship. 



In a survey aimed at improving the general magnetic charts and not at the 

 study of geological detail, a lesser accuracy both of position and field is sufficient, 

 or at any rate must be tolerated. Here the main need for accurate positions 

 comes from the desire to determine the secular variation. In ten years the field 

 components change by an amount of the order of 500 y whilst the field gradient, 

 after smoothing out local anomalies, is of the order of 5 y/km. Thus the mean 

 of a number of repeat observations will give a useful determination of the 

 secular variation even if their positions are subject to errors of 20 km, as is 

 easily possible in aircraft flights in remote regions. The density of observations 

 needed is discussed by Serson and Hannaford (1957), 



In all recent investigations one of two methods has been used for the measure- 

 ment of the strength of the field. The most convenient for use from a ship is one 

 depending on proton magnetic resonance, though fluxgate methods, devised 



