SECT. 1] THE MAGNETIC FIELD OVER THE OCEANS 177 



latitude 52° 24'S and returned along the coast of North America. He obtained 

 observations almost daily, of which van Bemmelen's collection gives only a 

 selection. On his return he published the first chart showing lines of equal 

 declination (isogonic lines) ; this was for 1700 and covered the Atlantic Ocean. 

 It was followed in 1702 by a chart covering the whole world, but with no lines 

 of equal declination over most of the Pacific (Chapman, 1941 ; Bullard, 1956). 



Observations of declination, and occasionally of inclination, continued 

 through the 1 8th century and led to a gradual improvement of the magnetic 

 charts. The first charts of the horizontal component of the field and of the total 

 force were those of Hansteen published in 1819. 



After the change from wooden to iron and steel ships, it became impracticable 

 to make accurate magnetic measurements at sea, except in specially con- 

 structed non-magnetic vessels. The most successful of these were the Galilee 

 (1905-1908) and the Carnegie (1909-1929), Their tracks are shown in Fig. 1 ; the 

 results from these cruises still constitute the main evidence for the behaviour 

 of the magnetic field over the oceans. Tracks of other ships making magnetic 

 surveys between 1839 and 1916 are given by Fleming (1937). After the destruc- 

 tion of the Carnegie by fire in 1929, no new ocean-going non-magnetic ship was 

 constructed and no systematic measurements were made at sea for many years. 

 Since 1956 this work has been taken up again by the Russian ship Zarya, which 

 has made a number of traverses of the Atlantic, Pacific and Indian Oceans. A 

 description of this ship and its work has been given by Ivanov (1960, 1961) and 

 Matveyev (1962) ; she is a schooner 37.5 m long and of 600 tons displacement. 



Extensive measurements of all three components of the field have also been 

 made in Canadian and United States aircraft (Vestine, 1960), and a large 

 programme, "Project Magnet", covering almost the whole of the oceans is at 

 present being carried out by the United States Navy. The intended tracks of 

 the project are shown in Fig. 2. 



The surveys from these special aircraft and from non-magnetic ships give 

 measurements of all three components of the field. Such measurements are 

 essential to a knowledge of the field as a whole, and are necessary if it is to be 

 divided into parts of internal and external origin (Chapman and Bartels, 1940; 

 Vestine et al., 1947). They do, however, require elaborate equipment and 

 sj)ecial ships or specially modified aircraft. Fortunately, much geological in- 

 formation can be obtained from the much simpler measurement of total force. 

 This can be made with an instrument towed behind a ship or aircraft at a 

 sufficient distance to avoid the magnetic disturbance due to the engines and 

 hull. 



2. Measurement of the Magnetic Field at Sea 



Three quantities are required to define the magnetic field at a point. At least 

 one of these must be the field strength or the strength of a component of the 

 field ; the other two can be field components, or angles giving the direction of 

 the field relative to axes fixed relative to the earth. There are thus four opera- 

 tions involved in the complete determination of the field at a point: (1) The 



7 — s. III. 



