10. THE MAGNETIC FIELD OVER THE OCEANS 



E, C. BuLLARD and R. G. Mason 



1. Historical 



It is believed that the compass was first discovered in China (Needhani, 

 1962). The earhest certain references to it are in the Wu Ching Tsung Yao, 

 edited by Tseng Kung-Liang and finished in 1044 and in the Meng CJihi Pi Than 

 of Shen Kua written about 1088. Needham considers it hkely that it had been 

 used for fortune telling for some hundreds of years before this. The first men- 

 tion of its use in a ship is by Chu Yii in a work written between 1111 and 1117. 



No connection is known between these Chinese discoveries and the first use 

 of the compass in Em-ope. The evidence has been discussed at length by 

 Crichton Mitchell (1932) who concludes that the first certain reference to it in 

 European literature is in two books written about 1187 by Alexander Neckam, 

 a monk of St. Albans, who treats the compass as a device well known in his 

 time. In the 13th and 14th centuries there are many references, particularly by 

 poets, to whom the idea of a needle that points the way in storm and darkness 

 has always appealed. For example, in 1375 the Scottish poet Barbour (Skeat, 

 1870) wrote of the journey of Robert the Bruce from Arran to Carrick in 1306: 



"Thai rowit fast" with all thar micht 

 Till that apon thame fell the nycht 

 That It wox myrk on gret maner 

 Swa that thai wist nocht quhar thai wer 

 For thai na nedil had na stane." 



In China in 1088 Shen Kua knew that the compass needle does not point to 

 the true north (or, as he would have said, to the true south). In Europe this 

 appears to have been unknown till late in the 15th century (Needham, 1962; 

 Crichton MitcheU, 1937). 



The first systematic observations at sea were made during the voyage of 

 Joao de Castro in 1538 to 1541 (Hellmann, 1897). He sailed around the Cape, 

 up the west coast of India and into the Red Sea ; during this voyage he made 

 43 determinations of declination. These and other early observations have been 

 collected by van Bemmelen (1899) and by Gaibar-Puertas (1953). From these 

 van Bemmelen has compiled maps showing the declination at 50-year intervals 

 from 1550 to 1700. As the observations at sea were made in wooden ships, they 

 were usually not seriously disturbed by neighbouring masses of iron, and it is 

 believed that the maps do give a good general representation of the declination 

 over a large part of the earth. 



It is not possible to give any account here of the voyages and magnetic 

 observations of the 16th- and 17th-century navigators. The voyages of Foxe 

 and James to Hudson's Bay at the beginning of the 17th century and of 

 Edmond Halley in the Atlantic in 1698 to 1700 are of particular significance. 

 The results of the former show that the north magnetic jDole was then, as it is 

 today, to the north-west of Hudson's Bay. Halley went from England to 

 [MS received October, 1960] 175 



