TERRESTRIAL MAGNETISM. 131 



them that the south end of the needle by which they 

 steered did not point precisely due south. We have even 

 the statement belonging to the 12th century, from one of 

 their determinations, of the amount of the " variation to the 

 south-east." ( 16 ) The extension of the use of the mariners' 

 compass was much favoured by the very ancient connection 

 of China and India with Java,( 161 ) and in a still greater 

 degree by the visits of people of Malay race to Madagascar, 

 and their settlements there. 



Although, judging from the present very northerly position 

 of the magnetic equator, it is probable that when the mis- 

 sionary Guy Tachard remarked the existence of horary 

 variations of the declination in 1682 at the town of Louvo 

 in Siam, that place was nearly out of the northern magnetic 

 hemisphere, yet it must be acknowledged that accurate 

 horary declination- observations were not made in the south- 

 ern magnetic hemisphere until a full century later. In 1794- 

 and 1795, John Macdonald followed the inarch of the 

 needle both at Tort Marlborough on the south-west coast 

 of Sumatra, and at St. Helena. ( 162 ) The results then 

 obtained called the attention of physicists to the great 

 decrease in the amount of the diurnal variation of the decli- 

 nation in the lower latitudes ; the differences between the 

 extremes at these stations amounting only to 3 or 4 minutes. 

 A more comprehensive and deeper knowledge of the phe- 

 nomenon was gained by the scientific expeditions of Preycinet 

 and Duperrey; but it is the establishment of magnetic ob- 

 servatories at three important points of the southern magnetic 

 hemisphere, at Hobarton in Van Diemen Island, at St. 

 Helena, and at the Cape of Good Hope (at which places 

 hourly observations on the variation of the three elements 



K 2 



