MAGNETIC VAKIATION. 127 



Although, judging from the present very northern position 

 of the magnetic equator, it is probable that the town of Louvo 

 in Siam was very near the extremity of the northern mag- 

 netic hemisphere, when the missionary father, Guy Tachard, 

 first observed the horary alterations of the magnetic varia- 

 tion at that place in the year 1682, it must be remem- 

 bered, that accurate observations of the horary declina- 

 tion in the southern magnetic hemisphere were not made 

 for fully a century later. John Macdonald watched the 

 course of the needle during the years 1794 and 1795 in Fort 

 Marlborough, on the south-western coast of Sumatra, as well 

 as at St. Helena. 62 The results which were then obtained 

 drew the attention of physicists to the great decrease in the 

 quantity of the daily alterations of variation in the lower 

 latitudes. The elongation scarcely amounted to 3 or 4 

 minutes. A more comprehensive and a deeper insight into 

 this phenomenon was obtained through the scientific expedi- 

 tions of Freycinet and Duperrey, but the erection of mag- 

 netic stations at three important points of the southern 

 magnetic hemisphere, at Hobarton in Van Diemen's Land, 

 at St. Helena, and at the Cape of Good Hope (where for the 

 last 10 years horary observations have been carried on for the 

 registration of the alterations of the three elements of ter- 

 restrial magnetism in accordance with one uniform method), 

 afforded us the first general and systematic results. In 

 the middle latitudes of the southern magnetic hemisphere 



62 Phil Transact, for 1795, pp. 340349, for 1798, p. 397. The result 

 which Macdonald himself draws from his observations at Fort Marl- 

 borough (situated above the town of Bencoolen, in Sumatra, 3 47' S. 

 lat.), and according to which the eastern elongation was on the increase 

 from 7 A.M. to 5 P.M., does not appear to me to be entirely justified. 

 No regular observation was made between noon and 3, 4, or 5 P.M., and 

 it seems probable, from some scattered observations made at different 

 times from the normal hours, that the turning hours between the 

 eastern and western elongation fall as early as 2 P.M., precisely the 

 same as at Hobarton. We are in possession of declination-observations 

 made by Macdonald during 23 months (from June, 1794, to June, 

 1796), and from these I perceive that the eastern variation increases at 

 all times of the year between 7h. 30m. A.M. till noon, the needle moving 

 steadily from west to east during that period. There is here no trace 

 of the type of the northern hemisphere (Toronto), which was observ- 

 able at Singapore, from May till September; and yet Fort Marlborough 

 lies in almost the same meridian, although to the south of the geogra- 

 phical equator, and only 5 4' distant from Singapore. 



