i8 



PRESIDENTIAL ADDRESS. SECTION A. 



method of reducing all observations to a common epoch; that 

 is to say, we must have the necessary knowledge of the secular 

 and the periodic variations. I shall begin, then, by stating 

 what means we in South Africa have for that. These periodic 

 variations can only be studied properly at a permanent station, 

 where continuous records of the various elements can be made. 

 The only permanent station at the present day in our part of 

 the world is the Royal Albert Observatory at Mauritius (1895). 

 There were permanent stations at St. Helena (1840- 1845) ai^<i 

 at the Royal Observatory of the Cape of Good Hope (1841-53). 

 Neumayer also speaks, in the introduction to Berghaus's 

 " Atlas des Erdmagnetismus," of such a station in Portuguese 

 West Africa at St. Paul de Loanda; it also has disappeared. 

 Continuous records were also taken for a few years at Dar-es- 

 Salaam. At the present day the most crying want for the 

 study of the magnetic state of this part of the world is the 

 establishment of such a station. A history of the Cape station 

 and the attempts to revive it will be found in the report of the 

 Kimberley meeting.* For the study of the secular changes a 

 permanent station is also necessary; it is now becoming more 

 and more evident, with the accumulation of fresh data, that 

 the secular changes are much more complicated than was pre- 

 viously supposed. If, for example, we are told that the de- 

 clination at Cape Town was 30° 2' W. in 1866, and 29° 2' W. 

 in 1897, it would not be correct to say that the secular change 

 in that period was a decrease in declination of i'.93 (approxi- 

 mately) per year; still less it is possible to state what the change 

 was before 1866 or after 1897. At the present day all we can 

 do is to observe as carefully as we can the mean values for 

 two successive years, and obtain from them the change in that 

 period. The following table gives the values of the changes 

 per year at the Royal Observatory, Cape Town: — 



Table No. i. 



Year. Declination. 



Yearly 

 Change. 



Dip. 



Yearly 

 Charge. 



In terms ot 

 _J 

 Hori 

 zontal 

 Interns. 



Yearly 

 Charge. 



* J. C. Beattie : " Magnetic Observations in South Africa." Report S.A. 

 Ass. for Adv. of Sc, Johannesburg (1905) and Kimberley (1906), p. 170. 



