PRESIDENTIAL ADDRESS. SECTION A. 1/ 



ous magnetic elements suffer change from day to day and from 

 year to year. 



The first of these changes discovered was the secular change 

 in declination. Gellibrand, a professor of mathematics at Gres- 

 ham College, pointed out that the declination at London did 

 not always have the same value; to uphold his view he published 

 the results of observations in 1580, when Borough and Norman 

 had found the value 11° 15' E.; in 1622, when Gunter deter- 

 mined it as 5° 56V E., and his own determination in 1634, when 

 the value was 4°"'6' E. The secular change in the dip was well 

 known to Wilke, who published, in 1768, the first dip chart. 

 Wilke pointed out that within 150 years the dip in London had 

 changed by more than 3°. 



A periodic, as distinguished from a secular change, was 

 discovered in 1722, when Graham, a London clockmaker, after 

 making a great many observations at different times of the day, 

 .announced that the declination had a daily change. His result 

 was confirmed by Celsius, in Upsala. in 1740. Since then 

 exacter observations have shown that the dip and the field 

 strength have also a daily variation. 



In addition to the various changes mentioned above, the 

 magnetism of the earth undergoes changes, sometimes local, 

 sometimes extending over the whole earth. These magnetic 

 storms, as they are called, were first brought into notice by 

 Baron Humboldt, who attempted, in i8c6. to make con- 

 tinuous observations of the declination in Berlin. It was not. 

 however, till 1828 that he was in a position to carry on this 

 work to his satisfaction; in that year he had three observatories 

 established, in Paris. Berlin, and Freyburg respectively. He 

 was able to show by comparing the records of these three 

 stations that a magnetic storm would sometimes be felt simul- 

 taneously at all the three stations; at other times, however, 

 one station might indicate a storm, while the two others were 

 undisturbed. 



This general description of the evolution of magnetism in 

 connection with the earth may be fittingly brought to a close 

 with a reference to the German mathematician and natural 

 philosopher. Gauss, who may be looked on as the father of 

 modern earth magnetic studies. Gauss put forward in his 

 " Allgemeine Theorie des Erdmagnetismus " (1839) what is 

 to-day known as the Gaussian theory of earth magnetism. 

 Previous to this date he had laid down a method of determining 

 the intensity of the earth's field in absolute measure, and had. 

 along with Weber, devoted much time and labour to improve 

 the methods of observation, and to collect the results which 

 were buried in different libraries and archives throughout 

 Europe. * 



I shall now proceed to give a summary of the work that has 

 been done in South Africa — meaning by that, roughly, Africa 

 south of the equator. Evidently, for such a great area, obser- 

 vations must be taken at many places, and as they cannot be 

 taken simultaneously the first essential is to have some accurate 



* Gauss : " Gesammelte Werke." Band V. 



