PRESIDENTIAL ADDRESS. SECTION A. 



19 



A glance at the above shows how the changes in the different 

 elements vary from year to year, and how necessary it is to 

 proceed with caution when using even an interpolation formula. 

 A further complication becomes evident when, instead of 

 confining" ourselves to one place, we examine the secular and 

 the periodic variations at different places during a common 

 period. I cannot, unfortunately, give you the results for two 

 separate stations in recent years, but must confine myself to 

 the secular changes observed at Cape Town and in St. Helena 

 about the middle of last century. The following table, in con- 

 junction with Table No. i, illustrates the point to which your 

 attention is called. 



Table No. 2. 

 5^. Helena. 



Year. 



Declination. 1 Chana;e. ! Date. 



Dip. 



Change. 



We thus see that the secular variation must be the subject 

 of special study before the observations necessary for a satis- 

 factory knowledge of the magnetic state of a country can be 

 reduced to the epoch. 



A considerable amount of information bearing on the secular 

 changes since 1600 in and around Africa has been derived from 

 observations made by travellers, from ships' logs, and 

 from the results of "The Gazelle," "The Challenger," 

 ■"The Discovery,"" the work done under Ross, and 

 the determinations made by officers of our own and other 

 navies. Most of these results deal with the declination, and I 

 should like to bring to your notice in particular the work of 

 \"an Bemmelen* He has. with great pains, gathered together 

 over 5,000 observations from different voyages, and has shown 

 the results of a series of isogonic charts of the world, for the 

 epochs 1500, 1550, 1600, 1650, 1700. Some of these charts you 

 see before you, in so far as they refer to Africa. In addition, 

 the isogonic charts for 1800 and 1885 are shown. A glance 

 en-'.Dles us to see many interesting changes which have taken 

 place during these periods. 



In Africa, south of the Zambesi, an attempt has been made 

 bv Beattie and Morrison to obtain the approximate values of 

 the secular changes. Certain stations in different parts of the 

 Union and Southern Rhodesia have been re-occupied during 

 the last ten vears, and it is proposed to continue similar obser- 

 vations in the near future, t The results obtained up to the 



* Van Bemmelen : Lnc. ut. 



t The money necessary for this part of magnetic work in South Africa has 

 been contributed by the London Royal Society, the Government of Cape 

 Colony, and the Carnegie Institution of Washington. 



