PRESIDENTIAL ADDRESS. — SECTION A. 25 



ditions the instruments and the methods used for the determina- 

 tion of the dip and the horizontal intensity were unsatisfactory, 

 and the vahies obtained for these elements can scarcely be 

 looked upon as anything but rough approximations to the true 

 results.* On the other hand, the declination results obtained 

 are satisfactory, and enable us to form a good idea of the value 

 of that element in Central Africa. Xone of these observers 

 has published a magnetic map of the Congo. Keeling has 

 used their results, however, to give the terrestrial lines in his 

 magnetic maps of Northern Africa. 



In Africa, south of the Zambesi, a considerable amount of 

 magnetic work has been carried out in recent years, chiefly by 

 Beattie and Morrison. Between 1898 and 1906 observations 

 were carried out at over 400 stations, several of which were 

 occupied more than once for the purpose of determining the 

 secular variations of the various elements. The results at 

 these repeat stations have already been mentioned. The obser- 

 vations were reduced to the epoch July ist, 1903. and magnetic 

 charts have been published j giving the true isomagnetics for 

 the surveyed regions at that date. Like all charts representing' 

 the results of a survey of this density, they show many irregu- 

 larities. The disturbances are greater in the Transvaal than in 

 any other part; a glance at any of the charts shows, however, 

 that there are other very disturbed regions, and the fact that 

 in many districts the lines appear less convoluted is probably 

 due to the number of stations there being fewer. The maps 

 which I have now to show you give, in addition to the true 

 isomagnetics, the terrestrial also. In determining the latter, 

 the observations made in Southern Rhodesia and in Bechuana- 

 land were not used; the stations in the other parts of South 

 Africa were divided into 15 districts, and a mean station ob- 

 tained for each, with mean values for the declination, the dip. 

 and the horizontal intensity. By the method of least squares a 

 suitable formula was obtained for each of the three elements, 

 and by help of these the terrestrial lines have been drawn, t I 

 might go on to give you details of the residual magnetic field 

 in this part of South Africa. This, however, would lead me 

 too far, and I shall content myself by referring you to the 

 published report of this survey. 



To complete the account of magnetic work in this part of 

 the world I have still to mention the observations carried out 

 in German East Africa by Maurer.§ those in Madagascar by 

 Colin,!! the work carried out by Chaves,*' and the observations 

 by Beattie and Morrison in 1909**, when over 300 new stations 



* Magnetic Observations in Egypt, 1895-1905, wdth a summary of previous 

 magnetic work in Northern Africa, bj- B. F. E. Keeling. Survey Department 

 Paper No. 6. Cairo, 1907. 



t Report of a magnetic survey of Soutli Africa, by J. C. Beattie. Royal 

 Society, London, 1909. 



I See Appendix to this Address. 



§ Keelinsf, loc. cit. 



\\ Terrestrial Magnetism, Vol. VIII., p. 187. 



■" Contribution aux etudes de magnetisme terrestre en Afrique, par F. A. 

 Chaves. Bulletin de I'lnstitut Oceanographique, Monaco, 1908. 



** The funds for this expedition were provided by the Carnegie Institution 

 of Washington, supplemented by grants from the London Royal Society, 

 and from Sir Lewis Michell and Dr. Jameson. 



