3— SOME RESULTS OF A MAGNETIC SURVEY OF 



NATAL. 



By J. C. Beattie, D.Sc. 



The observations on which the results of this paper are based 

 were taken in 1903. The thanks of the writer are due to the 

 Government of Natal for their help in this work ; and they are in 

 particular due to Mr. Massen, Surveyor-General, for his kindness 

 and for the trouble he put himself to in order to make the labour 

 of observing as pleasant as possible, and to Mr. Nevill, our Presi- 

 dent, who gave much valuable assistance and information. 



The instruments used were Dip circle 142 by Dover, and 

 Magnetometer 73 by Elliot ; a theodolite and a chronometer. It was 

 possible to rate the chronometer several times weekly by interchanging 

 signals with the Government Observatory in Durban. 



The latitude and the longitude of each station was determined, 

 and the magnetic elements observed were the declination, the dip, 

 and the horizontal intensity. 



From these latter the vertical, the total, the westerly, and the 

 northerly intensity were calculated. All magnetic results have been 

 reduced to the epoch ist July, 1903. 



To reach the conclusions which can be drawn from the whole 

 survey, the results were grouped thirty at a time. In this way a 

 mean value of the particular element concerned was obtained at an 

 imaginary station whose latitude and longitude was the mean of the 

 latitude and of the longitude of all the stations. 



There are two such mean stations for Natal, I., II., whose 

 latitudes and longitudes and the values of the magnetic elements 

 there, are given below : — 



Station Lat. Long D. H. B Z T X Y. 



L 29^43' S. 30'31'E. 23 45'W. 18127 61' 6'S. -32838 -37510 -16625 -07311 

 n. 27^39' S. 30' 3E. 2223'W. 18753 59°37'S. -32008 -37100 -17327 -07207 



Where D is the declination reckoned positive towards the W. 



H is the horizontal intensity. 



is the dip reckoned positive when the South Pole is below 

 the horizon. 



Z is the vertical intensity reckoned positive towards the zenith. 



T is the total intensity. 



X is the northerly intensity. 



Y is the westerly intensity. 



The next step was to form an equation — for each station used 

 in the group — of the form 



■V (^m - ^) + V (X„ - \) = e^ -e 

 Where $„ \^ are the latitude and longtitude of the imaginary station 

 $ \ ,, ,, an actual station 



e„ 9 the dips at the imaginary and the actual station 

 respectively. 



Similar equations were used for the other magnetic elements. 



X and y were determined by Caneby's method, and give 



the variation with latitude and longitude of the different elements. 



