118 

 MAGNETIC OBSERVATIONS. 



The Observations made on Terrestrial Magnetism were of two kinds, Absolute and Variation 



or Differential. 



ABSOLUTE OBSERVATIONS AND ADJUSTMENTS. 



The observatoi'j in which the absolute observations were made was a log hnt about 

 15 ft. (4'5 m.) X 8 ft. (2-5 m.) with a mud fireplace in one corner. No iron was used in its 



construction. 



Absolute observations were made in the neighbourhood of the observatory with satisfactory 

 results, no sign of any local magnetic influence being observed. 



Horizontal Intensity (X). 



The absolute value of the horizontal component of the Earth's magnetic force was found 

 by means of vibrations and deflections with the unifilar magnetometer No. 102, by Jones, 

 London. During every observation the bifilar was read at short intervals, and the mean of 

 these readings was assumed to correspond with the value of the horizontal force X found by 

 means of the absolute observation. 



The following are the instrumental constants of the unifilar which were ascertained at 

 Kew before its departure, and verified on the return of the instrument. 



Graduation of deflection bar : — 



Apparent distance from centre of instrument. True distance at tem23. 0° Cent. 



• 20 metre = 0-1 99925 metre 



0-25 „ = 0-249925 „ 



0-30 „ = 0-299925 „ 



0-35 „ = -,349925 „ 



0-40 „ = 0-399925 „ 



Deflection apparatus, angular value of one scale division = 2' 1". 



Vibration magnet, angular value of one scale division = 2' - 25. 



The deflecting magnet employed was marked — N 5. 



The suspended „ „ — N a. 



For deflecting magnet : 



Correction to 0° Cent. = • 000224 {t^ - 0^) + • 0000018 {t^ - Oy. 



Induction coefficient ^a = - 00000637. 



Log. tt'' K at 0° Cent. = 9 • 50076. ■ 



Dimensions of inertia cylinder : length = - 103617 metre. 

 „ „ ,, diameter = - 00998 metre. 



„ ,, „ weight = 68 - 2799 grammes. 



The following table gives the results of the observations, each value of X being obtained 

 from a pair of observations, one of vibration and one of deflection ; in being the magnetic 

 moment of the magnetic needle used, and X the Earth's magnetic horizontal force. 



