362 MAGNETIC SURVEY OF A PART OF THE SOUTHERN HEMISPHERE. 
“ 3. Elements of Calculation of the Intensity Observations. 
“ Fox No. 1. — For the observations with this instrument, the Cape of Good Hope 
has been taken as a base station, the intensity having been observed there both 
before and after the Expedition, so that any change in the magnetism of the de- 
flectors or needles can be detected. 
“ The intensity at Woolwich being assumed =1*372, it is necessary to get the cor- 
responding intensity at the Cape. This can be got independent of the dipping 
needles, by means of the absolute horizontal intensity and inclination observed at 
each station. 
“ The value of the horizontal intensity at the Cape is given as follows by observa- 
tions made at the observatory in February, March, April and May 1845 : — 
“ Observations of the Absolute Horizontal Intensity, at the Magnetic Observatory, 
Cape of Good Hope, 1845. 
Bar. A. 21. Suspended . . . length 3*00 inch . . . fl -f-jrJ “ 1*00084. 
Bar. V. Deflecting . . . length 3*67 inch . . . ^ = *00008 . . . log -r 2 . £=1*57254. 
Date. 
Angles of Deflection. 
Corrected 
time of 
vibration. 
Temperature during 
Bifilar readings at 60 
during 
Results. 
Dist. l - 2 ft. 
Dist. 1*3 ft. 
Deflection. 
Vibration. 
Deflection. 
Vibration. 
m. 
X. 
1845. 
Feb. 10, 11, 12. 
Mar. 10, 11, 12. 
Apr. 13, 14, 15. 
May 14, 15, 1 6 . 
6 05-1 
6 01-1 
5 57*5 
5 55-5 
o / 
4 47*5 
4 44-0 
4 41-2:] 
4 39-2 
4-4970 
4-5310 
4-5570 
4-5650 
O 
71*4 
71-4 
62-9 
60-7 
71*7 
71*9 
62-6 
59*9 
Scale dir. 
185- 9 
186- 0 
176-9 
177*2 
Scale dir. 
185-8 
185-8 
177*4 
177*5 
0-4118 
0-4064 
0-4019 
0-4001 
4-488 
4-480 
4-478 
4-482 
' 
>4-482 
“ The value of k is obtained by means of two cylindrical weights in the usual manner ; the value employed 
is the mean of several determinations. Bifilar magnetometer £=-000218, §'=•000218. Increase of reading 
denotes increase of force. 
Whence X = 4*482, 6 being =—53° 25'*5. 
“ The corresponding values at Woolwich are 
X=*3*7284, 0 being =+68° 57'*9. 
“ From these values of X and 0, we obtain the relative value of the intensity at 
the Cape (that at Woolwich being 1*372), 1=0*993. 
“ The relative intensity given by the needles of No. 1, from observations made at 
Woolwich and the Cape and given in the sequel, are as follows : — 
Needle 1 .{ Weight 1 gr * I = 0 ’ 996 ) 1*006. Needle 2.f VVeight 1 gr ' J =0 ' 994 |tOOO. 
lWeight2grs. I=1*017J LWeight 2 grs. I=1*006J 
* Contributions, No. VII.; Philosophical Transactions, 1846, p. 246. 
