354 
CAPTAIN B. W. CREAK ON THE MAGNETICAL 
Before proceeding to show the remarkable amount of the disturbances of the 
magnetic elements observed at Port Walcott, it must be remembered that the only 
means available for obtaining the observations was on board a composite-built* * vessel 
with steam machinery ; in fact, that the observers had nolens volens placed the ship, 
herself a disturbing magnet, between the instruments and the source of disturbance. 
It was therefore necessary, first of all, to determine to what extent this interposed 
magnet, the ship, disturbed the needles on board. For this purpose the ship was 
swung off Baudin Island in 19 fathoms of water on May 6, 1891 : (a) for values of 
Dip and Intensity on the eight principal points of the compass ; (5) simultaneous 
observations of the Standard and Fox compasses for deviation on all points of the 
compass. 
Adopting the methods and formula; described in the Admiralty ‘ Manual of 
Scientific Enquiry’ (Art. Terrestrial Magnetism) for 1886, the following table of 
Disturbance of the Dip and Total Force, caused by the Horizontal Forces of the 
ship, was computed :—- 
Ship’s head. 
Correction for Dip. 
i 
Correction for Total 
Force. 
met. units. 
North 
-6 17 
+ •074 
N.E. 
+ 1 27 
+ T92 
East .... 
+ 2 26 
+ •305 
S.E. 
+ 1 29 
+ •177 
South 
+ 0 17 
+ •014 
S.W. 
+ 1 48 
+ T18 
West ... 
4 2 26 
+ •217 
N.W. 
+ 0 49 
+ •157 
Next adopting the notation of the Admiralty ‘ Manual for Deviations of the 
Compass’ (1893), the following coefficients, representing the horizontal disturbances 
of the ship, were computed from the observed deviations :— 
Standard compass 
Fox compass 
B. 
C. 
D. 
+ 6 45 
o / 
-0 11 
o / 
+ 3 55 
+ 0 35 
-0 4 
+ 2 22 
Having corrected the Dips for effects of the ship’s horizontal forces, the resulting 
mean Dip was compared with the normal values at Baudin Island, and found to differ 
only 2’. The Vertical Force of the ship was therefore considered zero. 
Now at Port Walcott the coefficients B, C, D were found to be :— 
* Composite-bailt stipes have iron frames and wood planking. 
