496 STAFF CAPTAIN EVANS, E.N., ON THE CHANGES OF POLAR MAGNETISM 
These results are clearly attributable to the change in the general magnetism of the 
ship due to position ; the fore and aft force has been little affected, but there is a decided 
increase in the transverse force, = +-219. 
With respect to the compasses affected by Mr. Hopkins’s process, the results are widely 
different: — in seven months (end of January to end of August), as will be seen by the 
following Table, at the starboard steering — 33 has increased T83, (5 having scarcely 
changed in value; at the poop compass — 33 has increased *281, and +6 T62 ; at the 
port steering compass — 33 has increased -098, while the (5 has increased ’320; so 
that the increase of the (S in the poop compass is almost exactly the mean of the increase 
in the port and starboard compasses. 
1867. 
Port steering compass. 
Poop compass. 
Starboard steering compass. 
33 
e 
35 
e 
January 26 
-•078 
-176 
-•059 
+•203 
-046 
+•596 
February 28 
-•071 
-•128 
-•070 
+•230 
-•078 
+•625 
June 26 
August 29 
-176 
+•150 
-•153 
-•340 
+•360 
+'365 
-•229 
+•637 
r--212 
+•508 1 
r - -215 
+ 541 n 
j _ -136 
+-641i 
December 10 
1--218 
+•48,1 
l --221 
+•516 J 
L — -141 
+ • 617 } 
If we confine ourselves to the practical question of the amount of the semicircular 
deviation, and also of the direction of the ship’s force [starboard angle], or, in other 
words, the direction of the neutral or zero-points of the semicircular deviation, the com- 
parison is as follows : — 
Standard. 
Port steering. 
Poop. 
Starboard steering. 
1867. 
Maximum 
semi- 
circular 
deviation. 
Starboard 
angle. 
Maximum 
semi- 
circular 
deviation. 
Starboard 
angle. 
Maximum 
semi- • 
circular 
deviation. 
Starboard 
angle. 
Maximum 
semi- 
circular 
deviation. 
Starboard 
angle. 
0 
0 
0 
0 
0 
0 
0 
0 
January 
1... 
371 
173* 
47* 
157 
51 
164* 
42* 
181* 
» 
26... 
37 
170+ 
12 
246 
12* 
104* 
38* 
94* 
February 
28... 
37 
170 
8* 
241 
14* 
107 
41* 
97 
June 
26..., 
39* 
155 
23 
112 
August 
29... 
38* 
156 
13* 
139* 
30 
133 
42* 
110 
Deoember 
10... 
42* 
151 
33* 
112* 
3o^ 
111* 
40 
102 
From this comparison it will be seen how short lived is the apparent benefit derived 
from the process we have been considering. 
The nature of the changes in the polar force will perhaps be more clearly appre- 
hended if we represent them graphically, as in Plate XXXII., by taking the position of 
each compass as origin and laying down the points, of which X33 and are the coordi- 
nates, as derived from the observations on January 1st and 26th, February 28th, August 
29th, and December 10th, 1867. 
As regards each compass, a line drawn from the origin to one of these points repre- 
sents in amount and direction the polar force acting on that compass at that epoch ; and 
a line drawn from one point to the next succeeding in order of date represents, in 
amount and direction, the additional polar forces acting on that compass introduced in 
