492 STAFF CAPTAIN EVANS, E.N., ON THE CHANGES OF POLAE MAGNETISM 
Maximum of Semicircular Deviation. 
A „ 
r # n 
Poop compass. Starboard Steering compass. 
1866. 
May 29th ... 48f 39| 
July 12th ... 64£ 50 
August 10th . . . 55f 46^- 
In each case it will be seen there is a slight decrease in the interval comprising the 
4th August, but the decrease does not exceed half the increase in the preceeding interval ; 
and seems only the recovery from an anomalous increase in July, probably attributable 
to some external disturbing cause of the nature before mentioned, viz. the proximity 
of the two armour-plated ships in the Victoria Docks. 
The next and final trial was made between the 1st and 26th January 1867, at Sheer- 
ness : ten or twelve days were occupied in passing large electro-magnets along the out- 
side of the ship from the water-line to the top sides. Subsequently, and apparently as an 
after thought, — as no mention is made in either specification of internal demagnetization 
on so large a scale, — about five days more were occupied in applying the electro-magnets 
to the transverse iron beams of the poop and upper deck nearest the poop and steering 
compasses, and to two adjacent vertical iron stanchions supporting the upper deck. 
As no appreciable effect was produced on the Standard compass, or on any compasses 
except those in the immediate vicinity of the beams and stanchions operated on, we 
may have confidence in attributing the change which took place entirely to the latter 
operations. 
The following Tables, derived from the General Table, furnish the means of deciding 
this question. 
Table I. 
i At position of the 
undermentioned 
compasses. 
1 
(I.) 
(II.) 
(III.) 
(IV.) 
(V) 
(VI.) 
(VII.) 
B 
Part of 
semicircular 
deviation 
from 
Headward 
force. 
C 
Part of 
semicircular 
deviation 
from 
Transverse 
force. 
sin— I VS3 2 +S 2 
Maximum of 
semicircular 
deviation. 
e 
tan-i^ 
Starboard 
angle. 
J“ 
Mean vertical 
force of earth 
and ship, in 
terms of earth’s 
horizontal force 
as unit. 
Heeling 
error to 
windward 
for 1 ° of heel. 
/Standard 
-37 
+ H 
173f 
1-223 
•fl 18 
§ §g -| Starboard steering . . 
— 39f 
+15^ 
461, 
1611 
1-056 
+ 1 4 
| Poop 
— 48| 
+ 151 
56* 
164i 
1-469 
+2 1 
£ ^ Port steering 
-43 
+ 19* 
61 
157-i 
Table II. 
( Standard 
-36i 
+ 7 
401 
a »> i 
-( Starboard steering . . . 
- 2£ 
+381 
36f 
3’ -1 Poop 
- 31 
+ 121 
12 | 
zo ( Port steering 
- H 
-11 
11 
1701 
1-271 
+ 1 27 
94i 
1-250 
+1 45 
104| 
•986 
+0 37 
246 
