IN THE IEON AND AKMOIJR-PLATED SHIP NORTHUMBERLAND. 
497 
the interval. To avoid confusion in the figure, the lines from the origin are only drawn 
for the earliest date. The joints added to these lines, therefore, represent the additional 
forces introduced in each interval. 
This Plate shows strikingly the contrast between the comparatively small and regular 
changes in the polar magnetism of the Standard compass, and the large and irregular 
changes in the polar magnetism of the other compasses ; but at the same time shows 
strikingly the general analogy in the changes in these three compasses. 
An attentive study of it will also show, that while the changes in the polar magnetism 
of the Standard compass are almost entirely from an increasing attraction to the south 
side of the ship, in the other compasses this is combined with an increasing attraction 
to, or rather a diminishing repulsion from, the point abaft the poop compass to which a 
powerful north pole had been communicated. 
In the interval between the 28th August and the 10th December, a general headward 
force on the three stem compasses is also added. What was the cause of this force I 
am unable say ; it may possibly be connected with the introduction of the five iron 
beams in front of these compasses, and the general hammering to which that part of the 
ship was consequently exposed. 
The views which I have taken of the effect of the application of the electro-magnets 
are, I think, strongly confirmed by observations made on the 10th December 1867, at 
four points surrounding the position of the supposed north pole produced by them. 
Of these, the position of the poop compass was one ; the second was 18 feet abaft the 
poop compass ; the third, 7 feet 3 inches abaft the poop compass, and 4 feet to port of 
the midship line ; the fourth, 7 feet 3 inches abaft the poop compass, and 4 feet to star- 
board of the midship line. The values of © and (§, for these compasses were — 
33. <L 
No. 1. Poop compass — *215 +'541 
2. 18 feet abaft Poop compass — -641 + *482 
3. 7J feet abaft Poop compass, 4 feet to port . . . —’457 +‘260 
4. 7-j feet abaft Poop compass, 4 feet to starboard . — -458 +-692 
An inspection of these values will show that they indicate a generally attractive force 
to the stern and starboard sides ; but modified by a repulsive force emanating from a 
point nearly centrically situated with respect to the four compasses. We may, in fact, 
I think, infer that the mean of these values, or a © of —-443, and a 6 of +‘494,18 due 
to the general magnetism of the ship, and as regards (E partly to the new iron beams, 
and that the difference from these values as to each compass arises from local magne- 
tization. These differences — which are also multiplied by X(*860) to express them in 
terms of the earth’s magnetic force as unit — will be 
33. 
e. 
x33. 
A®. 
No. 1. Station (Poop) . 
. +-228 
+ •047] 
1 +-196 
+ •040 
2. „ 
. — T98 
— -012J 
1 — T70 
-010 
3. 
. --014 
-•234] 
-•012 
-•201 
4. 
. --015 
3 z 2 
+ -198J 
-013 
+ •170, 
