284 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
the average or normal amount in vessels of all sizes, and in only two vessels mentioned 
in that paper did D exceed 5°. 
In the iron-built armour-plated ships its average amount in the Standard Compass is 
about 7°, in the steering-compass about 10°, and in the main-deck compass about 12°. 
In the wood-built iron-plated ships the value of D is small. 
The following Table gives the value in different ships. 
Warrior. 
Black 
Prince. 
Achilles. 
Defence. 
Resistance. 
Hector. 
Valiant. 
Royal Oak 
(wood- 
built). 
Standard compass 
Starboard steering 
Main deck 
+ 8 27 
+ 11 56 
+ 11 43 
+ 7 38 
+ 10 32 
+ 13 16 
+ $ 58 
+ 8 51 
+ 12 13 
+ 7 o 
+ 10 16 
+ 14 35 
+ 6 17 
+ 8 28 
+ 14 0 
+ 5 24 
+ 8 24 
+ 9 47 
+ 4 54 
+ 6 52 
+ 8 05 
+ 3 09 
+ 1 47 
+ 1 28 
The large amount in the Standard and Steering Compass of the Warrior is doubtless 
owing to the rifle tower which is immediately before them, and which gives a -f a. The 
small comparative values in the Hector and Valiant to the iron-plating being extended 
from end to end in the ship giving a — a, and the absence of a complete transverse armour 
bulkhead, the existence of which in the Defence and Resistance, as well as in the 
Warrior and Black Prince, give large — e, and consequently large deviations in the bin- 
nacle and main-deck compasses. 
Between the Resistance and the Defence there is a remarkable difference. These are 
nearly sister ships, but with this difference, that from the different position of the mizen- 
mast in the two ships their standard and steering compasses are very differently placed 
with reference to the transverse armour bulkhead. In the Resistance the Standard 
Compass is exactly above the bulkhead at a height of 12 feet. The steering-compass is 
about 4 feet in front, and the same height above it ; while in the Defence these compasses 
are about 20 feet abaft it. 
Such a bulkhead, when magnetized at right angles to its plane, will produce a fore-and-aft 
force on all points in, or nearly in, the same plane in the opposite direction to the mag- 
netizing force. It will therefore, in the case of the standard and steering-compasses of 
the Resistance, introduce a —a as well as a — e, while it will produce little or no —a in 
compasses placed as in the Defence, and a much smaller — e. 
These differences do not show themselves in the value of D, which is in fact less in 
the Resistance than in the Defence, notwithstanding the much more powerful action of 
the forces which cause it. In order to see them, we must obtain separately the two 
parts of the quadrantal deviation D, or the value of a and e. This is done in the fol- 
lowing Table:- — 
