CHAEACTEE OE THE AEMOTTE-PLATED SHIPS OP THE EOYAL NAVY. 
275 
This can only be caused by horizontal induction in soft iron. E can only be caused by 
horizontal induction in soft iron unsymmetrically distributed, but of any shape ; an 
E may therefore be caused by the compass being placed out of the midship line and 
exposed to the influence of spherical or cylindrical masses, such as the iron gun-turrets 
of modern war-vessels. 
D, which in ordinary cases is always +, is caused by horizontal induction in soft iron 
arranged according to one or other of the following types : — 
Pig. 3. Pig. 4. 
+ a 
\\a 
In the figures -f -a represents masses of soft iron entirely before or entirely abaft the 
compass, as engines, boilers, funnels, iron masts, &c.; — a represents soft iron extending 
through the position of the compass, as the keel and hull of the ship, the screw-shaft, 
armour-plating, &c., the effect of the latter in almost all cases exceeding that of the 
former, so that a is in general negative; — e represents the effect of all the transverse 
soft iron, as the bottom of the ship, the iron decks (except where interrupted by hatch- 
ways near the compass), iron deck beams, and the engines, boilers, &c. ; -\- e represents 
the masses of iron, comparatively few in number, which lie to one side of the compass, 
as decks where the compass is in or over a hatchway, occasional guns, davits, & c. In 
every ship which has been examined, the effect of the transverse iron extending through 
the position of the compass exceeds that of any masses of iron wholly on one side, and 
e is negative and greater than a ; and as 2 = ^^, 2), and consequently D, are in almost 
all cases + . 
D and E do not change with a change of geographical position. 
In almost all cases in iron-built ships, not only is the direction of the needle directly 
affected by the iron of the ship, but a further prejudicial effect is caused by the soft iron 
diminishing the mean directive force of the needle, and so indirectly increasing the effect 
of all disturbing forces. This is shown by the factor X, which gives the mean value of 
the directive force, or rather of the northern component of the directive force in the 
ship, and which is almost always less than unity, the force on shore being considered as 
unity. 
The cause of this diminution will be seen by figs. 3 & 4. In fig. 4 a little considera- 
tion will show that both —a and — e diminish the directive force. In fig. 3 +a in- 
