CHARACTER OF THE ARMOUR-PLATED SHIPS OF THE ROYAL NAVY. 319 
GENERAL CONCLUSIONS. 
The following appear to be the principal conclusions to be drawn from the applica- 
tion of observation and theory to the magnetic phenomena in iron ships. 
1. The original semicircular deviation depends principally on the direction of the 
ship’s head in building, and consists principally in an attraction of the north point of 
the needle to the part of the ship which was (nearly) south in building. 
2. This attraction is caused by the subpermanent magnetism induced in the ship 
when building, by the horizontal force of the earth. 
3. If we consider separately, first, the effect of the subpermanent magnetism induced 
by the fore-and-aft component of the horizontal force, and secondly, the effect of the 
subpermanent magnetism induced by the transverse component of the horizontal force, 
the first is relatively less than the second. This, if the direction of the ship in building 
does not coincide with a cardinal point, modifies the direction of the semicircular devia- 
tion produced. 
4. A third part, being the remainder of the semicircular deviation, is independent of 
the direction of the ship in building. It is the effect of the subpermanent and transient 
magnetism induced in the ship by the vertical force of the earth, and it consists in an 
attraction of the north point of the needle to the bow or stern. 
In the usual place of the Standard Compass this part is, in the northern hemisphere, 
an attraction of the north point of the needle towards the bow ; but if the compass is 
placed nearly in front of a large vertical mass of iron, as the stern-post, it may be 
towards the stern. 
5. The first and second parts of the semicircular deviation diminish rapidly after the 
ship has been launched, the second generally most rapidly ; but after a time, which 
may be taken roughly as a year, if the ship has been allowed to swing on all azimuths, 
they attain a very fixed and permanent amount, from which they do not afterwards vary 
to any great extent. 
The third part changes little, if at all, so long as the ship remains in the same latitude. 
6. The changes which take place in the semicircular deviation of a ship built East 
and West are generally relatively greater than in one built North and South. 
7. The transient magnetism induced by the earth’s horizontal force adds to the effect 
of the subpermanent magnetism induced by the same force, when she is on the stocks, 
and afterwards when her head is in the same direction in which it was while building. 
8. The effect of the subpermanent and transient magnetism induced by the hori- 
zontal force when the ship is on the stocks is principally, and if the ship is built on a 
cardinal point entirely, to produce a diminution of the directive force on the needle, 
and very little, and if built on a cardinal point not at all, to produce deviation. 
9. The same effect (nearly) is produced at a subsequent time if the ship’s head is 
placed on the direction in which it was while building. 
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