IN THE lEON SHIPS OF THE EOTAL NAVY. 
343 
when at the Cape of Good Hope, should have been reduced in the inverse proportion of 
I'OOO to 1-174, or about one-sixth, whereas by observation it had more than doubled; 
the polarity of the funnel and machinery, following the changes of magnetic dip (the 
vessel was some months employed in high south magnetic latitudes), was reversed, and 
thus their force of magnetism ( — B) was added to that of the hull of the ship ( — B also), 
instead of being subtracted from it, as had been the case in England. 
The Simoom, a nearly sister vessel to the Vulcan, but with +B, and also observed 
at the Cape of Good Hope, has the conditions reversed, the semicircular deviation being 
smaller than its theoretical value, for the opposite reason ; viz. that the force of the 
machinery which had been added to that of the hull in England, changing its sign, was 
subtracted from it at the Cape of Good Hope. 
On the Nature of the Magnetism in Irmi-huilt Ships. 
The magnetic influence of steam machinery having been reviewed, the nature of the 
magnetism of h’on-built ships can be entered on free, to a certain extent, of conditions 
arising from this extraneous source of compass error, and those examples fairly elimi- 
nated where it tends to embarrass the discussion. 
The investigation of the coefiicient D, or the disturbance arising from the horizontal 
induction of the soft iron in the ship, when extended over the numerous examples 
recorded in Table I., ofiers several novel and suggestive points of inquiry: the chief 
characteristics are, — 
1. That it has invariably a positive sign, causing an easterly deviation in the N.E. 
and S.W. quadrants, and a westerly deviation in the S.E. and N.W. quadrants. 
2. That its amount does not appear to depend on the size or mass of the vessel, or 
direction when building, or on the iron beams. 
3. That a gradual decrease in amount has occurred, when examined over a number of 
years, in nearly every vessel that has been reviewed. 
4. That the value remains unchanged in sign and amount, on changes of geographic 
position, confirming theoretical deductions. 
5. That a value for this coefiicient, not exceeding 4°, and ranging between that 
amount and 2°, may be assumed to represent the average or normal amount in vessels of 
all sizes. 
The following examples support these propositions : — 
1. The value not depending on the size of the vessel. 
Great Eastern, of 22,000 tons . +4 8 
Himalaya . „ 3,453 „ . +3 18 
Assistance . „ 1,820 „ . +3 36 
Trident, of 850 tons 
Oberon, „ 649 „ 
Onyx, „ 292 „ 
+ 3° 26 
H-3 23 
+ 3 42 
As also, that the efiect of iron beams, which has been assumed as a large element of 
disturbance, cannot be traced. 
