IN THE lEON SHIPS OE THE EOTAL NAVY. 
353 
pensation; but especially that on the early voyages vigilant supervision should be 
exercised in the determination of the compass disturbances. 
Another important point, which is generally neglected when compasses are adjusted 
by the aid of magnets in a newly built iron ship, is rendered manifest from the circum- 
stances just detailed, namely, the necessity of the errors of the compass arising from the 
iron in a ship being determined and placed on record prior to its adjustment. Without 
the knowledge, to be derived from these observations, of the magnetic force of the ship, 
or the local disturbing cause (if such exists), all future changes of magnetism, and con- 
sequent errors of the compass, are the merest guesswork, both to those who adjust and 
the authorities in charge of the navigation of the ship. 
Any future legislation for the secmity of the navigation of our mercantile marine with 
reference to iron-built ships, should secure the determination and record of these 
preliminary observations. 
I have alluded to the importance of the conclusions to be derived from a review of 
the examples and cases given in this Report; for although varying conditions of com- 
pass disturbance exist, and the inference is irresistible that they arise from the nature of 
the iron employed in the construction of the hull of the ship, there is no doubt that, by 
attention to a few leading principles in the building and equipment of iron ships, the 
larger and uncertain sources of error may be modified and reduced within limits both of 
fluctuation and amount, that will not seriously compromise the safety of the ship in the 
hands of an ordinarily prudent seaman. 
The points of practical import to which I would invite attention are,— 
1st. The best direction, with reference to the magnetic meridian, for the keel and 
head of an iron ship to be placed for building, to ensure the least compass disturbance. 
2nd. The best position and arrangement for a compass, to ensure small deviations, 
and permanency on changes of geographic position. 
3rd. The changes to which the compass is liable from various causes when the 
foregoing conditions are fulfilled. 
Before entering on these points, I would refer to an appended series of diagrams 
(Plate XVI.) illustrative of the polarity of the topsides (and the coefficients B and C), 
according to the direction of the build ; these diagrams forming the key to much that 
comes under reriew. 
1. On the best Direction for Building an Iron Ship. 
In those built head N.E., East, West, and N.W., strong south polarity (or an attractive 
force on the north end of the compass needle) obtains on one side of the ship adjoining 
the compass as usually placed between the middle section and the stern ; the resulting 
disturbance is not lessened as the compass is moved in a fore-and-aft line within these 
limits. 
In vessels built head S.E. and S.W., north polarity obtains under the same conditions. 
MDCCCLX. 3 B 
