Deviations of the Compass on Iron and Wooden Steam-ships. 535 
be assisted to. be ‘shaken down”? by the vibrations of the machinery 
in a short preparatory trip prior to the determination of her compass 
errors, or their compensation ; but especially that in the early voy- 
ages vigilant supervision should be exercised in the determination of 
the compass disturbances. 
- Another important point, generally neglected when compasses are 
adjusted by the aid of magnets in a newly built iron ship, is rendered 
manifest by the results of this investigation; namely, the necessity 
of the errors of the compass being determined and placed on record 
prior to the adjustment. Without the knowledge to be derived from 
these observations of the magnetic force of the ship, all future 
changes of magnetism and consequent errors of the compass are 
mere guesswork both to those who adjust, and those in charge of 
the navigation of the ship. ¥ 
It is recommended that, in any future legislation for the security 
of the navigation of our mercantile marine with reference to iron- 
built ships, the determination and record of these preliminary obser- 
vations should be secured. : 
_ The paper concludes by directing attention to the general prin- 
ciples of practical import which result from the investigation, viz. as 
to the best direction with reference to the magnetic meridian for the 
keel and head of an iron ship to be placed in building, to ensure the 
least compass disturbance; the best position and arrangement for a 
compass to ensure small deviations, and permanency on changes of 
geographic position ; and the changes to which the compass is liable 
from various causes on the foregoing conditions being fulfilled. 
For the best direction in building, it is shown that, from the nature 
of the polarity of the hull, and especially of the top sides in the after 
section of the ship and adjoining the compass, where usually placed, 
the latter is least affected in those vessels built in the line of the mag- 
netic meridian. 7 
For iron steam-vessels engaged in the home or foreign trades in 
the northern hemisphere, it is recommended, from the then antago- 
nistie magnetic influence of the hull and machinery, to build them 
head to the north: for iron sailing vessels, from the top sides, in the 
usual position of the compass, being magnetically weak if built head 
to the south, the latter direction is to be preferred. 
The seleetion for the position of the compass depends on the 
direction of the ship during building; in those built head to north, 
it must be removed as far from the stern as convenience will permit ; 
in those built head to south, as near to the stern as convenient, but 
avoiding especially, in all cases, proximity to vertical masses of iron. 
In ships built head east or west, there is little choice of position: in 
those built on the intercardinal points, a position approximating to 
the stern when the action from the top-sides—to be determined expe- 
rimentally—is at a minimum, is to be preferred. 
Ample elevation above the deck and exaet peaten in the middle 
line of the ship, are primary conditions to be observed; and no com- 
pass should be nearer iron deck beams than 4 feet. As every piece of 
iron not forming a part of, or hammered in the fabrication of the 
