SUPPLEMENT. 
37 
The irregularity of the compass being one of the important 
objects of the expedition under the command of Capt. Ross, it 
became his duty to examine the various reports and publications 
on the subject, and to endeavor to ascertain how far the different 
systems given to the public are correct, and the rules for cor¬ 
recting the deviation of the variation to be depended upon. 
From the experiments that were made, the following facts were 
ascertained :—- 
1. That every ship has an individual attraction, which affects 
the compass on board of her, and to ascertain the exact quantity 
of its effect, though possible, requires the most particular care 
and the nicest attention. 
2. The effect of this attraction being different in different 
ships, and not always progressive, but often irregular, no general 
calculation will therefore apply in the case of all ships, so as to 
ascertain it for the purposes of correction, and consequently all 
the rules hitherto given for obtaining it, particularly in Arctic 
climates, cannot be relied on. 
3. As six compasses were compared with each other on board 
the Isabella, and found to agree in the same place, and as they 
were all found to disagree when placed in different situations 
between the stern and the foremast, it is evident that the devia¬ 
tion in any ship will vary according to the situation of the com¬ 
pass at the time of using it; and, therefore, as the point of 
change will not be the same at every part of the ship, all obser¬ 
vations must be made in the same place, where the point of change 
has been obtained, since to that only will this correction apply. 
4 The deviation does not always continue the same under the 
same apparent circumstances, but varies according to the point 
in which the ship’s head lies. 
5. The deviation appeared to be materially affected by heat 
and cold, as well as by variations in the humidity and density 
of the atmosphere. 
6. The direction of the wind seems to have an effect in dis¬ 
turbing the regularity of the deviation. 
7. The force or quantity of the dip possesses a decided influ¬ 
ence over the force or quantity of the deviation. 
