62 
MR. AIRY ON THE OBSERVED DEVIATIONS OF THE COMPASS 
on the arrangement of the mass of iron, and having relation only to the ship’s 
capacity for induced magnetism, but in no way related to terrestrial magnetism. 
If we can reconcile the observations made in the same ship at various localities by 
making H and S constant, then the subpermanent magnetism is truly permanent. 
In any case, N must be constant for the same ship. 
Perhaps the process of obtaining the various elements of a ship’s magnetism will 
be rendered a little more intelligible by exhibiting the work in a single instance. 
Iron-Steamer “Trident,” examined at Greenhithe, 1852, September. 
1. Deviations as registered, substituting only the 0, 1, 2, 3, &c. points for N., N.b.E., 
N.N.E., N.E.b.N., &c., and the signs + and — for E. and W. 
Apparent 
azimuth 
of ship’s 
head. 
Deviation. 
Apparent 
azimuth 
of ship’s 
head. 
Deviation. 
+ 
- 
+ 
- 
P 
o / 
O i 
P 
o / 
o / 
0 
2 55 
16 
2 30 
1 
3 40 
17 
0 0 
2 
9 15 
18 
2 30 
3 
12 32 
19 
5 50 
4 
15 50 
20 
9 10 
5 
17 40 
21 
11 52 
6 
19 30 
22 
14 35 
7 
19 5 
23 
16 47 
8 
18 40 
24 
19 0 
9 
17 15 
25 
20 22 
10 
15 50 
26 
21 45 
11 
14 10 
27 
22 10 
12 
12 30 
28 
20 0 
13 
11 30 
29 
16 30 
14 
9 20 
30 
13 0 
15 
5 55 
31 
7 32 
The sum of the + deviations is +205° 12'; the sum of the — deviations is — 203° 58'; 
the algebraical sum of all is +1° 14'; which implies a mean error of +0° 2'. Applying 
the correction — 0°2' to every deviation, the next table is formed. 
