278 STAFF COMMANDER EVANS AND ME. A. SMITH ON THE MAGNETIC 
Constant Deviation. 
A. 
The values of A, when the compass is placed in the middle line of the ship, and when 
the deviations have been observed with every care, are always so small, that the values 
which appear in the Tables may be considered rather as errors of adjustment and 
observation than as real values. In fact it may be inferred that in all cases where the 
compass is in the middle line of the ship, we may consider A as zero. It results from 
this, and is important in practice, that we may safely take the mean of the compass 
bearings of any object, on four or more equidistant compass courses, as the correct 
magnetic bearing ; observing, however, that if we observe on four points only, and D be 
large, these ought to be either the cardinal or the quadrantal points. 
Semicircular Deviation , 
B sin C cos g. 
The points which require attention are, — 
1. Its original value and its connexion with the direction of the ship in building, and 
the position of the compass in the ship. 
2. The changes which take place after launching. 
3. The subsequent changes. 
4. The changes which take place on a change of geographical position. 
1. In wood-built ships, as maybe seen by an inspection of the Deviation Tables given 
in the work of the late Captain E. J. Johnson, R.N., on the deviation of the compass, the 
direction of the force causing the semicircular deviation is in northern latitudes nearly 
towards the ship’s bow. In iron-built ships it is nearly to that part of the ship which was 
South in building ; or, in other words, the starboard angle as given in the Tables, is nearly 
the same as the azimuth of the ship’s head to the East of South in building ; thus, — 
Starboard angle, or direction 
Direction of bead in building. of semicircular deviation. 
Orontes . . N. 66° W. or S. 246° E. 235° 
Tamar . . . West or S. 270° E. 279° 
The case of the armour-plated ships is an interesting exception to this rule. Such 
ships are generally plated after launching, and in a different position from that of 
building. In these ships the angle of the semicircular force is generally intermediate 
between the angle of the ship’s head to the East of South in building, and the like angle 
in being iron plated ; thus, — 
Warrior . 
Black Prince 
Defence . 
Resistance 
Valiant 
Direction of bead 
in building. 
N. 3 E. or S. 177 E. 
S. 20 E. 20 
S. 47 W. 313 
Direction of bead 
in plating. 
N.W. or S. 225 E. 
South. 0 
S. 19° E. 19 
West. 
270 generally to westward 
Direction of 
Semicircular 
Deviation. 
195 
8 
0 
f 313 
* 1282 
