320 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
10. This diminution of the directive force is greater if the ship has been built East 
and West than if built North and South. 
11. The deviations in an iron ship which has been built East or West are more preju- 
dicial than in a ship built North or South in the following respects: — 
1. They are less symmetrical and regular, and therefore more perplexing to the 
seaman. 
2. They change more relatively after launching. 
3. They diminish the directive force more when the ship is on particular points. 
12. When a ship has been built head North, the upper part of the stern and the 
lower part of the bow are strongly magnetized ; the upper part of the bow and the 
lower part of the stern are weakly magnetized. When a ship has been built head 
South, the upper part of the bow and the lower part of the stern are strongly, the 
upper part of the stern and the lower part of the bow are weakly magnetized. 
Consequently in ships built head North, a compass placed near the stern will have a 
large semicircular deviation. 
13. In the last case there will be a large downward force on the north point of the 
needle, which will produce a large heeling error. In ships built head South, both the 
last errors will probably be small. 
14. On the whole, for compasses to be placed in the after part of the ship, the best 
direction for building is head South. For compasses near the centre of the ship, the 
directions head North and head South are nearly equally good. 
15. The diminution of the mean directive force is the mean of the diminution caused 
by the transient magnetism induced by the horizontal force when the ship’s head is 
North or South, and that induced when her head is East or West, i. e. it is the mean of 
the thrust from the north end and from the north side. 
16. The quadrantal deviation is caused by the excess of the latter over the former, i. e ., 
by the excess of the thrust from the north side over the thrust from the north end. 
17. The diminution of the directive force and the amount of the quadrantal deviation 
are nearly the same at the same level in different parts of the ship. They increase in 
descending from the position of the Standard Compass to the compasses on the upper 
and main decks. They diminish with the lapse of time. 
18. By substituting wood for iron in the part of the deck below and above the compass, 
and within an angle of 35° 15' of the vertical line passing through the compass, and 
having no masses of iron with their centres within 54° 15' of the same vertical line, the 
directive force is increased and the quadrantal and heeling error generally diminished. 
19. In selecting a place for the Standard Compass, care should be taken to avoid as 
much as possible the proximity of the ends of elongated masses of iron, particularly 
if placed vertically ; or, if they cannot be avoided, then a place should be selected where 
they diminish instead of increasing the semicircular deviation. 
The neighbourhood of rifle and gun turrets in ships carrying them should be as much 
as possible avoided. 
