105 
variation of the magnetic needle. 
throughout hoth sides, so that the focus of attraction occurs 
in midships, a compass placed on the midship line of the 
deck (drawn longitudinally) will be free from any anomaly 
from one end of the ship to the other, when the course is 
north or south ; but on every other course an anomaly will 
generally appear, increasing as the angle between the ship's 
head and the magnetic meridian increases, until the error is at 
a maximum, when the course is east or west. 
The unequal distribution of iron in the ship, on board of 
which I made all my experiments, prevented the above effect 
from being realized. A blacksmith's shop was situated be- 
tween decks, on the larboard side of the fore hatchway. It 
was lined with sheet iron, and besides the armourer's forge, 
vice, &c. contained a large quantity of other iron. The effect 
of this, together with the anchors, windlass necks, and other 
iron, was very remarkable on a compass placed in different 
parts of the deck near the foremast. [[See Observations, 18, 
1 9, and so of the prefixed table.]] 
li. Asa compass placed on the midship line of the deck is 
subject to no anomaly fore and aft, in certain ships, on a north 
or south course [[Inference No. io[], and as a compass in 
either side of the ship, opposite to the focus of attraction, 
shows no anomaly on a west or east course [[Inference No. 8[], 
the intersection of the line joining the two situations in oppo- 
site sides of the ship with the midship line traced fore and aft, 
will probably point out a situation directly over the top of 
the focus of attraction, where no anomaly on any course what- 
ever will appear. 
The Esk, in which I made my magnetical observations, had, 
as above stated, an armourer's forge near the larboard bow, 
mdcccxix. P 
