38 
SUPPLEMENT. 
8. Although the points of change found with the compass in 
the same part of the ship will remain the same, unless some ma¬ 
terial alteration is made in the stowage of the iron on board, yet 
the amount of the deviation, with the ship’s head on any point 
of the compass, will bear a proportion, though not a regular one, 
to the increase or decrease of the variation or dip, by both of 
which it appears in some degree to be governed, though the 
points of change are not ; as they seem to be independent of any 
influence, except that of the ship’s attraction or magnetism, 
which is not of equal force in every part of the same ship, nor, 
perhaps, alike in any two. It is, however, presumed that the ex¬ 
periments and observations that have been made will be sufficient 
to correct the errors in the mariner s course, which have so often 
proved fatal, and which have hitherto been attributed to defects 
in the compass, to currents, and other unaccountable causes. 
In conclusion, it only remains further to explain the manner 
of finding the point of change in deviation. 
Let the bearing of one, or the transit of two distant objects, 
(whose true bearing from the ship, or from each other, is known,) 
be taken, with the ship’s head at several points of the compass ; 
if they all agree, the ship has no deviation; but, if not, the one 
which is found to agree is the point of change. 
To find the deviation for the point steered, let the bearing of 
the same object be taken, with the ship’s head on the point of 
the course steered ; and add, or subtract, the difference between 
them, as it increases or decreases the variation. 
To find the deviation at sea when a distant object is in view, 
the true magnetic bearing of which is not known, let a boat be 
sent out of the ship’s attraction, to take the bearing of the ob¬ 
ject, and then the bearing of it is to be taken from the ship, in 
the manner before described. But even when no distant object 
is in view it can be, done in fine weather with smooth water, by 
veering a boat (copper fastened) astern with a compass. The 
ship is then to steer on different courses, (the boat always keep¬ 
ing her masts in one,) until the compasses of the £hip and boat 
agree. If there is no difference between them on any point, the 
bhip has no deviation. But whatever difference is found between 
