SUPPLEMENT. 
39 
them on any point, that is the ship’s deviation for that particular 
point, and must be added, or subtracted, to correct the ship’s 
course on that point, according’ to the true magnetic course of 
the boat. And, in like manner, the respective differences found 
on the several points are to be applied to each. On w hatever 
point the courses of the boat and the ship agree, when her masts 
are in one, that is the ship’s point of change. The result of ob¬ 
servations made with the ship’s head on this point will give the 
true variation of the compass ; but if observed on any other 
points, the error of variation will be according to the amount of 
the deviation, or differences found on those points respectively, 
between the course of the ship and the boat, and must be applied 
_]_ or —, as the case may require, to correct it. The variation 
may be observed, either before or after this process, for finding 
the ship’s point of change and deviation ; and if amplitudes, or 
azimuths, are taken at different parts of the ship, the difference 
between the azimuth compass (wherever it may stand,) and the 
compass the ship steers by, ought always to be taken, and ap¬ 
plied in like manner to obtain the true variation. 
It would be a great benefit to navigation, if the bearings of 
remarkable headlands and other objects on the coasts of differ¬ 
ent countries were correctly taken, and inserted in the pub¬ 
lished charts. Thus a ship, able to approach near enough to 
take the transit bearing of any two such objects, whose relative 
situations were exactly laid down, could thus know at once her 
deviation, or whatever course she was steering, if the true vari¬ 
ation was on the charts, since it would be the difference between 
that and the true transit bearing, as laid down on the chart, 
taking into consideration, at the same time, the known variation. 
Men of war, and indeed all ships, should, at every opportunity, 
try the deviation, and ascertain these points of change, and that 
being once found, no change should be made in the stowage or 
position of any of the larger masses of iron on board. 
It only requires, however, a careful perusal of some parts of the 
memoir of Commander Ross, to discover at once how very slight 
was the knowledge which Capt. Ross possessed of the actual dis¬ 
coveries that Commander Ross had made in magnetical science, and 
