514 
APPENDIX. 
[ Errors in variation. 
pass diminished together, as nearly in equal proportions as the accuracy of 
our observed variations could be depended on ; and 1 therefore considered 
the connexion between them to be so far certain, as to make the dip one 
datum in reducing the observed to the true variations. 
Another point of equal importance remained to be known : the com- 
pass stood right in both hemispheres when the ship’s head was at North or 
South, and erred most on one side when the head was West and on the 
other when it was at East ; but what was the proportion of error at the in- 
termediate points, between the magnetic meridian and East and West ? 
Unfortunately, the direction of the ship’s head when observations were 
taken, had not been particularly marked in the first part oi the voyage, 
nor always in the latter part ; and in gathering it from the courses steered 
when under way, and from the direction of the winds and tides when at 
anchor, there was often a good deal of uncertainty ; but it was evident, 
that the quantity of error increased as the angle between the ship s head 
and the magnetic meridian became greater. After some consideration, it 
appeared to me that the magnetism of the earth and the attraction forward 
in the ship must act upon the needle in the nature of a 'compound force ; 
and that the errors produced by the attraction should be proportionate to 
the sines of the angles between the ship’s head and the magnetic meridian. 
I tried this upon many observations where the direction of the head was 
least doubtful, and found the differences to correspond as nearly as could 
be expected, and sometimes exactly ; it therefore seemed probable that 
the error produced at any direction of the ship's head, woidd he to the 
error at East or West, at the same dip ; as the sine of the angle between 
the ship's head and magnetic meridian, teas to the sine of eight points, or 
radius. According to this, when the error was ascertained at any 
given direction, more especially at East or West where it was greatest, 
it might be found at any direction required, by inspection in the Traverse 
Table. 
Soon after my arrival in England, application was made to the Lords 
Commissioners of the Admiralty to have experiments tried on board some 
of His Majesty’s ships, that this law might be verified ; and they were 
pleased to order them at Sheerness, Portsmouth, and Plymouth. I was 
present at the two first ports, when a series of observations were made in 
