5)2 
APPENDIX, 
[ Errors in variation. 
No. II. 
On the errors of the compass arising from attractions within the ship, and 
others from the magnetism of land ; with precautions for obviating their 
effects in marine surveying. 
Several instances have been mentioned in the course of this voyage, 
where the compass showed a different variation on being removed from 
one part of the ship to another ; thus observations on the binnacle gave 29i* 
off the Start, where the true variation was about 25~° west, whilst others 
taken upon the booms before the main mast, sixty-eight miles lower down 
Channel, gave only 24“ ; and in the experiments made with five compasses, 
Vol. I. p. 18, the mean variation at the binnacle was 4° 37' greater than on 
the booms. Finding that the situation of the compass was an object of im- 
portance, I determined very early in the voyage to place it always upon the 
binnacle ; both when taking bearings for the survey, and when observing 
azimuths or amplitudes ; nor in any observations taken by myself, was it 
ever displaced except by way of experiment ; but the officers occasionally 
observed from different parts of the ship, when the sun could not be seen 
from the binnacle, until they were convinced that such observations were 
of no utility, either to the survey, or for ascertaining the true variation. 
It soon became evident, however, that keeping the compass to one spot 
was not sufficient alone to insure accuracy; a change in the direction of 
the ship’s head was also found to make a difference in the needle, and it 
was necessary to ascertain the nature and proportional quantity of this dif- 
ference before a remedy could be applied. This inquiry was attended with 
many difficulties, and no satisfactory conclusion could be drawn until a 
great variety of observations were collected ; it then appeared, that when 
the ship’s head was on the east side of the meridian the differences were 
mostly one way, and when on the west side they were the contrary, whence 
I judged that the iron in the ship had an attraction on the needle, and drew 
