18 
A VOYAGE TO 
{From England. 
1801. and in the last, was upon the booms. In order to ascertain clearly 
what effect this change of place did really produce, I took observa- 
Monday27. tions a few days afterward with every compass on board, and Mr. 
Thistle did the same upon the booms, ten or twelve feet before the 
main mast, where the compasses were as far removed from any 
quantity of iron, as they could be placed in any part of the ship. 
The head was south-west by the steering compass, our latitude 
was 38 0 1' north, longitude 14 0 18' west, and the results were as 
under. 
Variation from an azimuth compass by Wal- 
i i i xt r > . Binnacle. Booms, 
ker, marked No 1 : mean of both sides 
of the vane, - - - 25 47 22 17 W. 
From a ditto marked No. 2, - 25 35 19 15 
ditto marked No. 3, - - 24 4,1 21 27 
Walker's meridional compass, - 25 46 — — . 
Ditto used as a common azimuth, 25 51 20 35 
Compass made by Adams, - 25 44 21 9 
Means, 25 34 20 57 W. 
Thus a change of place from the binnacle to a little before the centre 
of the ship, produced an alteration of 4 0 37' in the mean variation, the 
same way as, but a less quantity than Mr. Thistle had found it off 
the Start, when the ship's head was west. The true variation I judge 
to have been 23 0 , and that the observations on the booms showed 
2 0 too little, and those on the binnacle 2^* too much. The error in 
excess, upon the binnacle, appeared to continue so long as the ship 
was in the northern hemisphere and the head to the westward ; but 
it diminished gradually as we approached the equator, and the ob- 
servations on the binnacle and booms then nearly coincided. This 
example is sufficient to show the impropriety of allowing a variation 
upon the ship's course, from observations taken elsewhere than at 
the binnacle. 
