C 348 3 
XVIIL Observations on the Permanency of the Variation of the 
Compass at Jamaica. In a Letter from Mr. James Robertson 
to the Right Hon . Sir Joseph Banks, K. B. P. R . S. &c. 
Read June 12, 1806. 
SIR, 
As any improvement, or discovery in the arts and sciences, 
will, I am persuaded, experience your favourable reception, 
I have the honour of submitting to your consideration a dis- 
covery I have made on a subject, the state of which can only 
be ascertained by observations made from time to time, as it 
is not regulated by any known law of nature: I mean the 
variation of the magnetical needle. 
This discovery may not only excite others to make, and 
repeat, observations in different parts of the globe, but, by 
causing this changeable quality to be better understood, may 
contribute to the benefit of navigation, and commerce, as well 
as to the advancement of a more particular knowledge of the 
subject. 
It has hitherto been considered, that the variation of the 
magnetical needle is not fixed in any particular place, but is 
constantly varying, in a greater or a less degree, in all parts 
of the world. I have discovered an exception to this supposed 
general property of variation ; and, as it may be, perhaps, the 
first that has been made, it will require proportionally strong 
