Cape de Verde IJlands, by night or day, being •well affured, with that
variation, that you are to the Eajlward of them. Another, in his
Chart *, lays down this variation ninety leagues to the
Weftward of them. Such a difagreement as this, is a ftrong
proof of the uncertainty of both.. However, I have no.
doubt, the former found here, as well as in other places,
the variation he mentions. But he ihould have confidered,
that at fea, nay even on land, the refults of the moft accurate
obfervations will not always be the fame. Different
compaffes will give different variations ; and even the fame
compafs will differ from itfelf two degrees, without our being
able .to difcover, much lefs to remove, the caufe.
Whoever imagines he can find the variation within a
degree, will very often fee himfelf much deceived. For,
befides the imperfet&ion which may be in the conflruition
of the inftrument, or in the power , of the needle, it is certain
that the motion of the fhip, or attraction of the ironwork,
or fome other caufe not yet difeovered, will frequently
occafion far greater errors than this.. That the variation
may be found, with a fhare of accuracy more than fufficient
to determine the ihip’s eourfe, is allowed; but that it can
be found fo exactly as to fix the longitude within a degree,
or fixty miles, I ahfolutely deny.
* Mr. Dun.
CHA P .
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