A VOYAGE TO 
BO 
November Whoever imagines he can find the variation within a 
%- • degree, will very often fee himfelf much deceived. For, 
befides the imperfection which may be in the conftruCtion 
of the inftrument, or in the power of the needle, it is cer¬ 
tain that the motion of the fhip, or attraction of the iron¬ 
work, or fome other caufe not yet difcovered, will frequent¬ 
ly occafion far greater errors than this. That the variation 
may be found, with a fliare of accuracy more than fnffi- 
cient to determine the fhip’s courfe, is allowed ; but that it 
can be found fo exactly as to fix the longitude within a de¬ 
gree, or fixty miles, I abfolutely deny* 
CHAP* 
