112 



magnetic latitudes. This conversion, indispens- 

 able for the study of the complicated laws of 

 magnetism, is, on the contrary, very problema- 

 tic, when we compare observations of the mag- 

 netic dip made in meridians very remote from 

 each other ; and when we consider the magnetic 

 equator as a great circle, without inflection, and 

 without irregularity of curve. 



Notwithstanding the considerable degree of 

 perfection, which Mitchell and Nairne had at- 

 tained in the construction of dipping needles, 

 these instruments, before the year 1791, had not 

 reached that degree of exactness,lwhich they 

 have now attained. If La Caille, Dalryrnple, 

 Cook, Bayly, and Lord Mulgrave have succeed- 

 ed in obtaining excellent results, it is because 

 these able observers made numerous compara- 

 tive experiments, and took the average of a 

 great number. The compasses employed in La 

 Perouse's expedition were those made use of by 

 Captain Cook in his last voyage round the 

 world. We must suppose, that these instru- 

 ments were out of order, or of difficult use : for 

 the dips observed on board the Astrolabe often 

 differ five, six, or eight degrees from those ob- 

 tained the same day on board the Boussole. 

 This uncertainty had induced the celebrated 

 Borda to apply himself, conjointly with Mr. Le 

 Noir, to the improvement of the dipping needle. 

 This navigator, to whom astronomy is indebted 



