104f 



Limits of error 

 in obferving 

 bearings on 

 £bip-bJ3id. 



DIFFEREVCES IN THE MAGNETIC NEEDLE, 



Head E. Head SW b. S. 



April 15th, ' 

 5^ PM 



Tbcy may 

 amount to two 

 or even three 

 degrees. 



Refults (imilar 

 to thofe firft 

 fiated. 



The peaked 



hummock N 12" W. after tacking N 18» W 



Former ex- 

 treme, a 



projedion S 59 E - S 64- E 



— Naked fandy 



patch, dif- 



tant3| - N33 E - N 31 E. 



From fome little change of place after tacking the fliip, and 

 from the part whole bearing was fet not being perhaps the 

 individual fpot in both inftances, the difference between the 

 feparate bearings in any fet will not be always the fame: to 

 thefe caufes for error alfo may be added inacurracies in taking 

 the angles arifing from the motion of the (hip and compafs, 

 from the view of the objefl being obftru6ted by the rigging, 

 mafts, or ftiip's upper works, and from too much hafte to get 

 the bearings before the (hip's place was materially altered. 

 Even in the Table of azimuths and amplitudes greater accuracy 

 than one degree muft not be looked for; and in fliip-bearings 

 two or even three degrees is not, I believe, too great an 

 allowance for error, unlefs in very favourable circumftances. 



Without attending to fmall differences, it is evident that the 

 bearings correfpond with the obfervation in requiring a lefs 

 eaft variation to be applied when the fhip's head was eafterly, 

 aJid a greater when it was to (he weftward, in order to get at 

 the true direction of the objed *. When examining the north 



and 



* As afpecimen of the plan I followed in protrafting fuch bear- 

 ings as the above, take the fet of April 15, AM, when the true 

 variation appears to have been 4'> £. On the firll bearing the fliip's 

 head was fix points on one fide of the meridian, and on the fecond 

 it was three points on the other fide, the mean is one point and an 

 half on the eatt fide ; now for this one point and an half I allow 

 l" of error, which, as it is on the eaft fide of the meridian, and 

 the variation is catlerly, muft be fubtiafted : the variation then to 

 be allowed upon the mean between the bearings before and after 

 ♦acking will be 3" E, from which the true bearings will itand as 

 follows : 



April 



