JQO DJTFBRENCES IN THl MAGNETIC NEEDLE. 



P. S. I negle6led to mention the following circumftance in 

 the body of the letter. The drawing inftrument, or wordle, 

 is made of flee! ; and is it not probable that this tool, polTef- 

 fing a flight degree of magnetifm given to it by fri6lion or 

 otherwife, affifts in producing the neceflary arrangement, by 

 acting upon healed and flender wires, while their molecules 

 are in a violent motion from the preffure of the inftrument it- 

 felf? This fuppofition has fome claim to plauiibility ; becaufe 

 a weak magnet will impart a portion of the fame virtue to a 

 bar of tempered fteel, the particles of which are in a ftate of 

 vibration ; for a rod of this metal will acquire a degree of po- 

 larity, provided it be firuck on the end with a hammer when ' 

 its axis lies parallel to the dipping-needle. 



III. 



Concerning the Differences in the magnetic Needle, on Board 

 the Invefiigator, arifing from an Alteration in ihe DireBion of 

 the Ship's Head. By Matthew Flinders, Efq. Com- 

 niandtr of his Majefiy's Ship Invefiigator. From the Philofo" 

 phical Tranfudions, 1805. 



The magnetic W HILST furveylng along the fouth coaft of New Holland, 

 needle is af. j,^ ,3^3^ ^,jj ^^^^ I.obferved a confiderable difference in the 



fectcd at lea . ' 



by the pofition diredtion of the magnetic needle, when there was no other 

 of the fliip's apparent caufe for it than that of the (liip's head being in a 

 different diredion. This occafioned much perplexity in laying 

 down the bearings, and in allowing a proper variation upon 

 them, and put me under the neceffity of endeavouring to find 

 out fome method of corre6ling or allowing for thefe diffe- 

 rences; for unlefs this could be done, many errors mufl una- 

 voidably gel admiffion into the chart, . I firft removed two 

 guns into the hold, which had flood near the compaffes, and 

 afterward* fixed the furveying compafs exa6tly a-midfhips 

 fcarcely from Upon the binnacle, for at firfl it was occafionally fliifted to the 

 5«k[°" °" weather fide as the ftiip went about ; but neither of thefe two 

 arrangements produced any material effed in remedying the 

 difagreements. 



The following table contains the obfervations for the varia^ 

 tion of the compafs in which the differences are mofi remark- 

 able, and from which I fliall beg to point put fuch inferences 

 as I think may be drawn fronxthem. 



TABLE^ 



