JJQ DIFFERENCES IN THE MAGNETIC NEEDLE. 



Thefe obfervations, particularly thofe of July 28, feem to 

 be decifive in fhowing that the variation is more weflerly 

 when taken upon the binnacle of a fliip whofe head is weft- 

 ward in north latitude, than when obferved in the center of 

 the (hip, which is a ftrong confirmation of the fuppofitions 

 before given ; but the obfervations on the change of the fhip's 

 head are too few to be fatisfaftory. Almoft every fea officer 

 can tell whether he has obferved the variation of the compafs 

 to be greater when going down the EngliQi Channel thaa 

 when coming up it: and indeed it would be very eafy for a 

 fliip lying in harbour to afcertain the point beyond controverfy. 

 Remarks and Should this point be well eftabli(hed, f think it would follow, 

 SSeTaw? "^^^ ^'■^"^ ^ ^'S'^ ^°"''^ latitude where the differences are great 

 on one fide, they are moft likely to decreafe gradually to the 

 equator, and to increafe in the fame way to a bigii north lati- 

 tude, where they are great on the other fide; thus the fmaller 

 differences on the north coaft of New Holland will be accounted 

 for. I ftiall leave it to the learned on the fubjedof magnetifm 

 to compare the obfervations here given with thofe made by 

 others in different parts of the earth, and to form from them 

 an hypothefis that may embrace the whole of the phenomena : 

 the opinion I have ventured to offer is merely the vague con- 

 jecture of one who does not profefs to umlerfiand the fubject. 

 Inftances of the Some account of the magnetifm of Pier Head, upon the eaft 

 aSed bylLal^^^^ ^^ ^^^'^ Holland, may not perhaps be thought an unap- 

 magnetifm. propriate conclufion to this Paper. I was induced to attend to 

 this from the following palTage in Hawkesworth, Vol, III. 

 p. 126. •' At fun-rife I went afliore," fays Captain Cook, 

 Pier Head. '* and climbing a confiderabie hill," Pier Head, '* I took a 

 view of the coaft and the ifiands that lie off it, with their 

 bearings having an azimuth compafs with me for (hat purpofe; 

 but I oblerved that ihe needle differed very confiderably in 

 its pofition, even to thirty degrees, in fome places more, in 

 others lefs ; and once I found it differ from itfelf no lefs 

 than two poin(s in the diftance of fourteen feet*. I took up 



fome 



as far as 50' 



Local deviation '" In a (at of angles taken near the head of Arnhem north baj-, 

 ''S^^yj'^lt^^ on the vireft fide of the gulph of Carpentaria, I found the needle 

 of the theodolite had been drawn 50° from its proper direction. 

 The fliore confifted of grains of iron ore caked ir,to a fcony mafs j 

 and a piece of it, when cipplitd to the needle, ch-evv it fix or eight 



decrees 



