I. The 



I3IPFERENCES IN THE MAGSETXC NEED Lfi. 107 



' In the latter of thefe obfervations, the differences arifing 



from a change In the diredlion of the fliip's head is lefs confi- 



derable than in the higher latitudes ; indeed, on approaching 



the line of no variation upon the fouth coaft, the differences in 



the variation were fiBaller than before and afterwards; but 



that thefe differences fliall be greater in a large variation and 



fmaller In a lefs, both places being equally diftant from the 



magnetic pole, I will not venture to alferf. The inferences Generallnfe 



that I think may be fafely drawn from the above obfervations compa'fs wa 



are as follows : ift. That there was a difference in the diredion found to deviate 



of the magnetic needle on board the Inveftigator when the *"°!^.'",^'y ^^^ 

 n . , , , . ^"C (hip s head 



inip s head pointed to theeafl, and when it was direfted weft- waseafterly or 

 ward. 2d. That this difference was eafterly when the fhip's ^efterly; the 



.,, •.. n inii- n north end being 



head was pointed to the wefr, and welterly when it was eaft. farther from the 

 '3d. That when the (hip's head was north or fouth the needle '^'P'* l^""^ «^i*" 

 took the fame diredion or nearly fo that it would on (bore; and deviation was 

 fhewed a variation from the true meridian, which was nearly proportional to 

 the medium between what it fliowed when eaft and when weft, [jj^ hearfrom 

 4th. That the error in variation was nearly proportionate to the N. or S. 

 the number of points which the fliip's head was from the north 

 or fouth. Conftant employment upon prafilce has not allowed 

 me to become much acquainted with theories, but the little 

 information I have upon the fubjeft of magnetifm has led me 

 to form fome notion concerning the caufe of thefe differences, 

 and although moft probably vague and unfcientific, I truft for 

 the candour of the learned in fubmitting it^ as well as the 

 inferences above drawn, to their judgment, 



Jft. I fuppofe the attradive power of the different bodies in Theory pro. 

 a fliip, which are capable of affefling the compafs, to be col- ^j] \^^ 

 lefled into fomething like a focal point or center of gravity, the fliip afts 

 and that this point is nearly in the center of the fliip where the ''•'« »"« «> ^gH«» 

 Ihot are depofited, for here the greateft quantity of iron is col- 

 lected together. 



2d. I fuppofe this point to be endued with the fame having a diffe- 

 kind of attradion as the pole of the hemifphere where ^^"jin°'r 'as 'the 

 the fliip is; confequently, in New Holland the fouth end Ihip is near the 

 of the needle would be at traced by it and the north end ^- *"■ S- P"'e «f 



^ the terreftrid) 

 repelled. • masaetifm, 



3d. That the attra<5tive power of this point is fufiiciently 



fiiong in a fhip of war to interfere with the action of the 



3 magnetic 



That 

 iron in 



