A Theory of the Variation of 

 the Magnetical Compafs. By 

 Mr. Ed. Halley, Fellow of the 

 Royal Society. 



TH E Variation of the Compafs (by which 

 I mean the Deflection of the Magnetical 

 Needle from the true Meridian) is of that great 

 Concernment in the Art of Navigation, that the 

 neglect thereof, does little Ids than render ufe- 

 lefs one of the nobleft Inventions Mankind ever 

 yet attained to. And for this caufe all Ships of 

 Confequence (efpecially thofe bound beyond the 

 Equator) carry with them Inftruments on pur- 

 pofe to obferve this Variation : That fo the 

 Courle iteer'd by the Compafs, may be reduc'd 

 to the true Courfe in refpect. of the Meridian. 



Now although the great utility that a perfect 

 Knowledge of the Theory of the Magnetical 

 Direction would afford to Mankind in general, 

 and efpecially to thofe concern'd in Sea Affairs, 

 feems as lufficient incitement to all Philofophical 

 and Mathematical Heads, to take under ferious 

 Confideration the feveral Phenomena* and to en- 

 deavour to reconcile them by fome general Rule : 

 Yet fo it is j that almoft all the Authors, from 

 whom a Difcourfe of this kind ought to have 

 been expected, pafs by in filence the Difficulties 

 they here Encounter. And thofe that mention 

 this Variation : By affirming it to proceed from 

 Caufes altogether uncertain (as are the caiual ly- 



