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LIX. Remarks on an important Error in a Tallefor computing 

 local Attraction, circulated ly Order of the Board of Longi- 

 tude, 



To Mr. Tillach. 

 . Deal, May 1, 1820. 



Sir, — A YOUNG nautical friend of mine, whose perseverance 

 in acquiring a complete knowledge of the scientific principles of 

 his profession is only equalled by his diffidence and modesty, 

 consulted me a few days back on a difficulty he had met with in 

 the practical application of the table alluded to in the head of 

 this article ; and I soon perceived that it was not one that could 

 be removed, as it arose out of some erroneous principle employed 

 in the computation of the numbers in question ; and as I con- 

 ceive it to be of tlie highest importance that no error of this kind 

 should be allowed to slide unnoticed into our nautical practice, 

 I have been induced to draw up the following remarks, which 

 mav be the means of checking the evil that might otherwise arise 

 out of the use of this table. 



The subject of local magnetism has of late attracted the at- 

 tention of many of our most eminent navigators and philoso- 

 phers, and various observations have accordingly been made in 

 different quarters of the globe, the results of wliich show that in 

 the northern hemisphere, the north end of the needle is drawn 

 towai'ds the vessel, and in the southern hemisjjhere, the south 

 end ; or we may say generally, that the end of the needle which 

 dips is always attracted bv the iron of the vessel, it appears 

 also by the observations of Captain Flinders, that the effect in- 

 creases in both hemispheres as the dip increases; a fact which 

 has been amply verified by the experiments of Captain Ross in his 

 recent vovage to the arctic regions. 



In consequence of this change of effect in parting north and 

 south from the magnetic equator, it seemed to follow of necessity, 

 that the attraction vanished entirely where the dip is zero ; and 

 such is the principle laid down by Captain Flinders, who how- 

 ever had obviously failed in establishing the law of increase of 

 effect in different latitudes. 



The recent voyages of discovery towards the north having 

 thrown additional light on this subject. Dr. Young, the learned 

 Secretary to the Board of Longitude, undertook to establish a 

 new law of increase, and computed a table for the use of navi- 

 gators, which is published or circulated h\ order of the menibers 

 of that board, wliereby the correction for local attraction is re- 

 duced to a simple logarithmic operation, the dip being Jirst as- 

 certained. It is to this table I am desirous of calling the atten- 

 tion of your nautical readers. 



We 



