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IX. — On the Determination of the Intensity of the Earth's Magnetic Force in ahso- 

 luteMeasure, by means of the Dip-Circle. By Humphret Lloyd, D.D.^ D.C.L.^ 

 F. R. SS. L. ^ E. ; V. P. R LA ; Corresponding Member of the Royal Society of 

 Sciences at Gottingen; Honorary Member of the American Philosophical So- 

 ciety, of the Batavian Society of Sciences, and of the Socieie de Physique et 

 d'Histoire Naturelle of Geneva, Sfc. ^x. 



Kead January 11, 1858. 



1 HE force exerted by the Earth upon a magnet is usually found by suspending 

 the bar horizontally, and observing its time of vibration. The result thence 

 obtained is the product of the horizontal component of the Earth's magnetic 

 force by the magnetic moment of the magnet ; and before we can know the value 

 of either of the factors which compose it, observation must furnish another re- 

 sult in which they are combined diiferently. This is effected, in the method 

 of Gadss, by using the same magnet to deflect another, similarly suspended, 

 and by observing the angles of deflection at known distances: this observa- 

 tion gives the ratio of the magnetic moment of the deflecting magnet to the 

 horizontal component of the Earth's force, and the two factors are therefore 

 absolutely known. 



This method, although much improved by the labours of Lamont and 

 others, has one insurmountable imperfection. The total force must be inferred 

 from its horizontal component, by multiplying by the secant of the inclination ; 

 the relative error of the deduced force, arising from a given error of incli- 

 nation, varies, therefore, as the tangent of that angle, and when the inclination 

 approaches to 90° it becomes very considerable. The method is, accordingly, 

 unsuited to the high magnetic latitudes. 



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