XI. ON THE DETERMINATION OF THE TOTAL INTENSITY 

 OF THE EARTH'S MAGNETIC FORCE IN ABSOLUTE 

 MEASURE, BY MEANS OF THE DIP-CIRCLE. 



Transactions of the Royal Irish Academy. Vol. XXIII. 



THE 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 result in which they are combined differently. 

 This is effected, in the method of (rauss, by using the same 

 magnet to deflect another, similarly suspended, and by observing 

 the angles of deflection at known distances : this observation 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 

 Lament 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 inclination, 

 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. 



I was induced to consider the means of supplying this defect 

 some years ago, upon the occasion of the Arctic Expeditions of 

 1845 and 1848 ; and I then suggested another process, by which 



