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XXXII. An Account of a Method of determining the Total 

 Intensity of the Earth's Magnetic Force in Absolute Measure. 

 By the Rev. H. Lloyd, D.D., President of the Royal Irish 

 Academy*. 



THE ordinary process for the determination of the earth's 

 magnetic force, it is well known, consists in observing 

 the time of vibration of a freely-suspended horizontal magnet, 

 whose moment of inertia is known ; and then employing the 

 same magnet to deflect another, similarly suspended, and ob- 

 serving the angle of deflection at a given distance. From 

 these two observations the horizontal component of the earth's 

 magnetic force is deduced; and the total force is thence in- 

 ferred, by multiplying by the secant of the inclination. 



This method is inapplicable in the high magnetic latitudes. 

 The relative error of the force, arising from a given error of 

 inclination, varies as the tangent of that angle ; and where 

 the inclination approaches 90°, it becomes so great as to ren- 

 der the result valueless. I was induced to consider the means 

 of supplying this defect, upon the occasion of the expedition of 

 Sir John Franklin to the Polar Sea in 1845 ; and I have been 

 recently led to re-examine the problem, on account of the two 

 Arctic expeditions, under Sir James Ross and Sir John Ri- 

 chardson, which are now in course of preparation. 



The object to be attained is to determine the total force 

 directly, without the intervention of its horizontal component. 

 The ordinary inclinometer will serve for this purpose. The 

 statical method, in which the position of the dipping-needle is 

 observed under the combined action of magnetism and gra- 

 vity f, will enable us to determine the product of the earth's 

 total magnetic force into the moment of free magnetism of 

 the needle ; and the ratio of the same quantities may be ob- 

 tained (as in the case of the horizontal component) by removing 

 this needle, and employing it to deflect another substituted in 

 its place. 



Let us suppose, for generality, that the needle moves in 

 any vertical plane, inclined to the plane of the magnetic meri- 

 dian by the angle « ; and let R denote the earth's magnetic 

 force, X and Y its horizontal and vertical components, and m 

 the magnetic moment of the needle. Then the effective mag- 



* From the Proceedings of the Royal Irish Academy, January 24, 1 848. 



f The principle of* this method appears to have been first suggested by 

 Mr. Christie, for the relative determination of the intensity; and it has been 

 since applied, under different modifications, by Mr. Fox and myself, to the 

 same purpose. Mr. Fox's mode of applying it, although not the simplest 

 in practice, is undoubtedly the best. 



