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January 24th, 1848. 



REV. HUMPHREY LLOYD, D. D., President, 

 in the Chair. 



The Rev. Richard Mac Donnell, D. D., having been called 

 to the Chair, the President communicated an account of a 

 method of determining the total intensity of the earth's mag- 

 netic force in absolute measure, applicable in the high magnetic 

 latitudes. 



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 

 Richardson, 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- 



VOL. IV. F 



