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LXIII. Illustrations of a Method for computing Magnetic 

 Declination^ on the principle proposed by Professor Gauss. 

 By Samuel Beswick*. 



THROUGH the kind suggestion of the Astronomer 

 Royal, I have been induced to examine attentively the 

 elegant and profound Essay of Professor Gauss on the subject 

 of Terrestrial Magnetism. For some time I have used a 

 method for computing magnetic declination, differing from 

 the one proposed by this eminent magnetician, though the 

 involved principle is precisely identical: but this fact was 

 wholly unknown until very recently. The method referred 

 to has been applied with varied success to more than three 

 hundred places, their localities being distributed over the four 

 continents and the principal oceans. A specimen of its ap- 

 plication to Greenwich, its antipode Sydney, and St. Helena, 

 is presented in this communication for the examination of 

 your readers. The principle of Prof. Gauss's method for 

 computing magnetic declination is given in these words (Tay- 

 lor's Scientific Memoirs, vol. ii.) : — 



" Consequently there are on the earth only two magnetic 

 poleSf apart from the possible case of local exception spoken 

 of in art. 13."— P. 223. 



" The two extreme values of V correspond in this point of 

 view to two poifits, inclosed by the zones, at which the hori- 

 zontal force is =0, and where therefore the whole magnetic 

 force can only be vertical ; these points are termed the magnetic 

 poles of the earth." — P. 195. 



" We proceed to develope the mode of subtnitting them to 

 calcidation. On the surface of the earth V becomes a simple 

 function of two variable magnitudes, for which we will take 

 the geographical longitude reckoned eastward from an arbi- 

 trary first meridian^ — and the distance from the north pole of 

 the earth (a complement of geographical latitude)." — P. 199. 



*' Resolving the horizontal magnetic force into two portions, 

 one of which, X, acts in the direction of the geographical 

 meridian, and the other, Y, perpendicularly to that meridian, 

 — and considering X as positive when directed towards the 

 north, and Y as positive when directed towards the west." — 

 P. 200. 



I propose to give a demonstration of the above method, 

 essentially differing from the one given by the Professor, and 

 such as 1 have used for years. 



* Communicated by the Author. 



