MAGNETIC VARIATION. 117 



• 



Passing to the consideration of the phenomena observed 

 in the variation of the magnetic needle, we must first notice 

 its alterations in respect to the different hours of the night 

 and day, the different seasons of the year, and the mean 

 annual values; next, in respect to the influence which the 

 extraordinary, although periodically recurring disturbances, 

 and the magnetic position, north or south of the equ^or, 

 exert on these alterations ; and, finally, in respect to the dif- 

 ferent lines passing through the terrestrial points at which 

 the variation is equal, or even null. These linear relations 

 are certainly most important in respect to the direct prac- 

 tical application of their results to the ship's reckoning, and 

 to navigation generally ; but all the cosmical phenomena of 

 magnetism, among which we must place those extraordinary 

 and most mysterious disturbances which often act simultane- 

 ously at very remote distances (magnetic storms), are so in- 

 timately connected with one another, that no single one of 

 them can be neglected in our attempt gradually to complete 

 the mathematical theory of terrestrial magnetism. 



In the middle latitudes, throughout the whole northern 

 magnetic hemisphere (the terrestrial spheroid being assumed 

 to be divided through the magnetic equator), the north end 

 of the magnetic needle — that is to say, the end which points 

 toward the north pole — is most closely in the direction of 

 that pole about 8h. 15m. A.M. The needle moves from east 

 to west from this hour till about Ih. 45m. P.H., at which 

 time it attains its most westerly position. This motion 

 westward is general, and occurs at all places in the northern 

 hemisphere, whether they have a western variation — as the 

 whole of Europe, Pekin, Nertschinsk, and Toronto — or an 

 eastern variation, like Kasan, Sitka (in Russian Ameri- 

 caj, Washington, Marmato (New Granada), and Payta, on 

 the Peruvian coast.* From this most westerly point, at 



Needle at London, 1722-1723, by Mr. George Graham). The change 

 of the variation depends "neither upon heat nor cold, dry or moist 

 air. The variation is greatest between 12 and 4 in the afternoon, and 

 the least at 6 or 7 in the evening." These, however, are not the true 

 turning hours. 



* Proofs of this are afforded by numerous observations of George 

 Fuss and Kowanko ; at the observatory in the Greek convent at Pekin ; 

 by Anikin at Nertschinsk ; by Buchanan Riddell at Toronto, in Cana- 

 da (all these being places of western variation) ; by Kupffer and Si- 

 monoff at Kasan ; by Wrangle, notwithstanding the many disturb- 

 ances from the Aurora Borealis at Sitka, on the northwest coast of 

 America ; by Gilliss at Washington ; by Boussingault at Marmato, in 

 South America ; and by Duperrey at Payta, on the Peruvian shores 



