102 On the Tmws of Terreilrial Magnetism 



reales^. The absolute cause of this corresponrlence between 

 them is unknown, as well as that of the aurora lorealis itself. 

 The influence of this meteor on the needle is commonly of short 

 continuance ; for, after being quickly agitated as long as this 

 manifests itself, the needle then returns to its usual position, and 

 proceeds again in the accustomed order of its motions : but it 

 also happens occasionally that it experiences a durable removal 

 from its jilace, some instances of which have been observed by 

 M. Cas-sini. 



To measure these variations, whether diurnal or annual, Cou- 

 lomb used a steel needle, suspended to an assen)blage (^t single 

 filaments of silk as taken from the natural investment ot the silk- 

 worm, and just sufiicient to suppdit it. At the two extremities 

 of the needle he fixed two metal arcs, carrying very fine circular 

 divisions. The whole apparatus was inclosed in a box glazed 

 above, in order that the needle might be observed without being 

 agitated by currents of air. Two microscopes fixed on the box, 

 and furnished with hair micrometers, could each be directed 

 to the divided arcs carried bv the needle, and thus showed its 

 smallest motions. M. de Prony used for the same purpose a 

 long magnetic needle, carrying on its length a telescope which 

 moved with it; the motion of which was observed on a distant 

 mark. M. Humboldt, who made a number of observations with 

 this apparatus, considers it verv exact. 



It is important, for the future progress of natural philosophy, 

 that we should determine with accuracy the present hitcnsity of 

 terrestrial magnetism, in the same manner as the weight of the 

 atmosphere and the actual temperature of diiferent climates have 

 been ascertained. By re|}eating the same observations after 

 some centuries, it would be known whether the magnetic force 

 varies in its energy, at it is certain it lias varied in its two di- 

 rections. 



The first method which presents itself to the mind, is to observe 

 the variation, the dip, and the intensity, immediately, by means 

 of three needles appropriated to this purpose, and then to pre- 

 serve them carefully for fiu'ther trials from time to time. As they 

 are liable to lose their magnetism in this interval, they might be 

 restored to the same state by a fresh process, using for this pur- 

 pose bars well couibined together, according to the method of 



* " The aurora borcalin is observed in Europe to Histiiri) the needle ex- 

 ceedinfjly; sometimes draw iui; ir several (lei^recs t'rnni its position. It is 

 alvviiys found to increase its deviation from llie meridian, and makes the 

 needle point more westerly. Tliis disturbance sometimes amounts to six or 

 seven degrees, and is generally observed to be greatest when the aurora 

 iurailis is most remarkable. No needle but a mas^netic one is alTected bv 

 the aurora borealis ; we may therefore conclude that there is some natural 

 tounexioii bet«ccij tliis n:ctcor aud magnetism." — Pt^ubison. 



the 



