304 Variations of the Terrestrial Magnetiim 



compare together the intensities observed in these dlf- 

 tert-nt places, the anomalies they exhibit announce in 

 some measure those which the inclination ought to ex- 

 perience. 



The cause of these anomalies becomes evident from what 

 has been already remarked ; they are merely the effect of 

 local circumstances, and arise from the small systems of 

 attraction hv which the general phajnomcna are modified. 

 This must be sensilile in particular for that part of America 

 which .M. Mnmboldl travelled over, and which is traversed 

 throughout its whole length by the grand chain of the Cor- 

 dillera of the Andes. It is also in these places that the 

 most considerable differences exist. Popayan, for example, 

 is situated near the volcanoes of Sotara and Pourace ; it is 

 joined to basaltic mountains abounding with magnetic iron. 

 Near Sulumito, to the east of Popayan, these basaltic co- 

 lumns have verv striking poles : in like manner Mexico is 

 situated at the height ol' I 160 toiscs on the ridge of the 

 cTand Cordillera of Lenschtitlan : the ground there is co- 

 vered with porous basaltcs and amygdaloids, which are 

 almost all cliarged with magncnc iron. Must not all these 

 causes have a sensible influence on the inclination of the 

 magnetic needle ; and must not the diiTerent dispositions of 

 the ferrnoinons masses, or their change of stale, in conse- 

 cjuence of the action of nature, produce also variations ? 

 M. Himi])oldt made on this point a decisive observation : 

 the earthquake of the 4th of November 1799 changed at 

 (ainiana the inclination of the needle. Otx the 1st of No- 

 vember it was 43" 63' ; on the 7th it. was otdy 42" 75' ; and 

 leu months after it returned to 42° 83' : but it did not re- 

 gain its former value ; the intensity of the magnetic force 

 tvas not chan«red bv the effect of this earthquake. 



k is proved, then, bv these observations, that local cir- 

 cumstances mav have on the inclination a sensible influ- 

 ence; and this influence is remarked in the countries tra- 

 versed bv M. ITumboldt*. 



It a])pears, therefore, that the mathematical hvpothcsis 

 which \vc have emploved rcallv expresses the law of nature 

 at least to the north of the n'.agnetic equator ; for, though the 

 first results observed towards the south seem to bend to it also, 

 the nncertaintv under which we are in regard to the true 

 c.uisL- of tfiese ])lupnomena must slop our conjectures, and 



■• We can (il)sen'e that the anomalies arc sensible in particular in the 

 iskiids. — Aoit t-y ihi- Juttiun uj Uie Memoir. 



prevent 



