358 Letter from Von Humboldt to Lalande. 



here fee that the magnetic power is fo different, that between 

 Paris and Cumana it decreafed from 245 to 229 vibrations in 

 ten minutes, though it does not decreafe with the inchna- 

 tion. This decreafe cannot certainly be afcribed to any 

 change in tlie goodr.efs of the needle, or to other accidental 

 caufes : for the fame needle made in the fame time at Paris 

 245 vibrations; at Girona, 23a; at Barcelona, 245; at Va- 

 lentia, 235 ; and, after a journey of feveral months, gave at 

 the fame places exaftly the fame number of vibrations as 

 before my departure. Thefe are always the fame, in the 

 open fields, in the houfe, or in a cavern, fo that the mag- 

 netic power in anv place is always the fame, and remains a 

 long time without change ; and feems to be a general power, 

 like that of gravity. 



I had the mortification of not being able at fea to make 

 any good obfervations of the declination. Notwithftandin^ 

 all the trouble T took, I could not find an azimuth compaft 

 on which I could depend within 40'. This is the true reafon 

 why I have made no mention to you of the declination at 

 fea. The point, however, where the inclination vanifiies, 

 certainly lies further to the weft than marked in Lambert's 

 chart, in the Berlin aftronomical almanac for 1779. A good 

 obfervation, made in the year 1775 aboard an Englifli fhip 

 from Liverpool, places this vanifliing point in latitude 29" 

 north, long. 66^ 40' weft. I obferved, with great care, the 

 declination at two places on the coaft of America with a 

 compafs by Lenoir, in which the needle is fufpended by a 

 •thread, according to the method of Prony and Von Zach. 

 At noon, 06lober 1799, it was at Cumana, and twenty 

 'leagues further eaft, 4* 13' 45" eaft : at Caripe, the chief 

 place of the Capuchin miflion among the Chaimas and 

 Caribs_, 3^ 15' eaft. 



During the earthquake at Cumana, on the 4th of No- 

 vember 1799, the inclination of the magnetic needle was 

 altered, but not the declination. The former, before the 

 fliock, was 44"'20 of the new divifion ; after the ftiock it 

 remairied at 43''-35. The number of the vibrations of the 

 dipping needle, however, was the fame as before, viz. 229 in 

 ten minutes. This, united to other obfervations, feems in my 

 opinion to prove, that during the earthquake this fmall part 

 of the ball of the earth was changed, and not the needle; 

 for, in diftrifts where no figns of an earthquake ever appeared 

 in the primitive chain of accumulated mali'es of granite, the 

 inclination was as great after as before. 



On account of the intereft which you take in every thing 



that 



