52 Baron Humboldt on Terrestrial Magnetism. 



German Ephemeris {Jahrhuchfiir 1836) proves graphically, 

 and by the parallelism of the smallest inflections of the horary 

 curves, the simultaneity of the perturbations at Milan and 

 Copenhagen, two cities having a diffei-ence of latitude of 10° 13'. 

 Mr. Gauss first made observations at the times which I pro- 

 posed in 1830, but with the intention of referring the angular 

 dimensions of magnetic declination to the smallest intervals of 

 time. (On the 7th of February 1834, alterations of six minutes 

 of the arc corresponded to a single minute of time.) INIr, Gauss 

 reduced the forty-four hours of simultaneous observations 

 to twenty-four hours ; and appointed six [seven ?] periods of 

 the year, viz. the last Saturday of each month consisting of 

 an uneven number of days, for the stations which are provided 

 with his new apparatus. The small magnetized bars which 

 he employs as magnetometers are of four pounds weight, and 

 the large ones of twenty-five pounds. The curious a2yparatus 

 of induction proper to render sensible and measurable the 

 oscillatory movements predicted by a theory founded upon 

 Mr. Faraday's admirable discovery, consists of two bars fast- 

 ened together, each of twenty- five pounds weight. I thought 

 it proper to mention the valuable labours of Mr. Gauss, in 

 order that those members of the Royal Society of London, 

 who have rendered most service to the study of terrestrial 

 magnetism, and who know the localities of the colonial esta- 

 blishments, may take into consideration whether bars of great 

 weight, provided with a mirroi', and suspended in a pavilion 

 carefully closed, should be employed in the new stations to be 

 established ; or whether Gambey's compass, hitherto uniformly 

 used in our present stations in Europe and Asia, should still 

 be employed. In discussing this question the advantages will 

 undoubtedly be estimated which, in the apparatus of Mr. 

 Gauss, arise from the smaller mobility of the bars by currents of 

 air, as well as from the facility and rapidity with which the 

 anoular divisions may be read in very short intervals of time. 

 Mv desire is only to see the line of magnetic stations extended, 

 whatever be the means by which the precision of the corre- 

 sponding observations may be attained. I ought also to men- 

 tion that two accomplished travellers, Messrs. Sartorius and 

 Listing, provided with very portable instruments of small di- 

 mensions, have very successfully employed the method of the 

 great geometrician of Gc)ttingen in their excursions to Na- 

 ples and in Sicily.* 



Your Royal Highness will, I hope, excuse the length of this 



• An abstract of a memoir by Prof. Gauss in which his apparatus and me- 

 thod of observation are fully described will be found in Lond. & Edinb. Phil. 

 Mag., vol. ii. p. 291, et seq. — Edit. 



