T E R 



T E R 



at intensity was established. Ait>-r that tint* extent* ve 

 term at observations on the dip and intensity were nude 



mmboldt in South America, and in lYance, Italy, 

 and Germany; by HKII-II. . I in,-, and Knii.ni in the 

 north of Euroi* and in Siberia, and by the last-incu- 



\ gentleman in the Pacific Ocean. .. loncl 



8abinr during liu voyages to tin- polar was and the equa~ 



Afhca and America ; I i the Kuwiaii ad- 



.-. igalion of the id by 



captaiiu King and Kit/roy in (lie survey of the coasts of 

 : and now, no observations on terrestrial 

 magnetism are considered complete unless all tin- tl.iv. 

 Uinn nln the declination, the inclination, and tin- inteii- 

 I determined at the same time at every -uinm. 



-iy to the time when Hmnholdt made his 

 niagnclic.il observation.- in South A>m n. . tlu> opinion 

 iienuty was a miiiinrimi at places when- tin- di]> 

 of the needle wan rero prevailed : and that philosopher, in 

 consequence, assumed unity as the measure of the inten- 

 sity at a certain station in Peru, where the dipping-needle 

 assumed a horizontal position. From the times in which 

 a certain number of vibrations were made l>y siu-h needle 

 at that station, and subsequently at Paris. Huiniioldt 

 found that the intensity at the latter place was equal to 



_' (that in Peru being unity) ; and M. Arago and t.'ol. 

 Sabine afterwards, by a comparison of experiments made 

 by themselves, ascertained that the inten.-ity in London 

 was (.in 1827) expressed by 1-372. The scale of intensity 

 which had been assumed by Humboldt continues to be 



. though it is now known that the zero of dip does not 



correspond to the minimum of intensity, and it becomes 



necessary to cmplo\ term- less than unity - the 



\ :it many "place., within und even I . uiud the 



L The intensity is subject to a beeular \iination; 



but of the amount and the law uition as yet no 



certain knowledge has been 



In the subjoin. .\luc-h, like the former, is 



a stereographical projeetion of the northern and southern 

 IsMiisphimn of the earth, are represented the pnncipal 

 uudynainical lin. i <(iial int. n the table 



and el y Col. Sabine in tin: Seventh Report of 



the British Association.' That chart ! from all 



the authenticated experiments on the intensity which had 

 been made between the years 1798 and lM3>i : but III the 

 cut, in order to avoid contiiMon. there are ii>en only the 

 lino eun.->|jondiiiLr to the intensity represented by 1, l''J. 

 ! 1. PO, 17. anil the points at which the inteiiMt v l^ Known 

 ID ! 1-s. which is the hi ..'.i. In all 



the tract between the curve lines marked ). the magnetic 

 intensity is less than unity: the breadth of this tract is 

 very unequal ; at points in long. 110" and ~*\' llir \\ . 

 the lines approaeh within :i or 4 degrees of one another, 

 while near the meridian c-.f (Greenwich they are separatid 

 by an interval of about 50 degrees. In the' middle of this 

 band of low intensity which surrounds the earth, it might 

 be expected that the intensity should he the lowest ; and. 

 in fact. M. Krman, in his leturn tci Kurope. found, at a 

 few di -'Wind of St. Helena, thai the intensity 



was 0743, which is the lowest u-t obtained from otw 

 tion. 



i no 







. A si 



thev 



m 



Some of the isodynamic lines in both hemispheres are 



>1ill wanting at places 

 !>osed to ])ass, but no doubt 

 .es which return into 

 MI will show that in each 

 ith two loops, or 

 .'mispherc. judg- 

 perceived that i!e western 

 ; ist be at a spot 

 .?7V(WW.'. and latitude about 

 >i Siberian 

 'ude is nn- 

 sill readily 



''''' '''' -!i. i -i h. mil ->\.< . ; i- cui > es, 



or. assume more 



hv estimation in 

 and m ln-itr. iM"' = l'3i" \\ . lat. W : the !'<. 



latti-r in the soul hern 

 prt 



!' 



o ! :ind is about i:i dt ..- 



the nearest distance in longitude between the Siberian and 

 the South American pole is about 120 denrc"s : the nearest 

 distances in latitude are about 11(1" and about 130" n 

 lively, so that those poles are not diametrically opposite to 



'(her. 



The intensity of magnetism at the New Holland 

 appears to be nearly ecjunl to that which has be> 

 at the North American pole, the observed intensity in 

 Van IJiemcn's Land nnil at New York being 1-S: and the 



the Siberian pole is nearly eipi.n to that : I 

 South Pkeifio Jiole, the observed intensity at Vilnisk in 

 Siberia being 1-70'. and the highest at present l<no 



rth 1'aeilic l-.eing |-~. The fact that these last in- 

 tensiti iha.ii the lonner is indicated by the loops 



about them being smaller than those about the two western 

 and Krman thinks it probable that the centres 

 nf imi'.nniie attraction, when' the loopy are small, may 

 depth below the surface than thev 

 . i/n titer 



'ronger and one ol w. 

 i nf the hemispheres on the north and 

 south oi the terrestria; it maybe infeircd, as i* 



