v NOTES. 



isodynamic lines are those from which the most fruitful results are to be er- 

 pected. Researches into the amount of the magnetic force at different points 

 of the earth's surface, and graphical representations of the results by line* 

 drawn through the points where the force has an equal intensity, have shown, 

 that there are two foci or points of maximum force in each hemisphere, and 

 consequently four on the whole surface of the globe. The isodyuamic lines 

 which surround each of the two points of maximum in an hemisphere, are not 

 circles, but are of an ovate form, having the larger axis in a direction which, 

 if prolonged, would connect the two foci by the shortest line, or nearly so, which 

 can be drawn between them on the surface of the globe. As the ovala 

 successively recede from the focus they correspond to weaker and weaker 

 degrees of force, each in its turn enclosing the ovals of higher intensity. 

 This continues to be the case until the two systems ot ovals encounter in a 

 point intermediate between the foci : the isodynamic line which corresponds 

 to the force at this point has consequently the form of a figure of 8, each of 

 the loops enclosing a focus with its surrounding ovals : this form is called by 

 geometricians a lemniscate ; there is but one such isodynamic line in the 

 extra-tropical part of each hemisphere , and it separates the isodynamics of 

 higher intensity than itself which are within the loops, each surrounding a 

 single point of maximum only, from those which correspond to weaker degrees 

 of force than that of the L-mniscate, and are exterior to it : each of the 

 exterior isodynamics surrounds both the foci, but without meeting or crossing 

 in the point between them : their general form is that of parallelism with tho 

 external figure of the lemniscate, but the inflections which produce the doublo 

 loop become progressively less marked in the isodynamics of weakest force. 



If the two foci in an hemisphere were points of equal force, the ovals sur- 

 rounding e^ch would be similar in force and area, and the point at which ths 

 two systems would encounter each other would be half way between the foci. 

 Such, however, does not appear to be the case : the intensity at one of the 

 foci is greater than at the other : it is so in both hemispheres, and the ratio 

 of the force at the major and minor focus appears to be nearly the same in 

 both. The focus of greater intensity in the Northern hemisphere is in Ncrt 1 ! 

 America, where its position has been ascertained, by a recent survey conducted 

 by Captain Lefroy of the Royal (British) Artillery, to be in the vicinity of 

 the S.W. shores of Hudson's Bay in 52 of latitude, (Phil. Trans. 1840. 

 Art. xvii). The weaker focus, of which the approximate position has been de- 

 termined by MM. Hansteen, Erman, and Due, is in the north of Siberia 

 about 120 of East longitude from Greenwich. The middle of the lemniscate 

 which encloses both, or the point where its loops are connected, appears to bo 



