Ivi NOTES. 



to the value of researches on he tterrestrial magnetic force, because the de- 

 terminations which are now made will be comparable with those which may 

 be made at future epochs. The unit of force in this scale is that amount of 

 magnetic force which, acting on the unit of mass through the unit of time, gene- 

 rates in it the unit of velocity. Taking the units respectively, as a grain, a 

 second, and a foot in British measure, the ratio of the force at the major 

 focus in North America determined by Captain Lefroy's survey is 13'9 ; 

 at the minor focus in Siberia, from the Observations of MM. Ilanstecn and 

 Due, it is 13-3 ; and at St. Helena, which is nearly on the line of least intensity, 

 and at its weakest par!",, and where cor.sc-quertly the force is nearly a 

 minimum on the surface of the globe, its value is 6'4. The observations 

 made in Sir James Clark Ross's expedition, referred to in page 175 of the 

 text, and which appear to have been made nearly in the centre of the highest 

 isodynamie oval, give 15'6 as the approximate value of the force at the major 

 focus in the southern hemisphere j whilst the observations of the same 

 voyage render it probable that the value at the minor focus does not much 

 exceed or much fall short of 14'9, which is in the same ratio to 15'6, as 

 13'3 to 13'9. It has been already noticed that the two foci are nearer to 

 each other in the south than in the north : assuming the general charge of 

 the two magnetic hemispheres to be the same, the greater proximity of the 

 two points of maximum in the south might readily be imagined to aug- 

 ment the force at both, whilst in the opposite geographical longitudes of 

 the hemisphere the intensity would be less than in the analogous positions 

 in the north : the isodynamie maps shew that such is the case, and give 

 reason to believe, that the charge of the two magnetic hemispheres may 

 not differ in the aggregate, although the distribution of the force at the 

 surface of the globe is not precisely similar. 



The view which we are now enabled to take of the magnetic system of 

 the globe, by means of the knowledge which we have acquired of the mag- 

 neto-dynamic variations at its surface, furnishes an explanation of many features 

 in the isoclinal and isogonic lines which had previously occasioned per- 

 plexity. We now perceive why the higher isoclinals should be ellipses 

 instead of circles, and why there should be inflection* at opposite points of 

 the ellipse nearer to one of its extremities than to the other, causing the 

 Jsoclinal line to resemble in its general form a curve inclosing a lemniscate, 

 of which one of the loops should be larger than the other. This remarka- 

 ble conformation of the isoclinal lines has been well described by Erman, 

 ia Posy;. Ann. der Phys. vol. rd. We now see also a confirmation of the 



