144 TERRESTRIAL MAGNETISM. 



Auroras at Toronto, were synchronous with magnetic 

 storms at Kerguelen Island in the southern hemisphere, 

 but not at Hobarton. Considering the all-pervading 

 character of magnetic as well as gravitating force in all 

 matter, it seems indeed difficult to form a clear concep- 

 tion of any obstacles to the propagation of magnetic 

 force through the globe, analogous to those which im- 

 pede the undulations of sound, or those which intercept 

 earthquake waves, so that at some nearly adjacent places 

 the earthquake shocks have never been known to occur 

 simultaneously. ( 184 ) May it be that the intersection of 

 magnetic impulses may oppose their propagation ? 



I have described both the regular and the seemingly 

 irregular movements presented by horizontally-sus- 

 pended needles. Having examined the regular periodi- 

 cally-recurring march of the needle, and found the mean 

 direction around, or, in other words, on either side of 

 which the oscillations have taken place say from one 

 solstice to the return of the same solstice in the follow- 

 ing year this mean direction is the magnetic meridian 

 of the place for the given year. The comparison of the 

 angle made by the magnetic meridians of different 

 places on the globe with their geographical meridians, 

 first led to the recognition of " variation-" (i. e. declina- 

 tion-) lines of strikingly different values (which Andrea 

 Bianco in 1436, and Alonso de Santa Cruz, the cosmo- 

 grapher of the Emperor Charles V., thus early attempted 

 to enter on charts); and subsequently to the happily 

 devised generalisation of isogonic curves, or lines of equal 

 declination, to which the grateful recollection of English 

 navigators long gave the historic name of "Halleyan 

 lines." Among these lines of various curvature, running 



