112 COSMOS. 



spots which are situated near one another never experience 

 the shocks simultaneously. 84 Is it possible that certain mag- 

 netic intersecting lines may by their intervention oppose all 

 further transmission ? 



We have here described the regular and the apparently 

 irregular motions presented by horizontally suspended 

 needles. If by an examination of the normal recurring- 

 motion of the needle we have been enabled from the mean 

 numbers of the extremes of the horary variations to ascer 

 tain the direction of the magnetic meridian, in which the 

 needle has vibrated equally to either side, from one solstice 

 to another, the comparison of the angles which the magnetic 

 meridian describes at different parallels with the geographi- 

 cal meridian has led in the first place to the knowledge of 

 lines of variation of strikingly heterogeneous value (Andrea 

 Bianco, in 1436, and Alonzo de Santa Cruz, cosmographer 

 to the Emperor Charles V., even attempted to lay down 

 these lines upon charts) ; and more recently to the success- 

 ful generalization of isogonic curves, lines of equal variation, 

 which British seamen have long been in the habit of grate- 

 fully designating by the historical name of Halleys lines. 

 Among the variously curved and differently arranged closed 

 systems of isogonic lines, which are sometimes almost parallel, 

 and more rarely re-enter themselves so as to form oval 

 closed systems, the greatest attention in a physical point 

 of view is due to those lines, on which the variation is null, 

 and on both sides of which variations of opposite denomina- 

 tions prevail, which increase unequally with the distance. 85 

 I have already elsewhere shown how the first discovery 

 made by Columbus on the 13th of September, 1492, of a 

 line of no variation in the Atlantic Ocean, gave an impetus 

 to the study of terrestrial magnetism, which, however, con- 

 tinued for two centuries and a half to be directed solely to 

 the discovery of better methods for obtaining the ship's 

 reckoning. 



However much the higher scientific education of mariners 

 in recent times and the improvement of instruments and 

 methods of observation have extended our knowledge of 



84 Cosmos, voL i, p. 208. 



85 Op. cit. vol. i, pp. 187189; vol. ii, pp. 657 659 and pp. 52 CO 

 of the present volume. 



