174 COSMOS. 



the horizontal deviation from the terrestrial meridian of the 

 spot. Their combined action may, therefore, be graphically 

 represented by three systems of lines, the isodynamic^ isoclinic, 

 and isogonic (or those of equal force, equal inclination, and 

 equal declination). The distances apart and the relative posi- 

 tions of these moving, oscillating, and advancing curves, do 

 not always remain the same. The total deviation (variation or 

 declination of the magnetic needle) has not at all changed, or, 

 at any rate, not in any appreciable degree during a whole 

 century, at any particular point on the earth's surface,* as, 

 for instance, the western part of the Antilles, or Spitsbergen. 

 In like manner, we observe that the isogonic curves, when 

 they pass in their secular motion from the surface of the sea 

 to a continent or an island of considerable extent, continue 

 for a long time in the same position, and become inflected as 

 they advance. 



These gradual changes in the forms assumed by the lines 

 in their transiatory motions, and which so unequally modify 

 the amount of eastern and western declination, in the course 

 of time render it difficult to trace the transitions and ana- 

 logies of forms in the graphic representations belonging to 

 different centuries. Each branch of a curve has its history, 

 but this history does not reach farther back amongst the 

 nations of the West than the memorable epoch of the 1 3th of 

 September, 1492, when the re-discoverer of the New World 

 found a line of no variation 3 west of the meridian of the 

 Island of Flores, one of the Azores.f The whole of Europe, ex- 



* A very slow secular progression, or a local invariability of the mag 

 netic declination, prevents the confusion which might arise from terres 

 trial influences in the boundaries of land, when, with an utter disregard 

 for the correction of declin tion, estates are, after long intervals, 

 measured by the mere application of the compass. " The whole mass ot 

 West Indian property," says Sir John Herschel, " has been saved from 

 the bottomless pit of endless litigation by the invariability of the mag- 

 netic declination in Jamaica and the surrounding Archipelago during 

 the whole of the last century ; all surveys of property there having been 

 conducted solely by the compass." See Robertson, in the Philosophical 

 Transactions for 1806, part ii. p. 348, On the permanency of the Com- 

 pass in Jamaica since 1660. In the mother country (England) the 

 magnetic declination has varied by fully 14 during that period. 



f I have elsewhere shown that, from the documents which have come 

 down to us regarding the voyages of Columbus, we can, with much 

 certainty, fix upon three places in the Atlantic line qfno declination 



