TERRESTRIAL MAGNE.ISM. 181 



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 po- 

 sitions 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 aU 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 Spitzbergen. 

 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 hnes 

 in their translatory 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 analogies 

 of forms in the graphic representations belonging to different 

 centuries. Each branch of a curve has its history, but this 

 history does not reach further back among the nations of the 

 West than the memorable epoch of the 13th 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. t The whole of Europe, excepting a small 



* 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 declination, estates are, after long intervals, rieas- 

 ured by the mere application of the compass. " The whole mass of 

 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 Philo- 

 sophical Transactions for 1806, Part ii., p. 348, On the Permanency of 

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

 the magnetic declination has varied by fully 14° during that period. 



t 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 of no declina- 

 tion for the l'3th of September, 1492, the 21st of May, 1496, and the 

 16th of August, 1498. The Atlantic line of no declination at that pe- 

 riod ran from northeast to southwest. It then touched the South 

 American continent a little east of Cape Codera, while it is now ob" 

 served to reach that continent on the northern coast of the Brtizik 

 (Humboldt, Examen Critique dr VHisi. de la Geogr., t. iii., p. 44 A!h \ 



