178 COSMOS. 



node near tlie Island of St. Thomas has moved 4 from east to 

 west between 1825 and 1837. It would be extremely import- 

 ant to know whether the opposite pole near the Gilbert Islands 

 in the South Sea has approached the meridian of the Caro- 

 linas in a westerly direction. These general remarks will 

 be sufficient to connect the different systems of isoclinic non- 

 parallel lines with the great phenomenon of equilibrium which 

 is manifested in the magnetic equator. It is no small ad- 

 vantage, in the exposition of the laws of terrestrial magnetism, 

 that the magnetic equator (whose oscillatory change of form, and 

 whose nodal motion exercise an influence on the inclination of 

 the needle in the remotest districts of the world, in consequence 

 of the altered magnetic latitudes, ')* should traverse the ocean 

 throughout its whole course, excepting about one-fifth, and 

 consequently be made so much more accessible, owing to the 

 remarkable relations in space between the sea and land, and to 

 the means of which we are now possessed for determining 

 with much exactness both the declination and the inclination 

 at sea. 



We have described the distribution of magnetism on the 

 surface of our planet according to the two forms of declination 

 and inclination ; it now therefore remains for us to speak of 

 the intensity of the force which is graphically expressed by iso- 

 dynamic curves (or lines of equal intensity.) The investigation 

 and measurement of this force by the oscillations of a verti- 

 cal or horizontal needle, have only excited a general and 

 lively interest in its telluric relations since the beginning of 

 the nineteenth century. The application of delicate optical 

 and chronometrical instruments has rendered the measur^- 

 ment of this horizontal power susceptible of a degree of accu- 

 racy far surpassing that attained in any other magnetic 

 determinations. The isogonic lines are the more important in 

 their immediate application to navigation, whilst we find from 

 the most recent views that isodynamic lines, especially those 

 which indicate the horizontal force, are the most valuable 

 elements in the theory of terrestrial magnetism.f One of 



* Humboldt, Ueber die seculdre . Verdnderung der magnetischen 

 Inclination, (On the secular Change in the Magnetic Inclination) in 

 Pogg. Annal.. bd. xv. s. 322. 



t Gauss, Resultatf. der Beob. des magn. Vereim, 1838, 21 : 

 Report on tJte Variations of the Magnetic Intensity, p. 68. 



