86 



COSMOS. 



the electro-magnetic activity is not limited to the gravitating 

 matter on our own planet. To adopt a different hypothesis 

 would be to limit cosmical views with arbitrary dogmatism. 

 Coulomb's hypothesis regarding the influence of the mag- 

 netic sun on the magnetic earth is not at variance with anal- 

 ogies based upon the observation of facts. 



If we now proceed to the purely objective representation 

 of the magnetic phenomena which are exhibited by our 

 planet on different parts of its surface, and in its different 

 positions in relation to tjie central body, we must accurately 

 distinguish, in the numerical results of our measurements, 

 the alterations which are comprised within short or very 

 long periods. All are dependent on one another, and in this 

 dependence they reciprocally intensify, or partially neutral- 

 ize and disturb each other, as the wave-circles in moving 

 fluids intersect one another. Twelve objects here present 

 themselves most prominently to our consideration. 



Two magnetic poles, which are unequally distant from the 

 poles of rotation of the earth, and are situated one in each 

 hemisphere ; these are points of our terrestrial spheroid at 

 which the magnetic inclination is equal to 90, and at which, 

 therefore, the horizontal force vanishes. 



The magnetic equator, the curve on which the inclination 

 of the needle nrO. 



The lines of equal declination, and those on which the dec- 

 lination (isogonic lines and lines of no variation). 



The lines of equal inclination (isoclinal lines). 



The four points of greatest intensity of the magnetic force, 

 two of unequal intensity in each hemisphere. 



The lines of equal terrestrial force (isodynamic lines). 



The undulating line which connects together on each me- 

 ridian the points of the weakest intensity of the terrestrial 

 force, and which has sometimes been designated as a dynamic 

 equator* This undulating line does not coincide either with 

 the geographical or the magnetic equator. 



The limitation of the zone where the intensity is generally 

 very weak, and in which the horary alterations of the mag- 



* See Mrs. Somerville's short but lucid description of terrestrial 

 magnetism, based upon Sabine's works ^Physical Geography, vol. ii., p. 

 102). Sir James Ross, who intersecte'd the curve of lowest intensity 

 in his great Antarctic expedition, December, 1839, in 19 S. lat. and 

 29 13' W. long., and who has the great merit of having first determ- 

 ined its position in the southern hemisphere, calls it " the equator of 

 less intensity." See his Voyage to the Southern and Antarctic Regions, 

 vol. i., p. 22. 



