TERRESTRIAL MAGNETISM. 93 



exclusively for the two points on the earth's surface at 

 which the horizontal force vanishes ; because (as has been 

 already remarked) these points, in which the maxima of 

 force are not situated, have occasionally, in modern times, 

 been confounded with the four points of maximum intensity 

 of the earth's force. ( 9 ) Gauss has also shown that it is 

 objectionable to give the name of "magnetic axis of the 

 earth" to the cord connecting the points above-named, at 

 which the inclination of the needle at the surface of the 

 globe is 90. ( 91 ) The various points and lines which 

 have been thus noticed are so connected with each other, 

 that we are enabled to describe the complicated phenomena 

 of terrestrial magnetism under three heads only, or as 

 belonging to three " elements" or manifestations of one and 

 the same activity, or force ; termed respectively, magnetic 

 "force," "inclination," and "declination." 



Intensity of the Magnetic Force. 



The recognition and examination of this, the most 

 important element of terrestrial magnetism, by the direct 

 measurement of the strength of the total force, did not 

 follow, until after considerable delay, the recognition and 

 examination of the direction of the same force measured in 

 the horizontal and vertical planes (declination and inclina- 

 tion). Experiments in which a magnetic needle is made to 

 vibrate for the purpose of inferring from the duration of the 

 vibrations the intensity of the magnetic force, were first 

 brought into use towards the close of the last century: 

 during the present century they have been constantly and 

 earnestly prosecuted. Graham, in 1723, measured the 



