62 TERRESTRIAL MAGNETISM. 



knowledge of terrestrial magnetism, an important catalogue 

 of southern stars, which it may be remarked, in passing, was 

 the first star- catalogue undertaken since the combination of 

 telescopes with measuring apparatus introduced by Morin 

 and Gascoigue.( 65 ) 



As the close of the 17th century had been marked by 

 progress towards a better knowledge of the position of the 

 declination lines, and by the first theoretical attempt to 

 determine their points of convergence as magnetic poles, so 

 the first half of the 18th century produced the discovery of 

 the horary periodic variation of the declination. Graham, in 

 London, in 1722, has the uncontested merit of being the 

 first to observe these variations with accuracy and perseve- 

 rance. Celsius and Hiorter, at Upsala, who were in episto- 

 lary communication with Graham, ( 66 ) further enlarged the 

 knowledge of the phsenomena in question. It was not until 

 the latter' part of the century, in 3784 1788, that Brug- 

 mauns and Coulomb, the latter gifted with a more mathe- 

 matical mind, penetrated more deeply into the essence 

 of terrestrial magnetism. Their acutely devised physical 

 experiments embraced the magnetic attraction of all matter, 

 the distribution of the force in a bar-magnet of a given 

 form, and the law of magnetic action at a distance. In the 

 methods adopted by them for obtaining exact results, they 

 sometimes employed the vibrations of a horizontal needle 

 suspended by a thread, and sometimes deflections measured 

 by a torsion balance. 



Science is indebted for the first knowledge of the variation 

 in the intensity of the earth's magnetic force at different points 

 of its surface, obtained by the vibration of a needle suspended 

 vertically and placed in the magnetic meridian, solely to the 



