BOOK XII. 



MAGNETISM. 



Recent Progress of Terrestrial Magnetisv., 



TX Chapter II., I Lave noticed the history of Terrestrial Magnetism ; 

 *- Hansteen's map published in 1819; the discovery of "magnetic 

 storms" about 1825; the chain of associated magnetic observations, 

 suggested by M. de Humboldt, and promoted by the British Associa- 

 tion and the Royal Society ; the demand for the continuation of these 

 till 1848; the magnetic observations made in several voyages; the 

 magnetic surveys of various countries. And I have spoken also of 

 Gauss's theory of Terrestrial Magnetism, and his directions and re- 

 quirements concerning the observations to be made. I may add a few 

 words with regard to the more recent progress of the subject. 



The magnetic observations made over large portions of the Earth's 

 surface by various persons, and on the Ocean by British officers, have 

 been transmitted to Woolwich, where they have been employed by 

 General Sabine in constructing magnetic maps of the Earth for the 

 year 1840. 1 Following the course of inquiry described in the part of 

 the history referred to, these maps exhibit the declination, inclination, 

 and intensity of the magnetic force at every point of the earth's sur- 

 face. The curves which mark equal amounts of each of these three 

 elements (the lines of equal declination, inclination, and force : the 

 isoffonal, the isoclinal, and the isodynamic lines,) are, in their general 

 form, complex and irregular ; and it has been made a matter of ques- 

 tion (the facts being agreed upon) whether it be more proper to say 

 that they indicate four poles, as Halley and as Hansteen said, or only 

 two poles, as Gauss asserts. The matter appears to become more clear 

 if we draw magnetic meridians ; that is, lines obtained by following 

 the directions, or pointings, of the magnetic needle to the north or to 



1 These maps are published in Mr. Keith Johnstone's Physical Atlas. 



