96 ON MAGNETISM. 



it must be stated that these diagrams are not, and could 

 not be, drawn simply from observations. They are 

 drawn from a theory (to be explained hereafter) founded 

 upon all the observations which could be collected be- 

 fore the preparation of the charts (published in 1840). 

 In general they represent very accurately the facts of 

 observation : but in later years some sensible but not 

 important inaccuracies have been discovered in the 

 southern hemisphere. 



In Figures 20 and 21, it is to be remarked that the 

 magnetic meridians might have been drawn through 

 any arbitrary points of the geographical equator; they 

 are in fact drawn through the points of east longitude 

 8, 18, 28, &c.: 8 and 188 being the points at which 

 the Magnetic Equator or line of no dip crosses the 

 geographical equator. The magnetic meridians cannot 

 generally be great circles of the sphere, because the 

 two Magnetic Poles through which all must pass are 

 not exactly opposite : they have moreover other irregu- 

 larities of form, which do not depend on the character 

 of the stereographic projection, but are equally con- 

 spicuous when the curves are traced on a globe. There 

 is no trace of more than one pole either in the north or 

 south. 



The form of the lines of equal dip is remarkable. 

 Commencing with the line of no dip or Magnetic 

 Equator, it is easily seen that it is not a great circle : its 

 greatest northerly distance from the geographical equa- 

 tor occurs at about 55 east longitude, or 47 from 

 its node (instead of 90), and its greatest southerly 



