11(> THIRD REPORT — 1833. 



direction and extent of chains of movmtains, and probably by 

 their geological structure. 



These normal lines may, to a certain extent, agree with the 

 lines of equal dip, which have already been delineated upon 

 some charts. In Churchman's charts they are represented in 

 the positions they would have on Euler's hypothesis of the earth 

 having two magnetic poles. The only use, however, of such 

 hypothetical representations is, that by comparison with actual 

 observation they become tests of the correctness of the theory, 

 or they may point out the modifications which it requires, in 

 order that it may accord with observation. In Professor Hans- 

 teen's chart the hnes of equal dip are projected from observa- 

 tions reduced to the year 1780. Considering how very deficient 

 we are, even now, in correct observations of the dip, I should 

 not be disposed to place much reliance upon the accuracy of 

 these lines, particularly where they cross great extents of sea 

 aflbrding no points of land necessary for observations of the dip. 

 Of these lines of equal dip the most important is the magnetic 

 equator, or that line on the earth at which the dipping needle 

 would be horizontal. The observations eivina^ this result can 

 of course be but few, and are therefore very inadequate for the 

 correct representation of this line. In order to obviate this 

 difficulty, M. Morlet made use of all observations not very re- 

 mote from the equator, determining the distance of that line 

 from the place of observation by means of the law, that the 

 tangent of the magnetic latitude is half the tangent of the dip, 

 which is derived from the hypothesis of two magnetic poles near 

 to the centre of the earth. By this means the position of the 

 equator was determined throughout its whole extent; and a 

 surprising agreement was fovind between the determinations of 

 each point by means of different observations, which shows 

 that, within certain limits near the equator, the hypothesis very 

 correctly represents the observations. This line exhibits in- 

 flections in its course which have been attributed, and probably 

 with justice, to the physical constitution of the surface in their 

 vicinity*. It has been considered also that a general resem- 

 blance exists between the isothermal lines and the lines of equal 

 dip on the surface of the earth f. 



All the lines to which I have here referred have been liitherto 

 represented on a plane, either on the stereographical, the glo- 

 bular, or Mercator's projection. Mr. Barlow has, however, 

 very lately represented the lines of equal variation on a globe, 

 from a great mass of the most recent documents connected with 



» Biot, Traite de Physique. 



f Hansteen, Edinhtrgh fhilosophical Journal, vol, iii. p. 127. 



