28 



TER RESTRl AL MAG NE TISM. 



[Sect. II 



results of the three eleaients finally corrected are entered 

 each m its proper geographical position, on maps on a 

 large scale, severally appropriated to the force, the in- 

 clination, and the declination. Each result has a small 

 characteristic mark denoting the ohservcr. Wlien ain 



portion of the globe is sufficiently covered hy the results 



of observations in proper distribution, the isopheenomena] 

 lines are drawn for that portion of the globe in eorre- 

 spondence with the observations, with a free hand, but with 

 a careful judgment, aided occasionally by a process of 

 calculation Vvhich it is not necessary here to describe. 

 From, those maps tables of double entry are formed, 

 having tlie latitude at the side, and the longitude along 

 the top of the page, and the values of the magnetic ele- 

 merits corrosponciing to the several latitudes and longi- 

 tudes are placed at the points of inLersection. By proper 

 car;- m tiie process, the step of iornung the tables from the 

 maps need involve no additional uncertainty whatsoever. 



M 



iB thus prepared will, when completed, 

 form an experimental theory of terrestrial magnetism, 

 in which the facts of nature will he shown with greater 



Sj in proportion as the observations are 



or less 



tne 



numerous, correct, and suitably distributed, and as they 



i 



are more or less corroctly represented in the maps. 

 Mathematical formula, based on general mathematical 

 views, having uuniorical coeificients of which the values 

 are derived from these maps, m.ay also serve for the com- 



gnetic elements at any geographical 



position on the surface of the globe ; and if the points 

 taken from the maps to serve as the basis of the numeri- 

 r*al valueB of the coefficients are sufficiently numerou 



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