TERRESTRIAL MAGNETISM. 227 



equator twice (in longitudes 6° and 186°) ; its points most di- 

 stant from the equator being in about 15° north and 15° south 

 latitude : and further that in the northern hemisphere there are 

 two minima, but in the southern hemisphere but one maximum. 

 According to a cui'sory computation, these minima and this 

 maximum are 



- 209-1 in 55° N. lat., 263° Ion. 



- 200-0 in 71° N.lat., 116° Ion. 

 + 277-7 in 70° S. lat., 154° Ion. 



These values are founded on the unity of our elements, and 

 must therefore be multiplied by 0-0034941 to obtain their ex- 

 pression in absolute measure. 



33. 



It has been already said that our elements are to be regarded 

 only as a first approximation. So considered, their agreement 

 with the obsei-vations in Art. 29 is sufficiently satisfactory. It 

 cannot be doubted that a much greater agreement would be ob- 

 tained by an improved calculation, even with these observations. 

 The only difficulty of such a calculation is its length, which 

 would be still alarmingly great, even supposing it abridged by 

 the introduction of such skilful methods as have been employed 

 by astronomers in correcting the elements of the planetary and 

 cometary paths. Although this difficulty might be easily sur- 

 mounted by dividing the work amongst a number of computers, it 

 does not ajjpear advisable to undertake such an amended calcu- 

 lation at present, when there is still so little certainty in the data 

 from a great number of places which it would be important to 

 employ. It will be preferable in the first place to pursue further 

 the comparison of the elements with observations, whence the 

 means will be afforded of giving much greater certainty to the 

 general maps, than has been hitherto possible by the exclusively 

 empirical mode. 



We may be allowed to give a few glances at the future pro- 

 gress of the theory, the perfect realization of which may indeed 

 be far distant. 



34. 



For the satisfactory refinement and completion of the ele- 

 ments, it will be requisite to make much higher demands than 

 have been hitherto complied with, as to the data furnished by 

 obsenation. Their accuracy at all the points employed ought 



