TERRESTRIAL. MAGNETISM. 18/ 



only sLx complete in the three elements. In these comparisons 

 we find in the dip differences of 13° between calculation and ob- 

 servation*. 



If these differences are greater than are admissible in a satis- 

 factory theory, one cannot avoid drawing the conclusion, that the 

 magnetic conditions of the earth are not such as to admit of re- 

 presentation by means of a concentration in either one or two 

 infinitely small magnets. It is not denied that with a greater 

 number of such fictitious magnets, a sufficient agreement might 

 be ultimately attainable ; but how far such a mode of soUing 

 the problem might be advisable is quite a different question. The 

 calcidations are extremely laborious even with two magnets; 

 with an increased number they would probably present insuper- 

 able difficulties. It will be best to abandon entirely this mode of 

 proceeding, which reminds one involuntarily of the attempts to 

 explain the planetary motions by continued accumulation of epi- 

 cycles. 



In the present treatise it is my pui-pose to develope the gene- 

 ral theoiy of terrestrial magnetism independently of all particu- 

 lar hypotheses as to the distribution of the magnetic fluids in 

 the body of the earth ; and to communicate the results which I 

 have obtained from the first application of the method. Imper- 

 fect as these results must be, they give an idea of what may be 

 hoped for in future, when trustworthy and complete observations 

 from all parts of the earth shall be obtained, and employed in 

 renewed and more refined attempts. 



1. 



The force which at each part of the earth imparts a certain 

 direction to a magnetic needle suspended by its centre of gravity, 

 (supposing it free from all extraneous influence, such, for ex- 

 ample, as that of another artificial magnet, or the conductor of a 

 galvanic current,) is termed the earth's magnetic force, in so far 

 as the source whence it is derived is to be sought for in the earth 

 itself. It may indeed be doubted, whether the seat of the proxi- 

 mate causes of the regular and irregular changes which are 

 hourly taking place in this force, may not be regarded as exter- 



* In the declination there is even a difference in one instance of 29 degrees ; 

 but it is proper to estimate the error of the calculation, not by the number of 

 degrees of declination, but by the true angular difference between the calcu- 

 lated and observed directions, which in the case in question is 1I§ degree*. 



