Secular Variations of the Magnetic Needle. 69 



By the theory of the earth, I mean that theory which supposes 

 the present condition of our globe to be the result of a primitive 

 solution of its materials by caloric and a subsequent cooling pro- 

 cess ; by far the greater portion of the earth being still in a state 

 of fusion, with an external solid crust of some forty or fifty miles 

 in thickness. 



With the direct evidence of the truth of this theory, the read- 

 ers of the Journal of Science are of course well acquainted. 

 Passing by this direct evidence, and the other numerous and 

 satisfactory applications of the theory to the solution of physical 

 problems, I propose in this paper to confine my remarks to the 

 bearings of the theory on the phenomena connected with the 

 variation of the declination of the needle. 



The observations of two or three centuries past demonstrate, 

 as is well known, the gradual westerly motion of the line of no 

 declination, at a rate which if uniform will complete an entire 

 revolution in about seven hundred years. The variations in the 

 position of the horizontal and dipping needles at any point on 

 the earth's surface, are doubtless dependent on the same physical 

 causes, and have a like period. 



Without going at all into the question of the nature of the 

 magnetic forces, it is a truth which we may take for granted in 

 the outset, that the position of the magnetic line at any place, 

 (that is, the position of the magnetic needle freely suspended by 

 its centre of gravity,) is the result of the combined action of all 

 the magnetic forces in the mass of the earth, whatever the nature 

 of these forces may be. This combined action may on familiar 

 dynamical principles be resolved into the two sets of magnetic 

 forces, namely, those contained in the solid crust of the earth, 

 and those exerted by the internal fluid mass. Considering the 

 former set by themselves, the needle freely suspended would 

 take the direction of the resultant (A) of all the magnetic influ- 

 ences in the solid crust. Considering the latter set by them- 

 selves, the needle would take the direction of the resultant (B) 

 of all the magnetic influences in the internal fluid mass. The 

 actual position of the dipping needle at any given time and place, 

 is in the direction of the diagonal between these two resultants. 



Now adopting for the present as true, the hypothesis that the 

 internal fluid mass has in reference to the external crust a west- 

 erly revolution once in about 700 years, it would seem that all 



