. 
“ 
’ 
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 tision, with an external solid crust of some forty or fifty miles 
in thickriess: 
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, [ propose in this paper to confine my remarks to the 
bearings of the theory on the — connected with the 
Variation of the declination of the needle. — a 
‘he observations of two or three centuries oat derieallvets, 
_ asis 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 
canses, and have a like period. 
Without going at all into the question of the mature of the 
magnetic forces, it isa 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 
