Geology and Magnetism. 497 



dered as the cause of the earth being an oblate spheroid. In the 

 first place, a globe constituted and placed under similar conditions 

 as our earth is, with its enveloping fluids, placed under the influence 

 of centripetal and centrifugal forces alone, according to the well- 

 known laws of physics, would not, nor could not, produce the obser- 

 ved figure, much less the observed variation of attraction towards 

 the poles. Its magnetism alone, without rotation, must necessarily 

 compress the poles, and also cause the inverse law of intensity in 

 the enveloping fluids." 



Passing over the proof Mr. Hopkins affords of magnetism 

 alone being sufficient to account for the oblate spheroidal 

 figure of the earth, and of its universal agency, we arrive at 

 the following evidence of the polarity of the atmosphere. 



" If the globe be a magnet, we ought to observe at the poles some 

 indications of the convergence of the air towards the poles, i. e. 

 something similar to the inverted conical appearances which we 

 observe in the ingredients at the poles of an artificial magnet, Plate 

 I. It is true that the air is an invisible substance, yet, as it oc- 

 casionally becomes saturated with visible fluids, it would, under 

 such condition, with the advantage of reflected light, which may 

 also be expected to vary according to the curves formed by the 

 currents, show the phenomena of convergences. This fact we have 

 in the aurora borealis and australis. These are the luminous ap- 

 pearances seen in the atmosphere connected with the poles of our 

 earth, the general appearances of which correspond to the curves 

 of convergence towards the poles. When these luminous pheno- 

 mena display unusual brightness and activity, the magnetic needle 

 is also found very fluctuating, both in dip and direction ; and also 

 the mercury in the barometer is subject to similar action ; this coin- 

 cidence of the variable movements indicates that they are produced 

 by the same cause, viz. the disturbance of the equilibrium of the 

 magnetic curves, or tension of the fluid. 



The aurora is not the cause of the needle being disturbed, nor 

 are its luminous rays required to produce a rise or fall of the mer- 

 cury in the barometer; on the contrary, the whole phenomena 



