496 Geology and Magnetism. 



the substance of the bar, and again to issue from its former outlet, 

 as exhibited by the arrows in the above diagram, it is reasonable 

 to suppose that the fluid will not be compressed more than the 

 transverse size of the bar will require. Taking this simple prin- 

 ciple of action as a guide, with its various consequnces under 

 different circumstances, we shall soon perceive that we not on- 

 ly account for the various phenomena of geology, but, in a word, all 

 phenomena connected with terrestrial physics ; and that we are 

 enabled also to reason from the known to the unknown, and actually 

 to predict facts before trial, not merely to satisfy curiosity, but 

 questions of practical utility, especially in mining. Indeed, theories 

 are not worthy of attention unless they can be fairly demonstrat- 

 ed and rendered practically useful." 



The phenomena of terrestrial magnetism is then exempli- 

 fied by Mr. Hopkins, by supposing an iron bar magnet to be 

 placed in a wooden globe, the poles of the magnet corres- 

 ponding with those of the earth. 



"In placing needles round this globe, we find them arranging 

 themselves in the exact order which they keep on the terrestrial 

 globe, both as to dip and direction; and as there is nothing to 

 disturb them on the wooden globe, their positions in the curves 

 of convergence are uniform ; their focus of convergence being equal 

 to the transverse area of the magnetic axis at both ends. 



We find in measuring the force along the magnetic curves, that it 

 varies inversely as the distance ; allowing for the influence of the 

 earth's magnetism and tracing the effects in a horizontal plane, we ob- 

 tain the following law, viz. that the force varies inversely as the square 

 of the distance from the surface of the ball in the equatorial plane, and 

 also in the meridian from the poles towards the equator ; being the 

 necessary consequences of the expansion and compression of the 

 magnetic fluid surrounding it, as shown in the sketches. This 

 is precisely the law of what is called gravitation, but the meri- 

 dional variation towards the poles of the earth has been ascribed to 

 the effect of a centrifugal force produced by the earth's rotation. 

 The assumed centrifugal effect of the earth's rotation is also consi- 



