236 Intelligence and Miscellaneous Articles. 



render the rock distinctly columnar. At the eastern end of the 

 mountain the stone is so close in texture as often almost to resemble 

 chert. In the next two quarries westward the rock is coarser, and 

 its jointing less regular. In the most westerly quarry the stone is 

 generally fresher in appearance, closer in grain, and greener in. 

 colour. All these stones are probably modifications of the same 

 original rock. Prom the chemical analysis of the rocks the author 

 concludes that, supposing them all to have had originally the same 

 composition as the unaltered rock in the most westerly quarry, that 

 at the extreme east of the mountain has lost about 3 per cent, of 

 silica, and the others have received respectively an increase of 1-35 

 and 0*77 per cent, of silica. The altered rocks contain an abun- 

 dance of quartz granules, due probably to the crystallization of pro- 

 gressively dissociated silica, as the specimens of rock in which these 

 granules occur do not contain a larger proportion of silica than 

 those in which its presence can hardly be detected under the 

 microscope. The proportion of alkalies in the different specimens 

 does not materially vary, 



Overlying the second quarry at the east end of the mountain is an 

 ash bed of reddish brown colour, containing more than 10 per cent, 

 of protoxide of manganese and nearly 20 per cent, of peroxide of 

 iron, and showing a great diminution in the percentage of silica 

 when compared with the associated crystalline rock. 



The author further described the characters of the uralite-por- 

 phyry of the Mawddach valley near Dolgelly, which is of a greyish- 

 green colour, spotted with black, and consists of a granular base 

 enclosing patches and crystals of uralite, the outlines of which are 

 sometimes sharp and well defined, but generally rounded and merging 

 into the general base. 



XXXII. Intelligence and Miscellaneous Articles. 



ON THE DETERMINATION OF THE POLAR DISTANCE IN MAGNETS. 

 BY R. BENOIT. 



T)OUILLET had the notion of making use of the equation of equili- 

 •*- brium of a magnetized bar placed in given conditions (into which 

 equation its polar distance enters), in order to determine the pre- 

 cise place occupied by each of the poles *. The following method is 

 both more direct and more general than that which he employed. 



Let AB, A'B' be two magnetized bars placed horizontally one 

 above the other so that their centres O, O' are in the same vertical 

 line. The first is fixed ; the second can turn freely about the axis 

 0'. As long as the distance separating them is sufficient, the 

 resultants of the reciprocal actions of their magnetic elements pass 

 very evidently through the poles A, B, A', B'. Now these actions 

 give rise (1) to symmetrical vertical forces destroyed by the weight 

 and the resistance of the suspending wire, and (2) to a horizontal 

 couple which tends to cause the movable magnet to rotate, and of 

 which the expression is easily found. 



Let m, m' denote the quantities of free magnetism which we may 

 * Comptes Rendits de TAcadhnie des Sciences, Feb. 5, 18P5. 



