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The author begins by an account of some experiments which he 

 instituted with a view to discover the cause of the irregularities in 

 the indications of the intensity of terrestrial magnetism given by the 

 vibrating magnetic needle. By inclosing the needle in a box sur- 

 rounded with water at different temperatures, the number of the vi- 

 brations did not appear to be affected by these differences of tempe- 

 rature when the heat was applied equally on all sides j but when par- 

 tially applied, irregular oscillations took place, apparently from the 

 currents of air set in motion by the inequalities of its temperature. 

 Hence the author recommends that for delicate experiments the 

 magnetic needle should be contained in a box of wood, or other im- 

 perfect conductor of heat ; or, for still greater security, that it should 

 be adjusted in a glass vessel exhausted of air. For experiments on 

 magnetic intensity at sea, he recommends placing two magnets at 

 some distance from the needle, in the line of its magnetic meridian, 

 and surrounded with water, in order to preserve a uniformity of tem- 

 perature. For increasing the action of terrestrial magnetism, he 

 suggests the employment of a bar or cylinder of wrought iron, placed 

 perpendicularly, or in the line of the dip, at right angles to the me- 

 ridian, so as to repel the north pole of the needle : and also sur- 

 rounded with water. 



The experiments made with an apparatus of this kind in some of 

 the deep mines in Conwall, did not lead to the conclusion that there 

 is any increase of magnetic intensity at the depth of 1000 or 1200 

 feet below the level of the sea ; but if any thing, rather the reverse j 

 but, on the whole, the discrepancy in the results was so great, that 

 no dependence can be placed on them as establishing a general fact 

 of this importance. 



It appeared also to the author that the direction of electrical cur- 

 rents under the earth's surface is greatly diversified ; although, when 

 taken collectively, the probability is that the tendency of the positive 

 currents is from east to west. 



The author then proceeds to state the results of his experiments 

 on the thermo-electricity of rocks. He found that compact slate was 

 an excellent conductor of electricity; and that the heated end gave 

 indications of positive electricity. Granite, on the contrary, at a 

 bright red heat, was almost incapable of conducting electricity, but 

 when vitrified became nearly a perfect conductor, owing probably to 

 the destruction of its crystalline structure. In general the end most 

 heated was negative, and the same was the case with porphyritic 

 feldspar. Greenstone and serpentine, which also occur in frequent 

 alternations in Cornwall, in like manner differ in their electrical pro- 

 perties ; the former giving out positive, and the latter negative elec- 

 tricity at their most heated parts. Many anomalies, however, oc- 

 curred in these properties, the results being frequently reversed 

 without any obvious cause. 



On the hypothesis of the existence of a very elevated tempera- 

 ture in the interior of the globe, it would necessarily follow from 

 the preceding experiments that electrical currents would be produced 

 from this cause, taking frequently different, and even opposite di- 



