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affording, when adjusted to the latitude of the place, sufficient mag- 

 netic power to render the effects distinct, he substituted for it an 

 artificial imitation, consisting of two magnets six inches long, so 

 placed, with respect to a revolving axis parallel to the axis of the 

 earth, as to imitate the position of the poles produced by thermo- 

 magnetism in his plate ; and making the apparatus revolve round 

 this axis, noticed the deviations produced thereby on a compass 

 placed horizontally over it. These deviations he then compares at 

 length with those actually observed, Istly, by Lieutenant Hood, in 

 1821, at Fort Enterprise, lat. 64 28' N.; 2ndly, by Canton in Lon- 

 don, in 1759 ; 3rdly, by Lieutenant Foster at Port Bowen, in 1825; 

 4thly, by Colonel Beaufoy at Bushy Heath, in 1820. The results of 

 this comparison are on the whole generally such as to indicate a con- 

 formity between the hypothesis and fact, with the exception of some 

 deviations from the exact times of maximum and minimum variation, 

 which could not but be expected. Those observations which afford 

 the least support to the hypothesis are those at Port Bowen ; but the 

 case is so extreme, with a dip of 88, that the author does not re- 

 gard them as essentially opposed to it, as modifying circumstances 

 must here have an overpowering influence. 



The author then considers the manner in which the distribution of 

 land and sea over the globe modifies the point of greatest heat, and 

 in consequence the place of the diurnal poles. He then observes, 

 that at the commencement of his experiments he had no idea of be- 

 ing able to reduce the deviations of the needle to so simple a law as 

 that resulting from a polarity in a particular direction, communicated 

 to the plate, but that he considered it of the greatest consequence to 

 ascertain whether the deviations at the outer edge of his plate had 

 the same general character with those within, at the line of junction 

 of the metals ; since these situations of the needle would correspond 

 to great elevations in the atmosphere and points near the earth's 

 surface, respectively as the character of the deviations turns out to 

 be the same in both cases ; so that in this respect the hypothesis, so 

 far as is known, agrees with observation. 



Mr. Christie proposed prosecuting these experiments with a hollow 

 copper shell filled with bismuth ; but from unequal thickness of the 

 copper, and imperfect contact, his experiments proved less uniform and 

 satisfactory in their results. One general effect, however, afforded 

 a striking correspondence with nature. The whole equator being 

 heated, and one part more than the rest, he uniformly found that, 

 the elevated pole being towards the north, the north end of the needle 

 deviated west, when the place of heat was on the meridian above the 

 horizon, and east, when below it ; which is precisely the character of 

 the diurnal variation, north latitude. 



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