358 Mr Christie on the Theory of the 



garding the earth and its atmosphere as such a combination, and 

 limiting our views to the intertropical zone alone, we should have 

 two magnetic poles produced on the northern, and two on the 

 southern sides of the Equator, the poles of opposite names being 

 diametrically opposite to each other. 



To apply this to the earth, it is necessary to know the times 

 of greatest heat in the twenty-four hours : this may be assumed 

 at three o'clock in the afternoon. The apparatus used by the 

 author not affording, when adjusted to the latitude of the place, 

 sufficient magnetic power to render the effects distinct, he sub- 

 stituted for it 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 pro- 

 duced by thermo-magnetism in his plate, and making the appa- 

 ratus revolve round this axis, he noticed the deviations produced 

 thereby on a compass, placed horizontally over it. These de- 

 viations he then compares at length, with those actually observed, 

 1,5^, by Lieutenant Hood, in 1821, at Fort Enterprize, lat. 64° 

 28' N. ; Mly^ by Canton, in London, in 1759 ; 3 J/?/, by Lieu- 

 tenant Foster, at Port Bowen, in 1825 ; ^:thly^ by Colonel Beau- 

 foy, on Bushy Heath, in 1820. The results of this compari- 

 son are, on the whole, generally such as to indicate a conformity 

 between the hypothesis and fact, with the exception of some de- 

 viations from the exact times of maximum and minimum varia- 

 tion, which could not but be expected. 



The author then considers the manner in which the distribu- 

 tion of land and sea over the globe modifies the point of greatest 

 heat, and, in consequence, the place of the diurnal poles. He 

 next observes, that, at the commencement of the experiments, he 

 had no idea of being able to reduce the deviations of the needle 

 to so simple a law as that resulting from a polarity, in a parti- 

 cular direction, communicated to the plate ; but that he consi- 

 dered it of the greatest consequence to ascertain whether the de- 

 viations on the outer edge of his plate had the same general cha- 

 racter with those within, at the time 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, 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. 



