40 Mr, Chnstk on the Magnclis^n of Iron [July, 



S Q N, and the arm A B attached to it with the iron plate on 

 the axis ; and that the centre of the needle might be in tne plane 

 of the plate, the compass box was moved in the direction of 

 the meridian. 



" Some of my first observations were made with the centre of 

 the plate in the equator, and I immediately found, that the 

 deviation due to rotation y instead of being 0, as in the cases when 

 the plate revolved in the planes at right angles to its present 

 position, was here considerable; and also that, that of the south 

 end of the needle was in the direction of the upper, or south 

 edge of the plate, contrary to what had been observed in the 

 same plane at the pole. This indicated that there must be, at 

 least, one point in this circle on each side of the pole, where 

 the deviation due to rotation was 0; and to determine nearly the 

 latitude of this point, I made observations at every 10° of lati- 

 tude on each side of the south pole. Before, however, giving 

 these observations, it is necessary that I should state the kind 

 of reliance I place on them as forming a complete set. In order 

 to make the observations near the pole, it was necessary to 

 adjust the instrument with the axis horizontal and pointing 

 east and west, and after having made the complete set, I 

 suspected that in the change from the one adjustment to 

 the other, the centre of the plate had been nearer to that 

 of the needle in making the observations near the equa- 

 tor, than those near the pole ; and that consequently, the devio" 

 tions due to rotation in the former case, were proportionally too 

 great. I was confirmed in this suspicion on comparing these 

 observations with those which 1 had, in the first instance, made 

 in lat. 0° and in lat. 90° ; and still further on comparing them 

 with others, which 1 subsequently made at the several distances 

 15, 17, 19, 20 inches ; in the corresponding situations. For 

 example, in my first observations, the deviations due to rotation 

 in lat. 0^ long. 0°, and in lat. 0° long. 180° were 3° 10', and 

 3° 14', giving a mean 3° 12' in lat. ; and in lat. 90 S, 1° 31'; 

 when the centres of the plate and needle had been carefully 

 adjusted to the same distance 13*2 inches, in the two cases ; 

 whereas the corresponding deviations in the table are o° 43' and 

 1° 29'/; and, by subsequent observations, I found the sum of 

 the deviations at the distances 15, 17, 19, and 20 inches to be 

 in these two cases, 7° 20' and 3° 32', to which 3° 12' and 1° 31' 

 are very nearly proportional. These difi'erences however do 

 not in the least affect the conclusions which I at the time 

 drew from this set of observations." 



" It appears, from these observations, that when the plate 

 revolves in the plane of a secondary to the equator and meri- 

 dian, 



" 1st. The deviation due to rotation is a maximum when the 

 centre of the plate is in the equator. 



