361 



was examined, it was found to exhibit phenomena in striking corre- 

 spondence with those at Hobarton. At Toronto also a double pro- 

 gression presented itself, of which the easterly extremes were attained 

 at nearly homonymous hours, as were also the westerly ; whilst the 

 hours of extreme elongation were nearly the same (solar) hours at 

 the two stations, but with this distinction, that the hours at which 

 the north end of the magnet reached its extreme easterly elongation 

 at Hobarton were the same, or nearly the same, as those at which it 

 reached its extreme westerly elongation at Toronto, and vice versa. 

 Pursuing, therefore, the ordinary mode of designating the direction 

 of the declination by the north end of the magnet in the southern 

 as well as in the northern hemisphere, the diurnal motion of the 

 magnet may be said to be in opposite directions at Hobarton and 

 Toronto ; but if (in correspondence with our mode of speaking in 

 regard to another magnetic element, the Inclination) the south end 

 of the magnet is employed to designate the direction of the motion 

 in the southern hemisphere, and the north end in the northern 

 hemisphere, the apparent contrariety disappears, and the directions, 

 as well as the times of the turning hours, are approximately the 

 same at both stations. 



The double progression in the diurnal variation, which was thus 

 so distinctly and concurrently marked at stations so distant from 

 each other, and at which the observations had been conducted with 

 an elaborate care which would admit of no doubt as to the depend- 

 ence to be placed on their general results, was at that time in great 

 measure an unexpected and even a startling phenomenon. In the 

 well-known description given by M. Arago (in the instructions drawn 

 up for the voyage of the 'Bonite' in 1836) of the general pheno- 

 mena of the diurnal variation in different parts of the globe, as then 

 known, they are represented as consisting of a single progression 

 only, with but one easterly and one westerly extreme, both occurring 

 during the hours of the day ; and no reference or allusion whatso- 

 ever is made to the existence of a double progression, or of a noc- 

 turnal interruption to the continuous motion in the one direction 

 between the two extremes*. That the diurnal motion must be a 



* From the omission on the part of M. Arago of any notice of a nocturnal 

 feature, it might perhaps be inferred that the diurnal variation at Paris is actually, 

 as described by him, a single progression: it seems very improbable, however, 



