470 Royal Society : — 



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 

 consequence, in some way or other, of the sun's action, could not 

 be doubted, from the fact that the period in which the variation 

 takes place is a solar day ; and whilst the progression was regarded 

 as a single one in the twenty-four hours, it accorded sufficiently well 

 with the prevailing notion, that the magnetic variations were pro- 

 duced by variations of temperature, to meet the general view, not- 

 withstanding the grave doubts and dissents which from time to 

 time had been expressed by those who more closely examined the 

 phenomena of particular localities. As the existence of a well- 

 marked double progression at some stations on the globe could, 

 however, no longer be disputed, the difficulty which now presented 

 itself was to explain in what way this apparently double action of 

 the sun was produced. 



On a careful examination of the diurnal motion of the declination 

 magnet on different days of the years referred to at the commence- 

 ment of this paper, it soon became obvious that, both at Hobarton 

 and Toronto, many days occurred in which the diurnal march was a 

 single progression, the nocturnal retrogression wholly disappearing ; 

 and that there were many more days in which this was more or less 

 approximately the case. It further appeared, on subsequently com- 

 paring the observations of the same years at both stations, that the 

 days most distinguished by a large and even sometimes an extrava- 

 gant interruption of the otherwise continuous single progression, 

 were generally the same days at both stations ; and by extending 

 the comparison to other though less complete series of observations 



* 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, 

 that this should be the case, since the observations at Greenwich and Kew have 

 shown that the progression is double at those stations. 



