403 



the southern as in the northern hemisphere, and in the tropical as in 

 the temperate zones. An ingenious explanation of these phenomena 

 has been suggested by Dr. Langberg of Christiania (Proceedings of 

 the Royal Society, vol. vii. p. 434) ; but whether this explanation be 

 or be not the correct one, the theoretical importance of the facts can- 

 not be doubted, inasmuch as they appear to be wholly irreconcileable 

 with the hypothesis which would attribute the magnetic variations 

 to thermic causation. We may ascribe to the general and almost 

 exclusive prevalence of the thermic hypothesis, and to its influence 

 on magnetic reasonings, that the well-known erroneous opinion was 

 so confidently promulgated by a deservedly high magnetic authority *, 

 that a line must exist surrounding the globe, in which the needle 

 would be found to have no diurnal variation. We have now, on the 

 contrary, reason to be assured, by the facts of the annual inequality 

 thus discovered, that there is no such line ; but that everywhere in 

 the regions of its supposed existence a diurnal variation subsists, 

 having opposite characteristics in opposite parts of the year as in- 

 fluenced by the sun's position on either side of the equator, and dis- 

 appearing only at the epochs when the sun passes from south to 

 north or from north to south Declination. 



Lunar Variation. But if thermic relations have failed to supply 

 a connecting link between the sun and those magnetic variations 

 which are, without doubt, referable to the sun as their primary cause, 

 the failure of that hypothesis is made still more obvious by the ex- 

 istence of variations governed by the moon's position relatively to the 

 place of observation. We are indebted to M. Kreil, now holding 

 the same position in Austria that I have filled in England, for the 

 first suggestion of the existence of a lunar-diurnal variation of one 

 of the elements, viz. of the Declination, founded on observations at 

 Milan and Prague; and in the Phil. Trans, for 1856, Art. XXII., 

 will be found an exposition of the facts of the moon's diurnal influ- 

 ence on each of the three magnetic elements at Toronto, viz. on 

 the Declination, Inclination, and Total Force. In the case of this 

 investigation, notwithstanding the smallness of the values concerned, 

 the instrumental means supplied to the Colonial Observatories have 

 been found competent to determine, with an approximation suffi- 

 * Arago, Annuaire, 1836, p. 284. 



