of Terrestrial Magnetism. 441 



From these facts we conclude that "a perfectly free needle 

 would describe during the day a species of double spiral pro- 

 duced by a compound circular motion having two periods, the 

 one diurnal, and the other semi-diurnal ; or of two periods, the 

 one while the sun is above the horizon, and the other while it is 

 below ; the excursions of which are in the proportion that the 

 diurnal arcs bear to the nocturnal, and have for principal axis 

 the local magnetic meridian." 



Lastly, there is a fundamental characteristic of all the prin- 

 cipal elementary periods, which consists in the maximum and 

 minimum being about 6 hours distant from one another. 



E. Total Force. 



Colonel Sabine has investigated whether the maxima and 

 minima of the total force vary during the year ; from the dis- 

 cussion of the observations at Hobarton and Toronto he has 

 arrived at the conclusion that the total force has its maximum 

 in the months of December and January in both hemispheres, 

 although these correspond to opposite seasons. Such a law cor- 

 responds too nearly with the change of the distance of the earth 

 from the sun to admit of our doubting that it depends upon it. 

 We have then that " the disturbing force of the sun increases 

 as its distance from us diminishes, and does not depend on the 

 temperature of the seasons." 



The exact determinations of this force hitherto obtaiued are 

 too few to enable us to get out the rigorous expression of this 

 law, that is, whether it is inversely as the square of the distance; 

 but the fact appears to be established, and will perhaps be 

 brought out by the discussion of other observations, particularly 

 if care be taken to eliminate from them the periodical changes 

 which depend on the seasons, and the secular changes. 



Paut III. 



In this part we will briefly discuss the different hypotheses 

 which have been proposed to account for the diurnal magnetic 

 period, and we will take this opportunity of referring to the 

 extraordinary variations. 



Justice must be done to the enlightened spirit of modern phy- 

 sicists, who, intent on the study of facts and their laws, care little 

 to construct hypotheses ; from this it arises, that whatever has 

 been proposed has been rather by way of conjecture than with 

 any real endeavour to establish a theory. We too, in the same 

 spirit, and merely for the purpose if possible of combining facts, 

 have supposed the sun to act as a great magnet. The explana- 

 tions hitherto proposed may be reduced either to thermo-electric 

 currents induced by the sun in the different strata of the earth, 



