36G 



general theory whereby the influence of the disturbances in different 

 parts of the globe may be predicated, their particular laws at every 

 station must be sought by a special investigation ; and no con- 

 clusion in regard to either of the components of the diurnal variation 

 is entitled to be viewed as final which has not been preceded by 

 such an investigation. 



It has appeared desirable to enter more at length into this pre- 

 liminary statement than may at first sight be thought to be required 

 by those who have followed the different stages of the inquiries re- 

 ferred to, because the interpretation, which was given so far back as 

 1845, of the diurnal variation at Toronto and Hobarton, has scarcely 

 received the consideration which might seem due to a laborious and 

 apparently successful analysis of the phenomena ; and there are some 

 eminent physicists who have framed or adopted theories for the ex- 

 planation of the diurnal variation, in which theories the existence of 

 a double progression as a universal and necessary phase is essentially 

 implied. Amongst these, the most prominent perhaps, and the one 

 which has obtained the widest circulation, is the theory of the 

 R. P. A. Secchi, Director of the Observatory of the Collegio Romano, 

 published originally in Italian in 1854 in -the ' Correspondenza Sci- 

 entifica' in Rome, translated into English in the edition of 1857 of 

 the late Dr. Nichol's * Cyclopaedia of the Physical Sciences,' and 

 more recently adopted in the third volume of M. de la Rive's ' Traite 

 d'ElectriciteY In M. Secchi's memoir, the diurnal variation, with its 

 double movement in the day and night, is ascribed to the direct 

 action of the sun as a distant and powerful magnet, influencing the 

 magnetic needle at different stations on the globe in a manner con- 

 tingent upon the direction of the magnetic meridian at each place, 

 and producing extreme deflections to the East and to the West twice 

 in the twenty-four hours, the turning hours being about six hours 

 apart, and stated to be appropriately represented by a formula of two 

 terms, one involving the sine of the hour-angle, and the other the 

 sine of twice that angle : the phenomena of the double progression 

 at Toronto and Hobarton are thus viewed by him as " Types of all 

 that happens beyond the limits of the torrid zone." 



If I have represented M. Secchi's views correctly, and I think I 

 have done so, the question between the conformity to nature of his 

 views and mine would be tested by the facts (when they should be 



