438 M. A. Secchi on the Periodical Variations 



malies is to be found in the following remarks, viz. that as the 

 origin of the periods which are superposed depends on the dif- 

 ferent hours at which the sun passes the magnetic meridian, the 

 semi-diurnal period has necessarily various positions with regard 

 to the diurnal ; and in this manner various inflections occur 

 which it would be difficult to account for in any other way. 

 For instance, the variations of the inclination given at the Cape 

 of Good Hope, p. 44, pi. 6, can easily be decomposed by an ex- 

 perienced eye into the usual principal periods, diurnal and semi- 

 diurnal, which gain alternately on each other with varying para- 

 meters in the different months. 



We may therefore finish this discussion with the following 

 general conclusion : that " the horizontal component, as well as 

 the vertical, maybe decomposed into a diurnal and semi-diurnal 

 period, which depend on the declination of the sun and on the 

 geographical latitude." 



C. Inclination and Total Force. 



Given the laws of the variation of the two preceding compo- 

 nents, that of the resultant or total force may easily be deduced ; 

 and the absolute magnetic inclination being known, the variation 

 of the inclination may also be deduced from the variations of the 

 horizontal and vertical force. 



General Law. — " The phases of the inclination are analogous 

 to those of the declination, but three hours earlier." 



Explanation. — If the maximum of the declination is at 2 h , the 

 maximum of the inclination would be at 23 h or thereabouts. 

 This may be seen in the Makerstoun curves, &c, and also in the 

 Hobarton curves, the explanation of which we will give presently. 



As for the other peculiarities, it will be sufficient to remark, 

 without analysing each casein detail, that in general the maxima 

 of the horizontal force coincide with the minima of the inclina- 

 tion. 



Colonel Sabine calls attention to the analogy that exists 

 between the variation of the inclination at Hobarton and Toronto, 

 places which are almost antipodal. In these localities the varia- 

 tion of the inclination in both its periods is the same at almost 

 precisely the same hours, with this difference only, that at Ho- 

 barton the south pole (the lower) is to be considered, and at 

 Toronto the north (the lower). 



The total force at Toronto is subject to two periods, viz. the 

 following : — 



Principal maximum at . 5 h 



Principal minimum from . . ]5 h to 16 h 

 Secondary maximum from . . 18 h to 20 h 

 Secondary minimum from . . 22 h to 23 h 



