436 M. A. Secchi on the Periodical Variations 



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

 value of the semi-diurnal period is a minimum at the equator, 

 and increases with the geographical latitude. The epochs of 

 the changes depend in this case, too, on the hours at which the 

 sun passes the magnetic meridian, and are somewhat advanced 

 in the summer of the hemisphere and retarded in the winter. 



If from the middle latitudes we ascend to the Pole, we shall 

 find that the curve of the bifilar appeal's to be in advance of that 

 of the declinometer : this peculiarity, which seems to complicate 

 the law which we have enunciated, depends entirely on the coeffi- 

 cients with which the diurnal and semi-diurnal period are alter- 

 nately affected according to the latitude. 



The following are the laws relative to the annual variation of 

 the diurnal means. So far as regards the monthly means, we 

 have already remarked that at St. Helena there is evidently an 

 annual period depending on the sun's declination ; and in order 

 to display the effects of the solar declination in the other obser- 

 vatories, it would be necessary to repeat the analysis made for 

 the magnetic declination. But unfortunately, really perfect 

 observations are as yet few, and hardly sufficient for the seasons. 

 As far as regards Hobarton, the march appears to be not very 

 unlike that of the declination. Thus during the summer, the 

 diurnal variation has its greatest extension, and becomes suc- 

 cessively less in spring and autumn, and is at its minimum in 

 the winter. 



So far as regards the absolute value of this component, it is 

 greater than the annual mean in summer, less in spring and 

 autumn, and a minimum in the winter. It would not be diffi- 

 cult to show, from the nature of this curve, that here, too, is to 

 be found the superposition of the two periods depending on the 

 solar declination and on the horary angle, which are added toge- 

 ther in summer, and of which one is subtracted from the other 

 in winter. At Toronto the effects are similar in the respective 

 seasons. At St. Helena the absolute maximum of the horizontal 

 force occurs in the months of February, March and April, and 

 the minimum in August and September. 



In these variations it is not easy to separate that which is 

 caused by the temperature from that which is strictly the mag- 

 netic period, — partly because the temperature exercises an influ- 

 ence on the bars, and if the variations are not accurately cor- 

 rected there is a danger of error, — and partly, because as the 

 temperature affects the force of all magnets, it may affect that of 

 the earth also. 



Besides, as the variation of this component depends both on 

 that of the inclination and on that of the total force, it is not 

 easy by means of the horizontal observations alone to determine 



