448 M. A. Secchi on the Periodical Variations 



subject. For the present we can only say with Colonel Sabine, 

 that so general a change in the march of all the magnetic ele- 

 ments demands a proportionate cause; and that as these do not 

 arise from the ordinary effects of the climate, which in these 

 years have exhibited no extraordinary change, it is necessary to 

 seek some other cause. 



Colonel Sabine also points out the singular coincidence be- 

 tween the years of the maximum and minimum of these mag- 

 netic changes and those of the maximum and minimum num- 

 ber of the solar spots, observed by Schwabe in these years. 

 This number was a minimum in 1833 and 1843, and a maxi- 

 mum in 1828, 1837 and 1848. From these and from other ob- 

 servations, Wolf has deduced a decennial period in the changes 

 of these spots; and it remains to investigate whether a like 

 march can be traced in the older magnetic observations. The 

 publication of Arago's observations has come opportunely for 

 this comparison. From the table at p. 500-501, vol. i. of his 

 Scientific Works, the declination-needle appears to have had a 

 minimum excursion in 1823 and 1824. Before that it was 

 greater, and having reached this minimum, it increased continu- 

 ously until it arrived at a maximum in 1829. These epochs 

 correspond with those derived from the period observed in the 

 solar spots, which was a maximum in 1828 and a minimum in 

 1823. From the Gottingen observations, we find a maximum in 

 the excursions of the declination-needle in 1836-37. This 

 maximum also coincides with a maximum of the solar spots in 

 Schwabe's table*. Hence Colonel Sabine thinks it not impossible 

 that changes in the solar atmosphere may extend their influence 

 to the earth in the form of magnetic action. 



The truth is, that to consider the whole complexity of mag- 

 netic perturbations as a mere meteorological effect, appears to 

 be assigning to them a cause not adequate to the effect. The 

 fact mentioned above, that the maxima of the perturbations at 

 Hobarton succeed each other with the same retardation as the 

 other magnetic phases, is one which cannot be explained cither 

 by the retardation of the effect of temperatures, or by the con- 

 densation of vapour. We cannot conceive how these should 

 account for the general retardation of one hour. It is then a 

 purely magnetic fact, the explanation of which depends on that 

 of the physical cause of solar and terrestrial magnetism. The 

 same may be said of the greater perturbations at the epochs of 

 the equinoxes, which certainly bear no relation to the state of 

 the atmosphere or to the solar heat. Colonel Sabine makes the 



* Results of the Magnetic Observations made by Gauss at Gottingen, 

 vide Taylor's Scientific Memoirs, vol. ii. part 1. art! 2. p. 57; see Hum- 

 boldt's Cosmos, vol. iii. p. 2!>2, Sabine's translation. 



