METEOROLOGY AND ALLIED SUBJECTS. 307 



(10) The mean length of the period from one maximum br minimum 

 to the next is as follows : 



Years. 



For the minima, 1712 to 1865 10. 96 



For the maxima, 1707 to 1870 10. 91 



Average 10 .94 



(11) With regard to the secular period, Fritz computes as follows : 



From 208 B. C. to 1848 . . . 

 From 397 A. D. to: 848.. 

 From 503 A. D. to 1848.. 

 From 807 A. D. to 1848.. 

 From 1401 A. D. to 1848. 



From the above he deduces the mean length of the secular period as 

 55.6 years, or exactly five eleven-year periods of 11.12 years each. 



He suggests that in the present relation between auroras and terres- 

 trial magnetism a portion of the periodical variability may be due to 

 the displacement of the existing distribution of terrestrial magnetism, 

 and be therefore to a large degree apparent. 



(12) In reference to the connection between auroras and the disturb- 

 ances of terrestrial magnetism, he concludes that auroras and magnetic 

 perturbations frequently occur simultaneously or follow each other 

 closely; that, however, the perturbations that coincide with aiiroras 

 are not observed at all magnetic stations, and that probably auroras 

 occur without being announced by perturbations in terrestrial mag- 

 netism. In most cases the aurora precedes the stronger deviations of 

 the needle. 



(13) Again, in reference to the daily magnetic periods, he finds that 

 the 10 p. m. maximum of auroras entirely corresponds with daily mini- 

 mum of declinations, which latter, like the maximum of auroras, comes 

 later with increase of latitude, and that, moreover, it agrees with the 

 minimum of inclination or with the maximum of the intensity for any 

 one single place, for instance, St. Petersburg, or with the secondary 

 maximum of inchnation of other places, such as Toronto, Hobarton, &c., 

 and the maxima of auroras only agree with the secondary maxima of 

 the disturbances. Herein it is established that the connection between 

 the changes in the terrestrial magnetism and the aurora is indirect, 

 and at present the appearances are that both these phenomena are in- 

 tiuenced by a common cause, or perhaps are due to it. 



(14) With reference to connection with annual periods of terrestrial 

 magnetism Fritz says that the relations are more intimate ; thus, the 

 daily variation of declination, as observed at Munich, Hobarton, Floe- 

 berg Beach, show annual changes in their amplitude coinciding with the 

 changes in frequency of auroras. 



