320 



METEOROLOGY AND ALLIED SUBJECTS. 



A further consideration confirms the conclusion that the aurora is a 

 % ery local matter, viz, that in the whole region of 71<^ to 55^ only three 

 nights occur on which the aurora was observed simultaneously every 

 where, and is even still doubtful in these three cases, whether the same 

 iiuroia was observed everywhere. (Z. 0. G. M., Vol. XV, 1880, p. 480.) 



W. Ellis has investigated the relation between terrestrial magnetism 

 and solar-spot frequency, using the Greenwich observations, 1840-1847, 

 and the Greenwich photographic records by the Brooks apparatus, 

 1848-1877, and, comparing these with Wolf's sun-spot numbers, the in- 

 timate relation of these phenomena is more clearly shown than ever 

 before. This is illustrated in the accompanying table, which gives the 

 dates of the epochs of maximum and minimum declination and hori 

 zontal force : 



On the average, the magnetic epochs follow the sun spots at an inter 

 val of 0.27 a year. The durations of the four periods are neanly iden- 

 tical for both sun spots and magnetic phenomena. 



The occasional sudden outbursts of magnetic and sun-si)ot energy 

 occur and continue nearly simultaneously. (Z. 0. G. /¥., XVI, 1881, 

 p. 489.) 



Lemstrom has developed a theory as to the origin of terrestrial mag- 

 netism based on the electric theories of Edlund, and confirms it some- 

 what by actual experiments. The assumption that the earth consists 

 of a glowing hot interior, surrounded by a cooler layer 50 or 60 kilo- 

 meters thick, leads to the conclusion that the magnetic forces must re- 

 side entirely in this outer layer, since glowing hot bodies cannot be 

 magnetized. I^bw, geological data show that the whole exterior shell 

 must contain about 2 per cent, of iron, or equivalent to a layer of mag- 

 netic substance 1 kilometer thick. This layer may be considered as 

 constituting a hollow sphere at a distance of 30 kilometers below the 

 earth's surface, and must, under the influence of a given force, exhibit 

 a magnetic moment, the same as if it were a solid sphere. Since, now, 

 the earth is revolving in a space full of ether, it must become magnet- 

 ized the same as if it were itself at rest, but the ether turning in an 

 opposite direction. This rotation must give rise, according to Edluud's 

 theory of electricity, to an infinite number of elementary-induced cur- 



