Bigelow — Meteorological Elements of the United States, 429 



[Semi-annual period in the solar prominences, the magnetic force, and 

 the direct type of the normal curve of the 26 - 68 day period.] 



MONTH 



JFMAMJJYASOND 



26. 68 DAY 

 PERIOD. 



12 3 4 5 6 7 8 9 10 II 12 13 14 



(1) 



DAI LY 



NUM BER 



OF SOLAR 



PROMINENCES. 



6.6 

 6.5 

 6.¥ 



6.3 

 6.2 

 6.1 

 6.0 





























• 









(2) 



AMPLITUDE 



NUMBERS 



HORIZONTAL 



MAGNETIC 



FOfl&E. 



220 

 2/0 

 ZOO 

 /90 

 180 



























(3) 



TRIAL 



NUMBERS 



OF THE 



DIRECT 

 TYPE-CURVE. 



■+ 20 

 + 10 

 

 - IO 

 -20 



























(1) Daily number of solar prominences. Table 2, 1872-1905. 



(2) Amplitude number of the European magnetic horizontal force. 

 Table 3, 1872-1905. 



(3) Trial numbers of the direct type-curve in the 26 "68 day period. 

 Bulletin No. 21, page 102. 



It is probable that this synchronism in these three semi-annual periods 

 depends upon the aspect of the earth to the sun in its orbit, the maximum 

 effects occurring at the time of the equinoxes. More prominences can be 

 seen at that time, because the sun's disk shows the two hemispheres more 

 evenly; the magnetic force is more disturbed; the direct type is at .a 

 maximum in the successive 26 '68 day periods. The physical explanation of 

 these effects is probably this, that the solar radiation is more favorable for 

 influencing the earth's magnetic field, through the process of ionization 

 which produces electric currents. The evidence is thab these are most 

 vigorous in the strata of the atmosphere near the earth's surface. It is also 

 found that the warm and cold masses of air move over the Missouri valley 

 in such a procession that the order reverses in the same seini-annual pei'iod, 

 when referred to the typical 26 - 68 day periodic curve. 



Am. Jour. Sci. — Fourth Series, Vol. XXV, No. 149. — May, 1908. 

 29 



