54 



CLIMATIC CYCLES AND TREE GROWTH 



He investigates the 27-day period in 28 years of daily records of the inter- 

 national magnetic character figure C, which is simply the magnetic disturb- 

 ance on the earth estimated by the various observers on a scale of to 2. 

 This disturbance is attributed with confidence to a solar origin. The 27- 

 day period is an approximate synodic rotation of the sun in the average spot 

 latitude that is found to occur in the late parts of the 11-year sunspot cycle. 



The character figures for each 27-day interval are in effect reduced to the 

 nearest sine curve of 27-day length, and the amplitude and phase derived. 

 In the harmonic dial 1 these amplitudes and phases are expressed on polar 



oec- 



Fig. 25 — Bartels' Harmonic Dial of International Magnetic Character Figure C. 

 (From Terrestrial Magnetism and Atmospheric Electricity, March 1935.) 



coordinates, the phase being assigned a certain direction from the pole, and 

 the amplitude being represented by the distance from the same. This is 

 repeated for the 378 rotations and a cloud of dots obtained, as in figure 25. 

 If these dots center on the pole, there is no "real" period. If, however, there 

 is a period present, the center of the cloud of dots will be some distance away 

 and will show the final total amplitude and phase by its direction and distance 

 from the pole. To show reality of a cycle under test, the data are subse- 

 quently massed by two's, three's, etc., in larger and larger averages (not 



1 In the nomenclature proposed in a preceding chapter this would be called a sum- 

 mation or integration dial as it is without a time scale, and figure 26 would be an ex- 

 tended dial. 



