330 
TERRESTRIAL AND SOLAR RELATIONS 
secutive means, H 0 , D 0 , V 0 for every day of the year, so that 
the consecutive mean plus the variation is the observed value, 
Ii o T- A H = H, Do + AD = D, Fo ■)■ A F — V. In passing 
the dates of excessive magnetic storms, it is proper to substitute 
a minimum A H min = 0.00025 C. G. S. It has been proposed 
to obtain the normal field by taking out the “quiet” days for 
Fig. 65. Example of the variations of the i?-component of the magnetic 
field in all latitudes, and showing the derived normal type curve in the 
26.68-day period (direct), beginning August 3.58, 1845. y = 0.00001 C. G. S. 
unit. 
each month, and computing the means from these selected 
days. Unfortunately the fact that a day is “quiet” does not 
guarantee that the day is near the normal, because “quiet” 
days are as likely to run on one side of the normal as are the 
rough or moderately disturbed days. The best daily and monthly 
means are derived by taking all the observations as they occur,