AND OF TERRESTRIAL MAGNETISM. 255 



only 0'135, and its deficiency, 0'522, is represented by the negative ordinate 

 marked 0. The algebraic difference of these ordinates, T186, represents the difference 

 in " character " between the representative disturbed and quiet days. 



It will be seen that the day which is three days prior to the representative 

 disturbed day is decidedly quieter than normal, and is less disturbed than the day 

 which precedes by three days the representative quiet day. On the other hand, 

 the day which is three days subsequent to the representative quiet day is decidedly 

 more disturbed than normal, and is less quiet than the day which is three days 

 sulwequent to the representative disturbed day. The latter result especially was 

 quite unexpected, in view of the frequent occurrences of sequences of disturbed days, 

 and still more of quiet days. A sequence of five, or even ten, successive O's in the 

 returns from an individual observatory is not unusual in months of minor disturbance. 

 The natural inference is that the proverb " the calm precedes the storm " has some 

 claim to recognition even in terrestrial magnetism. 



It may create surprise that the representative quiet day had so large a " character" 

 figure as 0'135. Days, however, of international "character" O'O are very rare. 

 There were only four, for instance, during 1906. The phenomenon is considerably 

 due to a few observatories where O's are assigned to only exceptionally quiet days. 

 On the other hand, if latitudes over 55 degrees were adequately represented, 0'0's 

 would be still rarer. 



A glance at fig. 2 will show that the 27-day period is just as prominent for quiet 

 as for disturbed characteristics, and that it can be traced backwards as readily as 

 forwards. The corresponding patches of curve associated respectively with the 

 disturbed and the quiet days, as it were, repel one another. This would probably 

 serve to prove the existence of pulses considerably beyond the range covered by 

 Table III. and fig. 2. 



9. One of the principal objects originally in view was to obtain a more exact 

 estimate of the length of the period by measuring the interval in days between the 

 crests of pulses remote from one another. But even in the 79- to 84-days' pulses 

 i.e., the third subsequent pulses the difference between the ordinates answering to 

 successive days has become very small, so that trifling accidental irregularities are 

 prejudicial to accurate time deductions. This difficulty will naturally tend to 

 disappear as the number of years for which international data are available increases, 

 and the power of the method will thus continually develop. 



In 6, it will be remembered, we obtained a result which suggested that the length 

 of the period increased with the amplitude of the selected disturbance. If, however, 

 this were the case, one would expect the interval between successive subsequent 

 pulses associated with the selected disturbed days to gradually diminish, and the 

 intervals derived from pulses associated with quiet days to be shorter than those from 

 pulses associated with disturbed days. These tendencies are not apparent in fig. 2. 



10. The fact that the rise in the " character " figure in the two days immediately 



