DAILY MAXIMA AND MINIMA. 



145 



The law of occurrence of minima in D is not so simple. At all seasons, it is true, the greatest frequency 

 of occurrence is from 5 to 7 p.m., the minimum in the diurnal inequality occurring at 6 or 7 p.m. ; 

 but there is, at least at Midsummer, a second maximum of frequency. This occurs, curiously enough, about 

 8 a.m., and so near the hour of the inequality maximum. The cause is probably the marked tendency 

 observed at times for Declination disturbances to bo particularly large and numerous from 5 to 10 a.m. 

 The figures for the "concentration" in the frequency, viz. Midwinter 54, Equinox 46, and Midsummer 30, 

 show a marked seasonal contrast to those for the D maxima. 



Comparing Tables LVI and XVII, we see that the H maxima mainly occur in the early afternoon, near 

 the hour of the inequality maximum. The figures for the " concentration," viz. Midwinter 50, Equinox 54, 

 Midsummer 51, indicate but little difference between the seasons. The time of greatest frequency of 

 occurrence appears to be somewhat later at Midwinter than at the other seasons ; this is in accordance 

 with what is seen in the case of the diurnal inequality. 



In the case of the H minima there is a distinct indication, at least in Equinox, of a double maximum in 

 the frequency of occurrence. The greatest concentration is found near the hour 4 or 5 a.m. of the 

 minimum in the diurnal inequality, but there is also a marked concentration near midnight. Taking the 

 three consecutive hours of greatest frequency as before, the figures for the concentration are : Midwinter 28, 

 Equinox 35, Midsummer 50. 



The figures in Tables LVIII and LIX for the maxima and minima in V are a little more irregular than 

 those for D and H. The sensitiveness of the V instrument was so small that the trace, when quiet, 

 appeared to the eye practically a straight line ; also the difference between the temperature corrections to 

 be applied at two hours was often in excess of the difference between the uncorrected values. Frequently 

 five or six ordinates had to be measured, and the final differences obtained were not infrequently 

 microscopic. Thus the results are less certain than for the other elements. There is clearly, however, at 

 all seasons a tendency for the maximum to be most frequent near midnight, and the minimum near noon, 

 i.e. near the hours of the inequality maximum and minimum. The figures for the " concentration " are 



45. A general idea of the incidence of maxima and minima will be most readily derived from Table LX, 

 which gives the mean results from all the observations. To obtain greater smoothness, the day has been 

 divided into two-hour periods, commencing with midnight. The figures are percentages of the total 

 number of maxima or minima. The maxima of frequency are indicated by heavy type. 



TABLE LX. Percentage Frequency of Occurrence from all Observations. 



