46 



DISCUSSION OF THE HORIZONTAL COMPONENT 



Annual Inequality in tJie Diurnal Variation of tlie Horizontal Force. The dis- 

 tinctive feature of the diurnal variation is shown in the annexed diagram (A), 

 constructed from the mean annual and half-yearly values given in the preceding 

 table, IV. It exhibits in the annual mean, as its characteristic type, a maximum 

 value about 6 A. M., a minimum value about 11 A. M., a secondary maximum value 

 about 3| P. M., and a secondary minimum about 9 P. M. For the half year when 

 the sun has north decimation, the morning minimum becomes smaller and the 

 afternoon maximum larger, thus increasing the diurnal range ; the converse takes 

 place in the other half of the year, when the sun has south declination. The 6 

 A. M. maximum remains nearly unchanged throughout the year. The average 

 summer range (April to September inclusive) is 0.0046, and the average winter 

 range (October to March inclusive) is 0.0025, both expressed in absolute measure. 

 The range between the morning maximum and the morning minimum is 0.0045 in 

 summer and 0.0036 in winter, as will be explained further on. 



