18 



AMPLITUDE OF THE SOLAR-DIURNAL VARIATION 



If we compare these ratios with the corresponding numbers in the preceding 

 tables, showing the bi-hourly distribution in regard to the number of disturbances, 

 we find, irrespective of the directions of the deflections, the 2 P. M. minimum 

 preserved ; the maximum occurs at 10 P. M. At Toronto, from a five years' hourly 

 series, commencing with 1844, these hours are respectively 1 P. M. and 9 P. M. 

 At Philadelphia, as at Toronto, the ratios are nearly invariable from 10 A. M. to 

 6 P. M., being then below unity ; and again from 8 P. M. to 8 A. M., when they 

 are above unity. 



The easterly maximum and the westerly minimum at 8 P. M. appear again as a 

 decided feature, and in general, the respective ratios exhibiting the diurnal dis- 

 tribution of the disturbances, both in an easterly and westerly direction, show 

 almost a perfect correspondence in regard to both number and amount. 



The next table exhibits the excess of westerly disturbance over easterly (the 

 sign indicating a defect, or excess of easterly over westerly) in the aggregate 

 values of the five year series, and in the last column, the mean effect of the same 

 at each even hour, is given as obtained by dividing the aggregate differential value 

 of the preceding column by the actual number of days of observation during the 

 whole period. The last column exhibits, therefore, the mean diurnal disturbance 

 variation. The number of days is very nearly 1500. 



The law governing the disturbances during a solar day is clearly shown, and 

 systematic in character. If we plot the disturbance curve on the same scale, or 

 actually superpose it on the curves of the regular diurnal variation, the difference 

 would hardly show to the eye. The diagram, showing the disturbance variation, 

 has, therefore, been plotted on a larger scale. (See Fig. 4.) 



