30 



DISCUSSION OF THE DISTURBANCES 



4 P. M. corresponds in time to the /in'/id/ml minimum at Toronto occurring between 

 2 and 6 P. M. Thus, the curves at the two stations, representing the diurnal vari- 

 ation of the disturbances (irrespective of increase or decrease) of the horizontal 

 force, is double crested with an exchange of the principal and secondary maximum 

 and also of the principal and secondary minimum. 



In the next Table, XIV., the diurnal variation of the disturbances is exhibited 

 separately for disturbances increasing and disturbances decreasing the horizontal 

 force. 



The disturbances increasing and those decreasing the horizontal force evidently 

 follow different laws; at Toronto they were found completely opposed; they are less 

 so at Philadelphia. The principal maximum of increasing disturbances (at noon) 

 seem to be contemporaneous with a secondary minimum of the decreasing disturb- 

 ances ; again the principal maximum of the decreasing disturbances (at 8 P. M.) 

 corresponds to a secondary minimum of the increasing disturbances. In reference 

 to the main feature, the maximum disturbance of those increasing the force and of 

 those decreasing the force, the Philadelphia ratios show even a greater resemblance 

 to the results at St. Helena and the Cape of Good Hope than to those at Toronto. 

 At the two southern stations the maximum in the disturbances which increase 

 occurs at 11 A. M. and the maximum in the disturbances which decrease occurs 

 about 6 or 7 P. M. (See Vol. II. of the St. Helena Observations.) 



Table XIV. contains also the hourly excess of the aggregate amount of the dis- 

 turbances which decrease the horizontal force over those which increase the same. 

 If we divide the numbers by the whole number of days of observation (nearly 1500) 

 we obtain the diurnal disturbance variation expressed in scale divisions. 



