190 



SPECIAL TYPE OF DISTURBANCE. 

 TABLE LXXII. Amplitude and Direction of Disturbing Force. 



\v- 





60-9 



To explain Table LXXII let us consider the mean result from all the observations, which may be 



assigned to a single representative occurrence of the phenomenon. 



In the figure, NS, EW are the geographical north- south and east-west directions, while nos represents 



the position of the Declination needle, n being the north pole. Taking 

 as point of departure the position existing at the commencement of 

 the first phase, the force required to produce the disturbance existing 

 at the end of that phase called for the action on the pole n, of a 

 force whose horizontal components were 60 -9y perpendicular to no, 

 and 15 '3y along no, in the directions shown, the vertical component 

 being 6 ly downwards. The resultant of the two horizontal com- 

 ponents amounted to 62 -87, and its inclination \f/ to no was 75 53'. 

 The total force AT, obtained by combining the horizontal resultant 

 with the Vertical Force 6 - ly, amounted to 63 - ly, and its inclination 

 X to the horizontal plane was 5 35'. The direction of AT pointed 

 below the surface of the ground. 

 For the second phase we take the position at the end of the first phase as point of departure. To 



produce the disturbance existing at the end of the second phase 



required the forces shown in the second half of the table. The 



accompanying figure illustrates the results in the horizontal 



plane answering to the mean or representative occurrence. The 



resultant of the Horizontal Forces, 55'2y, is inclined at 70 48' 



to on produced. It is thus not exactly opposite to the corre- 

 sponding force experienced during the first phase, the two being 



inclined at approximately 175. The Vertical Force in the 



second phase is nearly 3J times that in the first, so that the 



angle \, which the total disturbing force AT now makes with 



the horizon, is increased to 20 47' and points above the horizon. 

 There were only four occurrences in September, thus little 



significance attaches to the large size of the average disturbance 



for that month. 



107. The data on which Table LXXII depends were all got out before any anticipation was made as to 



