MAGNETIC DISTURBANCES AT KEW. 



301 



24 hours. 4 at 9 p.m. is, of course, 364 relatively to 356 at 8 p.m. The curve, as 

 viewed by an observer looking down on the horizontal plane, is described in the 

 direction of the hands of a watch. The motion is also in the direction of the hands 

 of a watch on disturbed days throughout the greater part of the day, but it is 

 distinctly retrograde for some hours after midnight. In the d q column there is an 

 apparent discontinuity in the value of (f> between 4 and 5 a.m. This really means 

 that p vanishes and the curve crosses the geographical meridian. 



The disturbed-day vector is in advance of the quiet-day vector at most hours of the 

 day. They are, however, practically coincident at noon and remain close together 

 until 6 p.m. 



25. On quiet days 6 increases from a minimum at 5 p.m. until at least 2 or 3 a.m., 

 and then remains nearly constant until 7 a.m. The rise to the maximum at 11 a.m. 

 and subsequent fall are rapid. The vector is directed above ground only from 8 a.m. 

 to 2 p.m. The total range in 6 is 75, from 45 below to 30 above the horizon. 



On disturbed days the range of 6 is much wider, being from 64 below to 82 above 

 the horizon, or in all 146; the rise from 6 p.m. to 6 a.m., and the fall from 6 a.m. to 

 6 p.m., are continuous. 



The mean values of 6 for the q, d, and d q columns respectively are 82, 99, and 

 94, so that the mean position is about as much above the horizon on disturbed days 

 as it is below it on quiet days. 



26. The absolutely largest and smallest values of p and R in Table XVI. are in 

 heavy type. The disturbed day value of p is larger than the quiet day value except 

 from 5 a.m. to 8 a.m. On neither type of day is there much variation of p between 

 5 p.m. and 3 a.m. On quiet days, owing to the comparatively small influence of V, 

 the value of II is usually but little larger than that of p ; both quantities have a 

 maximum at 1 p.m. and a minimum at 2 a.m. On disturbed days R is usually 

 considerably larger than p, and has its maximum 3 hours and its minimum 1 hour 

 later. The disturbed day value of R is always larger than the quiet-day value. 



27. If we regard 6 a.m. to 6 p.m. as " day," and ascribe half the sum of the values 

 at these two hours to "day" and half to "night," we obtain the following mean 

 values for p and R, employing q, d, and d q in the same sense as before : 



TABLE XVII. Comparison of Day and Night Values of Diurnal Inequality Vector. 



