Magnetic Disturbances at Greenwich. 317 



make it a condition that the qualifying movement took place 

 in a comparatively short definite interval of time, otherwise 

 complications might arise from the large size of the regular 

 diurnal range at certain seasons. 



§ 11. There is at least theoretically a good deal to be said 

 in favour of a limit varying in a definite way. In the average 

 year the regular diurnal variation is much larger in some 

 months than in others. In England the declination range in 

 December is only about a third of what it is in several 

 summer months. Again, the average range may be 40 or 50 

 per cent, larger for a year near sun-spot maximum than for a 

 year near the adjacent sun-spot minimum. Thus in the course 

 of an 11- year period the regular diurnal ranges in two 

 individual months may bear a ratio such as 4 or 5 to 1. 

 One cannot but doubt under such circumstances whether a 

 system which defines a day as disturbed, and the disturbance 

 as M, A, or G, by reference to an unvarying absolute scale 

 of magnitude is a natural one. If the immediate cause 

 alike of regular and irregular movements be electric currents 

 in the upper atmosphere, whose intensity^ is increased in 

 years of sun-spot maximum owing to the increased presence 

 of some ionizing or otherwise effective agent, it would seem 

 clearly better to take limits varying with the average diurnal 

 range for the individual month considered. Even if the 

 large disturbances have an absolutely distinct source, an 

 allowance would seem desirable for the movement that would 

 in the natural course of events have taken place between the 

 times when the maximum and minimum were recorded. 



§ 12. The considerations already advanced would suffice to 

 raise doubts as to whether the energy of the source to which 

 a particular storm is due can be supposed to answer in all 

 cases to the letter assigned to the storm on Mr. ElhVs and 

 Mr. Maunder's lists. But an even stronger reason for such 

 doubts remains to be mentioned. On Mr. Ellis's, or any 

 analogous system, a single to and fro movement may count 

 as a disturbance, whereas a dozen movements occurring in 

 close succession, of but slightly inferior amplitude, may fail 

 to be classed. Again, disturbances in which the extreme 

 range is the same may differ enormously in the number of the 

 oscillatory movements and in the rapidity with which they 

 take place. Thus a classification which takes account only of 

 the extreme range must in manv cases fail to arrange dis- 

 turbances according to their real intensity. Consequently 

 the conclusions which have been drawn as to differences 

 between storms of different intensities might have to be 

 modified if some more exact measure of energy were adopted. 



July 10, 1905. 

 Phil. Mag. S. 6. Vol. 10. No. 57. Sept. 1905. Z 



