Magnetic Disturbances at Greenwich. 309 



fluctuations of the early afternoon more frequently caught 

 the attention/' This suggests a more casual inspection than 

 even a severe critic would have ventured to suggest, especially 

 considering that the times were given to 0*1 hour in the case 

 of all the storms having a notably sudden commencement. 

 Also under the conditions mentioned the hours principally 

 overlooked ought surely to have been the forenoon hours. 

 But, as my Table I. shows, the percentage of occurrences 

 between and 11 a.m. is not conspicuously less, but even 

 absolutely greater, for the period 1882 to 1903 than the 

 period 1848 to 1881. Where the period 1882 to 1903 actually 

 shows a relatively smaller number of commencements is in the 

 late afternoon and evening hours. The chief difference between 

 the two periods consists in an alteration in the incidence during 

 the afternoon. It is also clear from my analysis that there 

 is a marked difference between the first and second halves of 

 the period 1882 to 1903, though one would infer that the 

 whole period was supposed by Mr. Maunder to have been 

 treated in a uniform way. 



§ 3. In discussing the diurnal inequality shown by his 

 data for 1848 to 1881 (which agree substantially with 

 those in my Table I.), Mr. Maunder says (I. c. p. 667) 

 " Table IX. rises, with a regularity which precludes the 

 possibility of accident, to a most unmistakable maximum at 

 18 hours (G P.M.)." This is surely not an accurate descrip- 

 tion of the facts. There seems a very poorly denned 

 maximum at 6 p.m., a sort of high level plateau extending 

 from about 4 to 7 p.m., with a sharp drop at 8 p.m. Also of 

 the three sub-periods into which 1848 to 1881 is divided in 

 Table L, only one gives a distinct maximum at 6 p.m. 



To explain apparently how this supposed maximum at 

 6 p.m. comes about, Mr. Maunder in his Table XV. 

 gives the " hourly distribution of small wave-movements, 

 1894-5/' The nature of these movements is not clearly 

 indicated, but as they are referred to on p. 668 as "isolated-" 

 they are presumably not the regular wave-movements con- 

 sidered by van Bemmelen. The diurnal variation in Table XV 

 is certainly very different from that assigned by van Bem- 

 melen (I. c. p. 81) for the regular wave-movements at Kew 

 for the year 1897. However this may be, I cannot regard 

 as satisfactory Mr. Maunder' s explanation that in getting out 

 the times of commencement of magnetic storms the eye was 

 caught by the movements whose incidence he records in his 

 Table XV. What that table shows is a high frequency of 

 easterly movements from 5 to 10 p.m., and a less but consi- 

 derable concentration of westerly movements from to 3 a.m. 



