AND OF TERRESTRIAL MAGNETISM. 271 



standard was intentionally changed in 1911 ; whereas the Greenwich standard has, I 

 believe, remained nearly uniform, a " 2," these Ix-in^ roughly equivalent to the 

 " magnetic storm " of ELLIS and MAUNDER. The number of magnetic storms in 

 Mr. MAUNDKK'S list averaged about 13 a year, while the number of 2's awarded to 

 the years 1890 to 1'JOO at Kew averaged about 44 per annum. 



Tin- investigation irf'ciml to ;ilx>vc was confined to days n 2 to n + 4, except that 

 lay n 11 was added for the Kew data. The results appear in Table X. The 

 absolute values are given of the mean " character " figure for the stated days of the 

 individual years. Values above the normal or mean value from all days are in 

 heavy type. The percentage figures for 1906 to 1910 express the arithmetic means 

 of the " character " figures in column n 2, Ac., as percentages of the corresponding 

 mean of the normal day values. The two last lines give comparative percentage 

 results for the 11 years 1890 to 1900, and the last five years of that period 

 respectively. 



Table X. confirms the physical reality of the difference between the two periods 

 1890 to 1900 and 1906 to 1910, but the percentage figures obtained for the later 

 period in columns n 2 to + 4 bear a remarkable resemblance to those applying to 

 the five years 1896 to 1900. 



In the 11-year period, 1890 to 1900, it was the contribution of the sunspot 

 maximum years, 1892 to 1894, which mainly determined the excess of the " character" 

 figures in columns n + s over those in columns ns. Since 1900 sunspot development 

 has been somewhat poor and irregular, and the results derived from the shorter period, 

 1906 to 1910, would naturally be less representative than those derived from 1890 

 to 1900. Still, it would be desirable to have results from several 11 -year periods 

 before dogmatising on this point. 



In the case of the Kew "character" figures for 1906 to 1910 there were thirteen 

 occurrences of " 2 " in days n + 3 and + 4, as compared with eleven occurrences on 

 days n 2, n 1, and n + 2, and nine occurrences on days n. But the number of 

 disturbed days (i.e., days of " 2 " and " 1 " combined) was most numerous on day n, 

 being greater by one for that day than for day n + 3. 



Day n 11, in 1906 to 1910, had only five occurrences of " 2," or nearly three below 

 normal, and occurrences of " " were five above normal ; whereas in 1896 to 1900, as 

 in 1890 to 1900, day n 11 had fewer occurrences of " 0" than normal. Taking the 

 whole 11 years, 1890 to 1900, day n 11, it will be remembered, had more 2's than 

 any other. This was the reason for including it in Table X. 



19. The Greenwich volumes of heliographic results give "corrected" as well as 

 " projected " areas of sunspots. The corrected areas allow for foreshortening, and 

 take as unit the one-millionth of the visible hemisphere. Projected and corrected 

 areas are also given for faculse. It was decided to replace the projected spot areas of 

 the investigation in 17 by corrected spot areas, projected faculie, and WOLFER'S 

 sunspot frequencies in turn. The fundamental days n, as in 2, were the 300 selected 



