424 On the 26-day Period of the Earth's Magnetic Force. [June 20, 



Postscript I. Received June 6, 1872. 



In determining the interval between the maximum of January 1844 and 

 that of January 1852, the epoch of maximum for 1844, deduced from the 

 26-day groups, was compared with that from the 26^-day group in 1852. 

 Either the latter should be corrected by + j'lA day = -f 2-J- days, or the 

 epoch found for the 261-day group in 1844 (22 - 9 day instead of 25 days) 

 should be taken. In either case the number of days intervening becomes 

 2911 days, which is only one day less than 112 periods of 26 days; or it 

 is equivalent to 112 periods of 25*99 days. 



If we try 113 periods, we obtain 25*76 days for the length of one, which 

 is less than that by any of the one- and two-year comparisons. The longest 

 interval, from January 1844 to January 1870, becomes (the above correc- 

 tion made) 9494 days, which number is equivalent to 365 periods of 

 26-01, or 366 periods of 25*94 days. 



The most probable values for the moment appear to be 



1844 to 1852 .... 25*99 days. 

 1852 to 1870 26*02 „ 



Postscript II. Received July 4, 1872. 



Since writing the preceding note, I have examined the observations of 

 the bifilar magnetometer made at different stations on the earth's surface 

 during the years 1842, 1843, 1846, and 1847, and found, as shown in my 

 paper already cited for the years 1844 and 1845, that the variations of the 

 daily mean horizontal force are the same at all stations at the same time : 

 this I consider a test essential in the first instance before seeking from any 

 observations other results which depend on the changes of mean values 

 from day to day. 



In the case of the observations for the years just mentioned, as in those 

 for 1844 and 1845, great disturbances are indicated always by a great 

 diminution of the mean magnetic force ; these diminutions are frequently 

 so great, that in the calculations for a single year they decide the epoch of 

 minimum. 



In my paper on the Horizontal Force of the Earth's Magnetism, I 

 stated that " a careful investigation of a much larger series of observations 

 leads me to believe that the period is variable within certain limits " * ; 

 whether this variation is due to a change of the solar meridian producing 

 the maximum after certain intervals, or to superposed regular or irregular 

 causes, I hoped, and hope yet, to determine. 



If the variation is one about a mean value, then that value will be best 

 determined by comparing the result of two well-marked groups of periods 

 separated by an interval of several years, as has been done in the preceding 



» Trans. Roy. Soc. Edin. vol. xxii. p. 544. 



