INYESTIGATION OF THE ELEVEN YEAR PERIOD 



CHANGE OF THE AMPLITUDE OF THE SOLAR-DIURXAL VARIATION' OF THE MAGNETIC 



DECLINATION, COMPRISING THE REGULAR AS WELL AS THE 



DISTURBED DIURNAL VARIATION. 



While the magnitude of the deflection is the only criterion for the recognition 

 of a disturbance, the adoption of any limit of deviation from the normal value for 

 the same hour, month, and year, must necessarily remain in some measure arbitrary, 

 or, in other words, there must always -remain, after the separation of the disturb- 

 ances, a certain small amount of their effect in the remaining regular diurnal 

 progression. General Sabine has shown that the results are not sensibly affected 

 by a small variation in the line of separation of the disturbed from the undisturbed 

 readings.' 



To effect the separation, I made use of Peirce's criterion," for the rejection of 

 doubtful observations, applying it, however, to observations following a law different 

 from the regular one.^ From an examination of 465 hourly observations, distributed 

 over different hours of the day and different months of the year, the following was 

 the limit of separation : — 



9^.3 from six months in 1840 

 8.1 " " 1843 



6.0 " " 1845 



The mean or 7.8 divisions, equal to 3'.6 of arc, has been adopted provisionally. 

 Accordingly, all numbers in the printed record of observations, differing 7.8 scale 

 divisions (or 10.3 divisions for June, and, up to July 18, 1840), from the mean 

 monthly value at each hour of observation, were marked in pencil. It was found 

 that the ratio of the disturbed observations to the total number was 1 : 9.6, or for 



' In the first discussion of the Toronto observations for the years 184.3, 1844, 1845, the limit of 3'.6 

 was adopted, corresponding to one disturbance in every 13.6 observations; in the second discussion 5'.0 

 was substituted as preferable. Phil. Trans., 1856, art. xv. 



' Gould's Astronomical Journal, vol. iv., No. 83, 1855. 



' A similar application was made in the discussion of Dr. E. K. Kane's magnetic observations at Van 

 Eensselaer Harbor, North Greenland, by Mr. Schott. Smithsonian Contributions to Knowledge, vol. x., 

 1858. 



