•. Bache on the Horhontal Component of Magnetic Force. 263 



5-5 is due to the loss 



Separation of the larger disturbances. — The observations having 

 been referred to a uniform temperature and corrected for pr(> 

 gressive change, Peirce's criterion was applied separately to each 

 month. For this purpose a sjstematic application was made 

 extending over the whole series of observations commencing 

 with the hour and the month of Julv, next with hour 2 and 

 August followed by hour 4 and September, and so on in regular 

 progression. This process eliminates from the result the diurnal 

 variation and the annual variation of the disturbances them- 

 selves. The value for O*" in July, 1840, was omitted as affected 



for the respective hour), also the number of observations in each 

 year subjected to the process. 



Limits of rejection hy Peirce's criterion. 



1843-44 28 313 



Mean value 39 Sura. 1488 



The limiting value derived from nearly 1500 observations is 89 

 scale divisions and the separate annual values show plainly the 

 effect of the eleven (ten ?) year period, the year 1843-44 being a 

 ininimum year. Certain limits in the adoption of a separating 

 value are allowable and upon trial as to the actual number of 

 Qisturbances separated, the value 33 scale divisions was finally 

 ?dopted. Any observation differing 33 divisions or more from 

 Its respective monthly mean was therefore marked and excluded 

 from the mean. 33 divisions equals 0-0012 parts of the horizon- 

 tal force and in the value of the absolute scale it amounts to 

 0-005. At Toronto the limiting value was 14 divisions =0-0012 

 parts of the horizontal force equal to 0-004 in the absolute scale 

 (vol. Ill of the Toronto Observations). 



Table No. VI, of the full memoir, shows the number of obser- 

 vations and the number of the larger disturbances, separated by 

 the value 33, as the limits for each month, year, and the whole 

 period. 



The total number of observations is 24,231, and of disturb- 

 ances, 1,698. The limiting value therefore separates one m 

 «very 14-3 observations. At Toronto, one in eveiy 12-o was 

 niarked as a disturbance. 



The larger disturbances having been excluded, new monthly 

 means were taken, and the process was repeated several times, 



