MAGNETIC SURVEY OF GREAT BRITAIN. 



97 



E denoting the error of observation, and E that due to the 



° 



imperfection of instruments. But 



E^ = ^, E, =-i, 



€ denoting the probable error of a single observation, and e^that 

 of a single instrument. Hence 



4 ^ 



E^ = - + — . 



(15) 



We have here taken no separate account of the error arising 

 from the variations of the dip, that error being inseparably 

 combined with the error of observation ; the symbol e^, there- 

 fore, in the -preceding, denotes the probable error resulting 

 from the two conjoint sources. 



In order to estimate the value of e^, I have taken the follow- 

 ing series of observations, made with the needles, L. 1, L. 4, 

 in Dublin, the longest series of observations made with the 

 same instrument at a single station in Ireland. The 1st column 

 of the table contains the dates of observation ; the 2nd the ob- 

 served dips (uncorrected) ; the 3nd the reduced dips, referred 

 to the 1st of January, 1836. In the 4th column are the differ- 

 ences between the partial results and the mean ; and in the oth, 

 the squares of these differences. 



Table XXVII. 

 Needle L. 1. 



VOL. vn. 



1838. 



