1870.] and Horizontal Force at Kew Observatory. 241 



19. The probable error of a single monthly determination of the hori- 

 zontal force derived from the seventy-two monthly determinations given in 

 Table VI., and after the application of the correction for secular change 

 has been applied, is +0*0021. There is, however, reason to believe that, 

 as in the case of the dip, the probable error is increased to some extent by 

 periods of disturbance, and the same method may be applied to test this 

 as was applied to the dip observations. Forming, therefore, a series of 

 seventy-two yearly values of the horizontal force, corresponding severally in 

 epoch to the seventy-two monthly values of Table VI., and deducting each 

 from the corresponding observed monthly value, we obtain, as before, a 

 series of seventy-two differences ; and w r e derive from these the following 

 modifications of the last two columns of Table VII. 



Table X. 



Date. 



Observed minus Calculated. 



April to September. ! October to March. 



July 1, 1863 



July 1, 1864 



July 1, 1865 



January 1, 1866 



July 1, 1866 



July 1, 1867 



January 1, 1868 



July 1, 1868 



January 1, 1869 



+ •0016 



— •OOI9 



-4- '0004. 

 -f-*ooo5 

 — '0008 



— •0014 

 -(-•oco6 

 «|-'odo6 



— "0009 

 + •0008 

 -|-'ooo8 



Mean 



-f-O'OCOZ 



4-o'ocoi 



Thus we see, as in the case of the dip, that the irregularity of the numbers 

 in these columns is much diminished, the result being, however, left the 

 same as before. Finally, if we deduce the probable error of a single ob- 

 servation by means of the series of differences so obtained, we find this to 

 be +0-0018 instead of + 0*0021, which it was before. 



III. Total Force. 



20. We find in Table VI. that the mean of the April to September 

 values of the horizontal component of the force in the last six years is 

 3*8346, corresponding in epoch" to January 1, 1866 ; and in Table I. that 

 the mean of the April to September values of the dip in the same six years 

 is 68° 6'*83. 



"We find also that the mean of the October to March values are for the 

 horizontal force 3*83/2, and for the dip 68° 6'*41, corresponding to epoch 

 July 1, 1866. 



