1876.] 



Magnetic Dip fyc. at Keiv Observatory . 



237 



In order to exhibit semiannual inequality in the horizontal force, 

 Table V. has been formed in a similar manner to Table II. 



Table V. 



Date. 



Correction 



for secular 



change. 



3-8682 



+ secular 



change. 



Observed 



values. 



Observed mimes calculated. 



April to 

 September. 



October to 

 March. 



July 1, 1869 

 Jan. 1, 1870 

 July 1, 1870 

 Jan. 1, 1871 

 July 1, 1871 

 Jan. 1, 1872 

 Julv 1, 1872 

 Jan. 1, 1873 

 Julyl, 1873 

 Jan. 1, 1874 

 July 1, 1874 

 Jan. 1, 1875 



- 0-0152 



- 0-0125 



- 0-0097 



- 00070 



- 0-0042 



- 0-0014 

 + 0-0014 

 4- 0-0042 

 + 0-0070 

 + 0-0097 

 4- 0-0125 

 4- 0-0152 



3-8530 

 3-8557 

 3-8585 

 3-8612 

 3-8640 

 3-8668 

 3-8696 

 3-8724 

 3-8752 

 3-8779 

 3-8807 

 3-8834 



3-8523 

 3-8579 

 3-8572 

 3-8597 

 3-8622 

 3-8659 

 3-8701 

 3-8724 

 3-8778 

 3-8776 

 3-8817 

 3-8838 



- -0007 

 -"•0013 



-"•0018 

 +"•0005 

 +"•0026 

 +"•0010 



+ -0022 

 ~ -0016 

 - -0009 

 •0000 

 - - -0003 

 + -0004 



Mean difference between the observed and calculated "1 

 values in the respective semiannual periods J 



•0000 



•0000 



Table IV. shows the mean annual secular change during the last six 

 years to have been precisely the same as during the six years previous, 

 and slightly greater than in the period 1857 to 1863, when it was 

 •0053. 



Table V. also agrees with Table VII. in Dr. Stewart's discussion, in 

 not exhibiting any trace of semiannual inequality, thereby differing from 

 Sir E. Sabine's paper, in which for the years 1857 to 1863 an inequality 

 amounting to *0026 was shown to exist. 



The probable error of a monthly determination of the horizontal force 

 as determined by least squares from the six years series, after the cor- 

 rection for secular change has been applied to each observation, is 

 + •0024. 



Total Poece, 



We find in Table III. that the means of the April to September 

 values of the horizontal component of the force in the six years is 3*8669, 

 corresponding in epoch to January 1, 1872, and in Table I. that the 

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

 is 67° 54''96, corresponding to the same epoch. 



We find also in Table III. that the means in the six years of all the 

 October to March values are for the horizontal force 3-8695, and for the 

 dip 67° 54'- 68, corresponding to epoch July 1, 1872. 



These values may be reduced to a common epoch by applying to the 

 former dip the correction of — 1'*07 as the proportional part of the 

 secular change as determined by the whole six years' observations. 



