1870.] and Horizontal Force at Kew Observatory, 239 



15. Forming now the following Table similar to Table II., we fail to 

 detect in it any trace of semiannual inequality. 



Table VII. 





Correction 



3 8360 ± 



Observed 

 values. 



Observed minus Calculated. 



Date. 



for secular 

 change. 



secular 

 change. 



April to 

 September. 



October to 

 March. 



July 1, 18G3 

 January 1, 18G4 .. 

 July 1, 1864 .... 

 January 1. 1865 .. 

 July 1, 1865 

 January 1, 1866 .. 

 July 1, 1866 .... 

 January 1, 1867 .. 

 July 1,1867 .., 

 January 1, 1868 .. 

 July 1, 1868 

 January 1, 1869 .. 



— 0-0152 



— 0*0125 



— 0*0097 



— 0-0070 

 —0*0042 



— 0*0014 

 +0*0014 

 +0*0042 

 +0*0070 

 +0*0097 

 +0*0125 

 +0*0152 



3-8208 

 3-8235 

 3-8263 

 3*8290 

 3*8318 

 3-8346 



3-8374 

 3*8402 



3*843o 

 3-8457 

 3-8485 

 3*8512 



3-8218 

 3-8214 

 3-8276 

 3-8293 

 3-8277 

 3-8335 

 3'8379 

 3*8402 



3-8453 

 3*8481 



3-8477 

 3-8510 



+ -COIO 

 -L-'OO 1 1 



— -0041 



+•0005 



+ -0023 

 — •0008 



— *0C2I 



+ -coo3 

 — -ooii 

 •ooco 



+•0024 



— *OCC2 



Mean difference between the observed and calculated 1 

 values in the respective semiannual periods J 



+ *ccco 



— 'OOOI 



16. Again, from the first six years we have a mean value of the hori- 

 zontal force equal to 3*8034, corresponding to the middle epoch April 1, 

 1860, and from the latter six years' observations given above, we have, as 

 has been shown, a mean value of horizontal force equal to 3*8360, corre- 

 sponding to epoch April 1, 1 806 ; also the secular change deduced from 

 the first six years is +*0053, while that deduced for the second six is 

 + •0055, the mean of the two being +*0054. 



If we apply this mean value of the secular change to the mean result 

 corresponding to epoch April 1, 1860 in order to bring it to epoch April 1, 

 1866, we obtain 3*8034 + 0*0324 = 3*8358, a value which agrees as nearly 

 as possible with that deduced from the second series, and corresponding to 

 the same epoch which, as we have seen above, was 3*8360. 



17. The coincidence of these two values naturally leads us to imagine 

 that the secular change of the horizontal force does not present the same 

 peculiarity as that observed in the case of the dip, and exhibited in the 

 diagram. 



In order to test this, let us form for the horizontal force the following 

 Table, similar to Table III. 



