870.] 



and Horizontal Force at Kew Observatory. 



233 



3. The absolute values of the dip corresponding to the beginning of 

 October in each of the years comprehended in the above Table and the se- 

 cular change in each year are as follows : — 



From April 1863 to March 1864 68 117 



18G4 ., „ 18G5 



1865 || 



1866 „ 



1867 „ 



1868 „ 



1866 

 1867 



1868 

 1869 



7 l 1 secular change —2-40 

 63 9*31 < 



„ -o-8i 



68 8-50 1 



,' : , „ -3'° 6 



68 5-44 



j- ,, „ —2-82 



63 2-62^ 

 68 2 



'49 



Mean of the six years corresponding to 1 

 middle epoch, April 1, 1866 J 



68 6-6 2 | 



with a mean annual se- 

 cular decrease of i ,m gz. 



4. The annual variation or semiannual inequality may be shown to be as 

 follows : — 



Table II. 



Date. 



Corrections 1 68° 6'-62 



for secular 

 change. 



July 1, 1863 

 Jan. 1, 1864 

 July 1, 1864 

 Jan. 1, 1865 

 July 1, 1865 

 Jan. 1, 1866 

 July 1, 1866 

 Jan. 1, 1867 

 July 1, 1867 

 Jan. 1, 1868 

 July 1, 1868 

 Jan. 1, 1869 



+ 5-28 



+4"3 2 

 4-3-36 

 + 2-40 



+ I-44 

 + 0-48 



— 0-48 

 -1-44 



— 2*40 

 -3-36 

 -4-32 

 -5-28 



+ secular 

 change. 



68 



68 



11-90 

 10-94 



9-98 



9'02 



8-o6 

 j- JO 

 6-14 

 5 -i8 



4"22 

 3-26 

 2*30 



1-34 



Observed 

 values. 



68 12*47 

 10-95 



9-77 

 8-85 

 893 

 8-07 

 5-40 

 5-48 

 2-27 

 2-97 



2-12 



68 



Mean differences between the observed and calculated \ 

 values in the respective semiannual periods J 



Observed minus calcu- 

 lated. 



April to 

 Sept. 



4-0-57 



— 0'2I 

 + 0-8 7 

 -0-74 



-1-95 

 -0-18 



— o 27 



October to 

 March. 



+ o-oi 

 -0-17 

 4-0-97 

 4-0-30 

 — 0-27 

 4-o-8i 



4-0-27 



5. We therefore deduce from these six years' observations the existence 

 of a semiannual inequality, in virtue of which the dip is on an average 

 0''27 lower in the six months from April to September, and 0'*27 higher 

 in the six months from October to March than is due to its mean value. 



This result is in the same direction as that found by Sir E. Sabine for 

 the six years ending March 1863, but is less in amount than the latter, 

 that determined from the first six years exhibiting a range of l'*31, while 

 that determined from the last six years only exhibits a range of 0' # 54. 



6. As already mentioned, the observations for the first six years were 



s 2 



