HorizontTaL Force. XXVil 
readings, which cannot be separated from that due to secular increase of horizontal 
force alone. 
The following Table exhibits the values of apparent secular change thus ob- 
tained, making use of the whole series of years :— 
TaBLE 9.—Apparent Secular Increase of Horizontal Force. 


Difference between the mean of Parts of Force. 
1842 and 1843 -003470 
18438 1844 -003885 
1844 1845 -0014385 
1845 1846 -001285 
1846 1847 °001243 
1847 1848 ‘000852 
1848 1849 -001863 
1849 1850 001394 
1850 1851 °001335 
1851 1852 -001268 
1852 18538 001787 
1853 1854 °001339 
1854 1855 ‘001136 
The first two values in the above Table are probably erroneous, and the mean 
of the other eleven gives ‘001358, which we may consider to represent the apparent 
mean yearly increase of horizontal force. While, however, this amount only ap- 
proximately represents the secular change of horizontal force, it represents actually 
the mean secular change that takes place year by year upon the readings of the 
bifilar from a combination of causes, and which must be eliminated from these 
readings in order that we may obtain the annual variation. This has accordingly 
been done, and the annual variation of horizontal force so obtained is exhibited in 
the following Table :— 
MAG. AND MET. oBs. 1847 To 1855. g 
