OF THE EARTH’S MAGNETIC FORCE AT BOMBAY. 
89 
of the constants used in reductions of the Horizontal-force observations has about the 
same effect ( + ‘0006) as the smallest of the above determinations, which have reference 
to observational errors only. 
IV. Total Force. 
20. With the following data : — 
Dip for epoch 1st October, 1868 =19° 4'*2 
Secular change of Dip per amium = + F'9 
Semiannual inequality of Dip : — excess from April to , 
September above the mean of the year . . . = + 0 ,- 3 
Horizontal Force for epoch 1st October, 1870 . . . = 8*0658 
Secular change of Horizontal Force per annum . . = + *0045 
Semiannual inequality of Horizontal Force : excess 
from April to September above the mean of the 
year = + *0009 
We find 
Dip for epoch 1st January, 1871 =19° 4'*2 
+2*25 xl , ’9=19° 8'*5 
Horizontal Force for epoch 1st January, 1871 . . . = 8*0658 + —*^ 
= 8*0669 
And by the formula E= c ^j, where R represents the Total Force, X the Horizontal 
Force, and Q the Dip : — 
Total Force for epoch 1st January, 1871 . . . . = 8*5391 
And by the formula ^R=~^+R tan 0dQ : — 
Secular change of Total Force per annum .... =+ *0064 
Semiannual inequality of Total Force: excess from 
April to September over mean of the year . . = + *0012 
21. Annual Variation . — In the annual variation of Total Force, derived by the 
formula last given from those of Dip and Horizontal Force in Tables HI. and XVII., 
the monthly excesses above the yearly mean are, for January to December respectively, 
as follows: *0025, -*0015, +*0008, +*0008, +*0004, +*0039, +*0029, -*0001, 
— *0004, — *0011, — *0015, and — *0015 ; and the quarterly means which give the greatest 
differences are : — February to April, *0000 ; May to July, +*0024 ; August to October, 
— *0005 ; and November to January, —*0018. 
