OE THE EARTH’S MAGNETIC FORCE AT BOMBAY. 
87 
Table XVII. 
Months 
Jan. 
Feb. 
March. 
April. 
May. 
June. 
July. 
August. 
Sept. 
Oct. 
Nov. 
Dec. 
Year. 
| Mean horizontal] 
1 fornfi J 
80631 
80639 
80664 
8'0664 
80663 
8-0701 
80675 
8-0648 
8-0651 
80650 
8-0651 
8-0659 
t Correction for] se- ] 
cular change ... J 
+9 
+6 
+2 
—2 
-6 
-9 
+9 
+6 
+2 
-2 
-6 
-9 
Corrected to epoch ] 
Oct.. 1, 1S70...J 
80640 
8 0645 
80666 
8-0662 
8-0657 
80692 
8-0684 
8-0654 
8-0653 
8-0648 
8-0645 
80650 
80658 
Annual variation . . . 
-•0018 
-0013 
+ 0008 
+ 0004 
-•0001 
+ ■0034 
+■0026 
-■0004 
— 0005 
-0010 
-0013 
-•0008 
The annual variation exhibits a semiannual inequality in which the horizontal force 
is, during the months March to August, -0011 greater than the mean of the year, and 
during the months September to February '0011 less. The quarterly means that give 
the greatest difference are as follows : — 
February to April *0000 
May to July ....... +-0020 
August to October — -0006 
November to January .... — -0013 
Without applying any corrections for secular change, the means of the values of 
Horizontal Force in Table XVII., for January and December, February and November, 
&c., will all correspond to the same epoch, October 1, 1870. Such means are com- 
pared below both for the uncorrected observations and for the observations corrected to 
the mean of each month. 
Table XVIII. 
Months. 
Uncorrected. 
Corrected. 
January and December 
February and November 
March and October 
April and September 
May and August 
June and July 
80697 
80706 
8 0722 
80716 
80717 
80755 
j 8-0701 
j 8-0719 
j 8-0736 
8-0645 
80645 
80657 
80657 
8-0655 
80688 
1 8-0645 
| 8-0657 
j 8-0671 
80667 
8-0649 
April to September 
October to March 
80729 
8-0708 
Both sets of numbers indicate a small semiannual inequality, in which the force is 
about a four-thousandth of itself greater in the half-year April to September than in the 
half-year October to March ; and the four-monthly means show that the force passes 
through its mean value in the months March, April, September, and October nearest to 
the equinoxes ; and thence, of necessity, the maximum occurs in the months May to 
August, about the summer solstice, and the minimum in the months November to 
