OF THE EARTH’S MAGNETIC FORCE AT BOMBAY. 
81 
rections for secular change to reduce the values for each month to the same epoch 
(October 1, 1870), being at the rate of 9" per month, are also shown, as are likewise 
the mean monthly values cleared of secular change and the monthly excesses above the 
mean of the year. 
Table XI. — Monthly Mean Values of Absolute Declination, as observed. 
Month 
Jan. 
Eeb. 
March. 
April. 
May. 
June. 
July. 
August. 
Sept. 
Oct. 
Nov. 
Dec. 
Year. 
Mean Declination. . . 
48 51 
49 1 
48 44 
47 55 
47 45 
47 50 
47 8 
46 55 
46 45 
48 22 
49 7 
49 24 
Correction for se- 1 
cular change . . . J 
+ 22 
+ 13 
+ 4 
- 4 
- 13 
— 22 
+ 22 
+ 13 
+ 4 
- 4 
- 13 
— 22 
Corrected to epoch ) 
October 1, 1870 J 
49 13 
49 14 
48 48 
47 51 
47 32 
47 28 
47 30 
47 8 
46 49 
48 18 
48 54 
49 2 
48 9 
Annual Yariation. . . 
+1 4 
+ 1 5 
+0 39 
-0 18 
-0 37 
-0 41 
-0 39 
-1 1 
-1 20 
+0 9 
+0 45 
+0 53 
Table XII. — Monthly Mean Values of Absolute Declination, reduced to Mean Monthly 
Reading of the large Declination Magnetometer. 
Month 
Jan. 
Eeb. 
March. 
April. 
May. 
June. 
July. 
August. 
Sept. 
Oct. 
Nov. 
Dec. 
Year. 
Mean Declination. .. 
49 6 
49 20 
49 41 
49 55 
49 43 
49 50 
48 55 
49 22 
49 26 
49 44 
49 46 
49 43 
Correction for se- 1 
cular change ... j 
+ 22 
+ 13 
+ 4 
- 4 
- 13 
- 22 
+ 22 
+ 13 
+ 4 
- 4 
- 13 
— 22 
Correctedto epoch 1 
October 1, 1870 J 
49 28 
49 33 
49 45 
49 51 
49 30 
49 28 
49 17 
49 35 
49 30 
49 40 
49 33 
49 21 
49 33 
Annual Variation. . . 
1 
-0 5 
0 0 
+0 12 
+0 18 
-0 3 
-0 5 
-0 16 
+0 2 
-0 3 
+0 7 
0 0 
-0 12 
The two sets of monthly differences represent the annual variation of declination, first, 
as affected by the annual variation of the diurnal inequality at the usual observation 
hour, and, secondly, after the elimination of that affection ; it is seen at a glance that 
the effect of the diurnal inequality is to alter both the character and range of the appa- 
rent annual variation, raising the range from O' 34" to 2' 25", and impressing upon the 
variation high values during the winter and low values during the summer months — 
effects quite consistent with the known character of the annual variation of diurnal 
inequality. The true (corrected) annual variation has a systematic character, with a 
double oscillation in the period, showing maxima near the times of the equinoxes, and 
minima about the times of the solstices. . 
13. That the Declination is greater near the equinoxes than near the solstices may 
also be shown without assuming a value for the annual secular change ; for if this 
change be uniform the means for the months January and December, February and 
November, &c June and July, will all correspond to the same epoch and be 
directly comparable ; these means are as follows : — 
