[ 75 ] 
III. The Absolute Direction and Intensity of the Earth's Magnetic Force at Bombay, 
and its Secular and Annual Variations. By Charles Chambers, F.B.S., Super- 
intendent of the Colaba Observatory. 
Received October 26, 1871, — Read January 25, 1872*. 
The observations discussed in this paper were taken at the Colaba Observatory during 
the years 1867 to 1874, and consist of observations of Dip, Declination, and Horizontal 
Intensity. The instruments with which they were taken were a Dip-circle by Barrow, 
with S-|-inch needles, and a Unifilar Magnetometer by Elliott Brothers, and both were 
examined and approved at the Kew Observatory before being sent out to India in the 
year 1867. Complete observations were taken regularly of the Dip and Horizontal 
Force twice a w r eek, and of the Declination once a week. 
Data for Investigation. — These consist of the monthly means of the determinations of 
Absolute Dip from April 1867 to March 1874, and the monthly means of the determr 
nations of Absolute Declination and Absolute Horizontal Intensity from July 1867 to 
December 1873. 
I. Dip. 
2. For some unexplained reason, but which is suspected to be owing to a sudden 
deterioration in the axles of the dip-needles, the quality of the observations began in 
March 1870 to be of a decidedly inferior order to that of the earlier observations; and 
the efforts made to obtain new needles, of the high-class character of those first supplied 
with the dip-instrument, have hitherto been unsuccessful, possibly because some secret 
process in their production has died with the maker. For this reason the first three 
years’ observations will alone be treated in some detail, and the monthly and annual 
means of the last four years will be simply recorded. 
Monthly Mean Values. — The following Table shows the monthly mean values of the 
Dip from April 1867 to March 1870 : — 
Table I. 
Year. 
April. 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Jan. 
Eeb. 
Mar. 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
19+ 
1867-68 
l'l 
19 
r-5 
0’-8 
2'-2 
2'-5 
2-5 
2'-8 
2'-7 
2'7 
3'-4 
3'-8 
1868-69 
4 1 
4 1 
4-3 
4 3 
4-2 
4-3 
4-2 
3-6 
3-5 
3 7 
4-8 
5 -2 
1869-70 
6 0 
6-2 
6-3 
6-8 
6-4 
5-7 
6 • 2 
6-5 
5 - 6 
5 -6 
5 '8 
6-0 
* I have taken the opportunity, -which the lapse of time affords, of extending the number of years of obser- 
vations treated in the paper from 3| to 6§. — July , 1876. 
MDCCCLXXVI. M 
