AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 209 
that have ever been made. The intercomparability of the results might possibly have 
been more perfect if one and the same instrument had been used throughout (an im- 
portant consideration in respect to the secular change) ; but, on the other hand, the 
probable accuracy of the general result as regards the mean inclination must be 
viewed as in some degree strengthened by the employment of different instruments. 
The following Table exhibits the fifty-two monthly results: — 
Table III. 
1845. 
1846. 
1847. 
1848. 
1849. 
0 / 
o / 
o f 
O ! 
January 
75 18*4 
75 13*8 
75 15-0 
75 20-3 
75 19-5 
February 
19-5 
14-2 
15-2 
18*7 
18-1 
March 
14-5 
13-8 
16-2 
17-2 
16-7 
April 
11-5 
14-3 
15-9 
18-0 
18-4 
May 
15-4 
14*4 
16-1 
17-2 
June 
15-0 
14-8 
13-1 
16-8 
July 
14-5 
14-0 
11-3 
16-4 
August 
14-6 
14-4 
12-6 
19-0 
September 
16-6 
15-7 
12-5 
17-3 
October 
14-3 
15-4 
17-6 
19-0 
November 
16-8 
15-0 
17-7 
19-4 
December 
15-2 
15-0 
17-0 
20-6 
On the hypothesis of the terrestrial magnetic inclination having been constant 
during the period of observation (z. e. constant in respect to secular change and an- 
nual variation, and subject only to irregular and diurnal fluctuations), we obtain from 
the results in the Table a mean inclination of 75° 16''09, with a probable error of 
4:0''20, and +l''46 as the probable error of a single monthly determination. These 
probable errors include of course the effects of irregular fluctuation as well as those 
of observation error properly so called, besides the possible influence of secular change 
and annual variation ; the two latter being excluded by the hypothesis. 
The errors remaining over on this hypothesis are shown in the following Table : — 
Table IV. 
1845. 
1846. 
1847. 
1848. 
1849. 
January 
+ 2-3 
-2*3 
/ 
— 1*1 
+ 4-2 
-I- 3-4 
February 
+ 3-4 
-1-9 
-0-9 
+ 2-6 
+ 2-0 
March 
-1-6 
—2-3 
+ 0-1 
+ M 
-1-0-6 
April 
-4-6 
-1-8 
-0-2 
+ 1-9 
+ 2-3 
May 
-0*7 
-1*7 
0-0 
+ 1-1 
June 
— 1-1 
-1*3 
-3*0 
+ 0-7 
July 
-1*6 
-2-1 
-4-8 
-h0*3 
August 
— 1-5 
-1-7 
— 3-5 
+ 2-9 
September 
-I-0-5 
— 0-4 
-0-6 
+ 1-2 
October 
— 1-8 
-0-7 
+ 1-5 
+ 2*9 
November 
+ 0-7 
— 1-1 
+ 1-6 
+ 3-3 
December 
-0-9 
— M 
+ 0-9 
+ 4-5 
The prevalence of — signs in the earlier months, and of signs in the later 
2 E 
MDCCCL. 
