524 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
from those for the barometer: it confirms generally the results at Trevandrum 
and Singapore. 
TABLE XI.—MontHiy MEAN VALUES OF HORIZONTAL FORCE (minus A CONSTANT) AT 











xX 
TREVANDRUM, 1842-45. 1:00= 10000" 
Monthly Means. (Monthly minus Yearly Means) plus 5:00. | 
MoNnTHS, | 
1842. 1843. 1844, 1845. 1842. 1843. 1844, 1445. | Means. 
January, f é 9-73) 12:57) 4:45 O66 | O13. | 11°43) 5:35. | 2B ey ae 
February, . : 6°58| 9:11] 5-60 | 10:23] 5-48 8:41} 6-14 | 6°70 | 6:68 
March, . : : 10:54} 4:13] 3:02 | 11:14] 8-94 3'86| 3:34 | 696 | 5:77 
April, . 5 : 746) .O pez td 785)| 5°36 0:23] 2:39 | 2:98 | 2-74 | 
Maya i : - 10:19| 0:25] 3-54 | 12:46] 7-59 0°63] 3:97 | 6:61 | 4°70 | 
June, ° ; F 7:97) 1:83] 6-82 | 13°66} 4:84 2:57| 7:28 | 6:89 | 5-40 
July, 1. ; : 3°99| 0:13] 6:10 | 13°62] 0°53 1:16) 646 | 6:01 | 3:54 
August, : - 7°98| 1:58] 4:13 | 12:87) 4:30 3°36 | 4:25 | 4:36 | 4:07 
September, . : 6°43} 2°58] 5°15 | 12°00) 3°91 4°55| 4°74 | 2:59 | 3°95 
October, 0 : 8-86| 4:68] 3:01 | 13°13) 5:91 6-61| 7°98 | 2°82 | 5:83 
November, . ‘ 9:49} 585] 4:14 | 17-27| 7:28 7:56| 746 | 6-06 | 7-09 
December, . . 10:93; 5°99] 6:97 | 15:58) 9°39 7°36| 5:28 | 3:47 | 6:38 





29. In examining the values projected in Plate XXIIL., it will be remarked 
that all the observatories show a large increase of force in the end of 1841, and 
that it is only at St Helena and the Cape, after June 1843, that any decrease of 
force is marked. The space included by these observations leaves the whole 
Pacific and South America untouched; but as far as they go they show in the 
end of 1841 an increase of force everywhere, and in the mean of the whole a 
considerable increase of force throughout the whole period.* It would be diffi- 
cult to make a satisfactory hypothesis as to the movements of the poles of force 
which alone would satisfy these results. 
30. ANNUAL VARIATION.—In order to eliminate the secular variation, we would 
require to know its law; this could only be obtained by an empirical process. I 
have preferred adopting the following method. The yearly mean corresponding 
to the middle of each month was obtained, as in the following formula, for the 
middle of July 1842 :— 
(1842) Jan.+2 Mar.+...... +2 Dec. +Jan. (1843.) 
* It may be supposed that the increase everywhere shown in the end of 1841 might have been 
in some way connected with the commencement of the series; but I think there is no ground for this 
supposition, The observations were commenced early in 1841; the magnets were removed monthly 
from their stirrups for observations of absolute intensity ; and any tendency to stretch in the wires 
would have been got rid of in this frequent manipulation. At Toronto, indeed, the second month 
projected (September 1841) shows a diminution of force. I think there is every probability that the 
increase was a real increase of horizontal force at all the stations. 

