AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 
213 
remarked on that occasion, that “the excess appeared to be too large to be caused by 
any conceivable error in the determination of the temperature correction of the 
magnet, or generally of the apparatus by which it was suspended.” The average 
difference between the summer and winter months, derived from the observations of 
the Bifilar in the single year referred to, was •00161 parts of the whole horizontal 
force, or '0056 in absolute measure. 
The question of an annual variation of the horizontal force appeared to me so im- 
portant either to verify or disprove, that, at my request. Captain Lefroy employed, 
during the years 1847 and 1848, a third method of experimenting, which, although it 
may not be quite so satisfactory in respect to the individual monthly results as the 
method of absolute determinations, in consequence of the magnetic moment of the 
bar not being subject to monthly examination, has yet the advantage of affording a 
third conclusion perfectly independent of the others, and but little inferior to the 
absolute method in proportion to the time of their respective continuance. One of 
the cylindrical magnets of 3‘67 inches in length, which had been employed in the 
North American survey, and appeared to have attained a state of steady magnetism 
(which however did not prove so thoroughly steady as was expected), was suspended in 
the usual manner in alight stirrup, with an attached mirror and a detached telescope. 
The horizontal force of the earth was measured at stated hours, twice in every day, 
at 10 A-M. and 5 p.m., by the times of vibration of the bar derived from four hundred 
vibrations observed in the usual manner, and reduced to a standard temperature and 
to infinitely small arcs. The magnetic moment of the bar was carefully examined 
before the commencement and after the conclusion of the series, viz. on the 31st of 
December, 1846, and on the 3rd of January, 1849, and also intermediately on January 
5th, 1848. The magnetic moment at these periods was as follows: — 
Loss of magnetism in 
nearly equal intervals. 
1846. December 31st = 0‘6104'l ^ 
^ , • -0104 
1848. January 5th =0*6000J^ 
1849. January 3rd =0-5913 /■0087 
The value of the magnetic moment has been assumed on the hypothesis of uniform 
loss of magnetism in the whole period, and has been computed for every day of ob 
servation. Now if we take the arithmetical mean of the absolute values of the force 
in the twenty-four months derived by this bar as the mean result corresponding to 
the 1st January 1848, and if for the purpose of eliminating secular change we com- 
bine the values in January 1847 and December 1848, February 1847 and November 
1848, &c., we obtain the excess or defect in the horizontal force in absolute measure 
for the months of winter and summer as follows : — 
