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XV. On the Variations of the Daily Mean Horizontal Force of the Earth's Magnetism 
produced by the Sun's Dotation and the Moon's Synodical and Tropical Devolutions. 
By J. A. Broun, F.D.S. 
Received December 15, 1875, — Read January 27, 1876. 
History of the Investigation. 
In a discussion of observations of the Bifilar magnetometer made at Makerstoun in 
1844 a series of remarkable oscillations of the daily mean horizontal force were 
remarked, which, as the maxima occurred near new moon, and the minima near full 
moon, were attributed to the synodical revolution of our satellite *. This conclusion 
was afterwards confirmed by the result of four years’ observations f. Kreil had found 
that the time of vibration of a horizontal needle was less near new moon and during 
the first quarter than near full moon and during the third quarter : this result he 
found only in the eight months November to June; while he obtained just the 
opposite result during the four months July to October J. 
In a much later investigation he found that his observations during the years 1842 to 
1847 confirmed the result obtained by me, but that those for 1840 and 1841, and 1848 
and 1849, were opposed to it, the magnetic force being greatest at full moon§. This 
change of law appeared to him connected with the decennial period. 
An examination of observations made under my direction at Trevandrum in the 
years 1855 and 1856, and of other series, induced me to doubt whether the oscillations 
seen in the Makerstoun observations for 1844 could be explained by the lunar action. 
The appearance of maxima at full moon in similar oscillations in the observations of 
other years, the great amplitude of the oscillation and its nearly equal magnitude at 
different points of the earth’s surface, induced me to seek for another cause. The only 
other apparent, with a period approaching that of the oscillation referred to, was the 
sun’s synodic rotation on his axis. Kreil had already thought of this cause as an 
explanation of the different results obtained for the moon’s action in summer and 
winter ][ : this he considered might be due to the different solar poles presented to the 
earth at these two times of the year ; but he soon perceived that any action due to the 
sun’s rotation would be nearly the same in the means for the two seasons, since the 
* Trans. Roy. Soc. Edinb. vol. xvi. part ii. p. 106 (read January 5, 1846). 
f Ibid. vol. xix. part ii. p. xxxvi. + Philosophical Magazine, vol. xvi. p. 241 (1840). 
§ Wiener Denkschriften, B. v. S. 36 (1852). It was only after the results of the present paper had been 
obtained that I became acquainted with this memoir. 
|| Phil. Mag. vol. xvi. p. 241 (1840). 
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