XV. On the variations of the daily mean horizontal force of the earth's magnetism produced by the suns rotation and the moon's synodical and tropical revolutions

Abstract

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. 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. 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.