232 Results of Monthly Observations of [Jan. 27, 



the middle of well-marked series, their different amplitudes and periods, 

 could not be explained except by the supposition that the solar action 

 was not continuous, but only by fits periodic. 



The author was induced to believe lately that these differences in the 

 oscillations were due to conjoint actions of the sun and moon; he 

 accordingly deduced the mean variations corresponding to three periods 

 of 26, 29*5, and 27*3 days, the times of rotation of the sun derived from 

 the magnetic observations, and of the moon's synodical and tropical revolu- 

 tions respectively. He finds that the combinations of these three series 

 of variations represent with considerable accuracy all the variations of 

 the daily mean horizontal force of the earth's magnetism during each 

 year ; so that the sun's rotation and the different positions of the moon 

 relatively to the sun and the plane of the equator (or of the ecliptic) are 

 found to produce all the differences in the amplitude and time, as well as 

 the apparent disappearance of the oscillation. 



Cases of considerable and sudden diminution of the earth's magnetic 

 force which happened in the years 1844 and 1845 are next examined ; 

 and it is shown that these changes occur at intervals of 26 days, or mul- 

 tiples of 26 days ; in one instance there are five successive recurrences at 

 the exact interval of 26 days. 



As this period is that of the sun's rotation relatively to ifie earth, it 

 appears to follow that the earth has some action on the sun, or (more 

 probably) on some ray-like emanation from the sun, which causes these 

 changes in the earth's magnetism. 



It is found also that these sudden variations occur more frequently 

 when the moon is at a considerable distance from the equator and the 

 ecliptic ; it would thus appear that our satellite has also an action on the 

 cause of the great terrestrial magnetic disturbances. 



II. " Results of the Monthly Observations of Magnetic Dip, Hori- 

 zontal Force, and Declination made at the Kew Observatory 

 from April 1869 to March 1875 inclusive/' By the Kew 

 Committee. Received November 25, 1875. 



This paper, containing the result of six years' magnetic observations 

 at the Kew Observatory, follows as the third of a series of publications, 

 the first of which was a communication to the Royal Society by General 

 Sir E. Sabine, published in the Philosophical Transactions for 1863, 

 page 273, which consisted of a discussion of certain selected magnetic 

 disturbances, a general discussion of declinational changes as recorded 

 by the magnetographs, and a discussion of the absolute determination 

 of Dip and Horizontal Force at the Observatory, for the six years April 

 1857 to March 1863 inclusive. 



The second publication appeared in 1870 as a paper in the ' Proceed- 



