REPORT OF THE SECRETARY. 17 



The first two of the papers of this series related to what is called the 

 eleven-year period of the variation of the needle, which corresponds 

 with the recurrence and frequency of the spots on the sun. The 

 third paper relates to the influence of the moon on the variation of 

 the needle. The fourth refers to the change in the horizontal part 

 of the force of the earth's magnetism coinciding with the eleven- 

 year period of the spots on the sun. The fifth relates to the effect 

 of the sun in producing daily and annual variations in the horizontal 

 component of the magnetic force. The sixth relates to the lunar 

 influence on the horizontal magnetic force. 



A particular account has been given of the result of all these inves- 

 tigations, which tend fully to corroborate the conclusions arrived at 

 from observations in other parts of the world, that both the sun and 

 moon are magnetic bodies, and exert an influence upon the polarity 

 of the earth ; and also that the magnetism of the sun has variations 

 in intensity which are in some way connected with the appearance 

 of spots on its surface, giving rise to the variations in those perturba- 

 tions of the needle which have been called magnetic storms, and 

 which present a periodical recurrence at an interval of about eleven 

 years. 



The influence of the moon is much less marked than that of the 

 sun, and appears to be more analogous to the temporary magnetism 

 induced in soft iron. 



Parts VII, VIII, and IX of this series, now in the press, are a con- 

 tinuation of the same subject. Part VII contains the discussion of 

 the effect of a change of temperature on the readings of the vertical 

 force instrument. 



If a magnetic needle could be supported perfectly free in space, so 

 as to assume the direction into which it would be brought by the 

 magnetic action of the earth, it would arrange itself in the line of 

 what is called the dip, or the inclination of the needle. At the mag- 

 netic equator of the earth such a needle would be parallel to the 

 horizon, but, departing from this line either to the north or the south, 

 the inclination would increase continually until we arrive at the mag- 

 netic pole, when it would be vertical. It is plain that the full magnetic 

 force of the earth, in the line of the dip, may be resolved into two* 

 others, viz., a horizontal force, or that which draws the ordinary 

 magnetic needle back to the meridian when it has been deflected 

 from this position : and, second, the vertical force which tends to 

 draw the end of the needle down into the line of the dip. The fre- 

 2 s 



