428 Dr. Lament on the Ten-year Period which exhibits itself 



second set of lines I advocate, chiefly with a view of stating the 

 question of their existence ; and though I should not have raised 

 the argument unless I had thought it both important, and likely 

 to be answered ultimately in the affirmative, I still hold the 

 opinion with some hesitation, with as much, indeed, as accom- 

 panies any conclusion I endeavour to draw respecting points in 

 the very depths of science, as, for instance, regarding one, two, 

 or no electric fluids ; or the real nature of a ray of light, or the 

 nature of attraction, even that of gravity itself, or the general 

 nature of matter. 



Royal Institution, 

 March 6, 1852. 



LIX. On the Ten-year Period which exhibits itself in the Diurnal 

 Motion of the Magnetic Needle. By Dr. Lamont of Munich*. 



THE remarkable experiments of Faraday relative to the 

 diurnal motion of the magnetic needle, have induced me 

 to submit the peculiarities of this motion to a closer examination, 

 and thus to obtain a clearer view of the facts which accompany it. 



Among these there exists at present one, which, so far as I 

 am aware of, has hitherto remained unrecognised, but which 

 seems of so important a nature that it cannot be omitted in any 

 theory which undertakes to render an account of the diurnal va- 

 riation ; it is the following : — 



The magnitude of the variations of declination have a period of 

 ten years. For five years there is a uniform increase, and during 

 tlie following five years a uniform decrease in the variations. 



With us the magnetic declination is a minimum at about 8 

 o'clock in the morning, and is greatest at 1 o'clock in the afternoon. 

 Subtracting the declination at 8 o'clock from that at 1 o'clock, 

 we obtain the magnitude of the diurnal motion. From the hourly 

 observations conducted in this observatory since the month of 

 August 1840, we ascertain the following to be the magnitude of 

 the diurnal motion for each month separately : — 



Magnitude of the Magnitude of the Magnitude of the 



diurnal motion. diurnal motion. diurnal motion. 



1841. , 1841. ; 



January + 3-72 July + 10-07 



February 5-13 August 9-86 



March 8-43 September 878 



April 11-49 October 6-82 



May 11-47 November 3-71 



June 11-49 December 2-89 



1840. ') 



August + 10-63 

 September 10-97 

 October 7-72 



November 4-40 

 December 3-51 



* From Poggendorff's Annalen, vol. lxxxiv. p. 572. 



