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XXXVI. On the Decennial Period in the Range and Disturbance of the Diurnal 
Oscillations of the Magnetic Needle, and in the Sun-spot Area. By J. A. 
Brown, F.R.S. (Plates XXXIX., XL.) 
(Read 15th May 1876.) 
1. The presentation to the Royal Society of Edinburgh of results relating to 
the decennial period, derived from observations of magnetic declination made 
during nearly a quarter of a century at Trevandrum, has seemed to me a 
favourable occasion for a determination of the mean duration of this period. 
Upon the explanation of the decennial variations depends the solution of 
several important problems in solar and terrestrial physics, and the first step 
towards this result is to ascertain the true mean duration of the period. Two 
markedly different results have been obtained, each of which has been accepted 
by men of the highest scientific reputation. 
2. Discovery of the Decennial Period of Magnetic Variations.— A century ago 
the varying positions of the magnetic needle were followed with much perse- 
verance by several men of science, but by none with more ardour than Van 
SwInDEN. Unfortunately, the needles employed were supported on steel . 
points, and, owing probably to weak magnetisation and the mechanical diffi- 
culties of construction, they gave widely different results at different places, 
and even at the same place; so that valuable series of observations made with 
similar instruments have lost, to a great extent, the weight which they would 
otherwise have merited. VAN SwiINpDEN, however, had needles from England, 
where much care had been bestowed on their construction; and he observed 
every hour from morning till evening during thirteen years, in eight of which 
three needles were observed simultaneously. The differences of movement of 
the three needles during magnetic disturbances show that we cannot consider 
the absolute amounts accurate ; the results, however, to which I am about to 
refer are independent of these differences. 
VAN SwINDEN followed with great interest every disturbance (afolement) of 
the needle, and he has given the number of days in each year from 1771 to 
1781 on which these afolements occurred. As he had not apparently any exact 
measure of what should be considered a day of disturbance, these numbers 
cannot be considered quite comparable. They show, however, fewest days in 
1771 and 1780-81, while the greatest number of disturbance days occurred in 
1773, 1774, 1775, and 1777.* Van SwinbeEn, however, obtained another result 
with specific limits. The north end of the needle is generally farthest west between 
* Analogie de l’Electricité et du Magnétisme, t. iii. p. 85. This volume contains the “ Dissertation 
sur les mouvements irreguliers de ]’Aiguille aimantée, par J. H. Van SwINDEN, La Haye, 1785. 
VOL. XXVII. PART Iv. 7k 
