564 MR J. A. BROUN ON THE 
1 and 2 p.m. VAN SwINDEN remarked that on many days without irregular 
movements the mean law was not followed, that the needle was farthest west 
before noon or after 4 p.m.; this irregularity is also due, however, to disturb- 
ances ; he then sought the numbers of irregular days in each year on which the 
needle attained the maximum before noon or after 4.P.m. From the table which 
he has given, he concludes—“ On voit par la combien le nombre de ces jours a 
cru de 1774 a 1776, et décru de 1776 a 1780. Y auroit-il quelque période de 
quatreans?”* Thatis ofeight years from minimum to minimum. An inspection 
of VAN SwINDEN’s numbers given afterwards (Art. 30), and a consideration of 
other facts, will show, I think, that VAN SwInpDEN had here discovered one of 
the results of the decennial period; he does not appear, however, to have 
examined the amounts of the diurnal range of the needle in each year. 
3. Neither Cassin1 nor GILPIN seem to have noticed the variation of the 
range of the diurnal oscillation from year to year; and ARaAGo’s observations 
from 1820 to 1830 were not reduced in such a manner as to show the decen- 
nial period till 1854, and even then it was not remarked.t 
4. The first who appears to have observed the fact, or thought it worth 
noticing, that the diurnal range of magnetic declination varied from year to 
year, was Gauss. In his discussion of the Gottingen observations for the three 
years April 1834 to March 1837, he remarked that the range of the diurnal 
oscillation of the magnetic needle between 8 a.m. and 1 p.m. was greater 
in each month of the second year than in the corresponding months of the 
first ; and again, greater in the third than in the second year; adding, “ But 
these differences are much too great for us to conclude that they are due to a 
secular increase, and it is much rather to be expected that by continuing the 
observations during several years an oscillation (Hinundherschwanken), cannot 
fail to present itself.” { 
5. In 1846 Dr Lamont added to the Gottingen results from 1834 to 1842 
his own, derived from the Munich observations from 1842 to 1845, and pointed 
out the very regular change of the diurnal oscillation during the ten years. The 
ranges showed the maximum in 1837-38, and Dr Lamont concluded there was 
reason to believe that the minimum was then attained (1846)§. The observa- 
tions of ten years did not prove, however, that the fluctuation predicted by Gauss 
was periodic ; and it was only in 1851, after the passage of a second maximum 
in 1848, that Dr Lamont concluded, with the aid of preceding series of observa- 
tions, the existence of a period occupying on the average 104 years. || 
6. Early in 1852 General Sir E. Sasine communicated to the Royal Society 
* Analogie de Elect. et du Mag. t. iii. p. 129. This result of Van Swinpen’s long and perse- 
vering labours seems to have been lost sight of. 
+ Ciuvres de F. Araco, t. iv. p. 501, Paris, 1854. + Resultate des Mag. Vereins, 1836, S. 54. 
§ Resultate des Mag. Obs, in Miinchen, 1846, S. 31. 
|| Poaeenporrr’s Annalen, B. 84, S. 572, Dec. 1851. 

