DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 575 
25. The following are the “relative” numbers (7) of sun-spots given by Dr 
WotrF for the years which we are now considering :* — 

Year. q. Year. 1. Year. tr. Year. 1. Year. r. 





1770 79 1780 73 1790 84 1800 18 1810 0 
1771 43 1781 68 1791 53 1801 39 1811 1 
1772 49 1782 33 1792 47% 1802 | 58 1812 5 
1773 40 1783 22 1793 40% 1803 65 2 1813 14 
1774 48 1784 5 1794 34 1804 75 1 1814 202 
1775 28 1785 21 1795 22 1805 501 1815 351 
1776 35 1786 87 1796 15 1806 259 1816 45 
ewer 63 1787 =| 105 LAE 8 1807 15% LOT. 44 
1778 95 1788 | 108 1798 4 1808 7 1818 34 
U7 @) 90 1789, 9 TAT 1799 10 1809 3 1819 22 








I do not think it possible to conclude, from the numbers which Dr WoLF 
has obtained and employed, that a maximum certainly happened in the year 
1804 rather than in the years 1805 or 1806, or a minimum in 1810 rather than 
in 1813 or 1814. Dr Wotr considers that the minimum occurred for 1798°5 
+0°5.t Mr Tuttze, on the other hand, finds 1796°5,f and Sir Joun HErRscHEL 
has remarked that 1800 was a year of minimum.§ On the whole, it seems 
extremely probable that there were comparatively few spots on the sun from 
1795 to 1800. This conclusion agrees with that which may be deduced from — 
GILPIN’s observations, since the increase in the number of spots, which should 
have corresponded to the increase of the diurnal oscillation of the needle in 
1797, must have been one which would have been shown distinctly only by a 
careful system of accurate sun-spot observations. 
26. We have still another phenomenon, the aurora borealis, which can be 
related to the magnetic disturbance. I have indeed shown “that the laws of 
the aurora borealis may be concluded from those of magnetic disturbance, and 
vice versa.|| That this should hold for the decennial period was at once evident, 
and different discussions have been performed to prove it. One of the most 
careful and satisfactory is that by Professor Loomis, who has considered only 
shown that they depend on few observations. He remarks that old observers directed their attention 
chiefly to large sun-spots; so that Fuavcrreuzs (one of the principal observers during the period in 
question) saw the sun frequently without spots, when many were seen by other observers (“ Einige 
Bemerkumgen iiber die zehnjihrige Periode,” 8.23). In an interesting investigation on the decennial 
period of aurorz, sun-spots, and magnetic variations, Professor Loomis has also pointed out the fewness 
of the observations employed by Dr Wo r, especially those for the years 1802 to 1806 (“ American 
Journal of Science,” April 1873). The numbers for some of these years Dr Woxr has himself 
marked (?) as doubtful, as in the above table. 
* “ Astronomische Mittheilungen,” xxxv. ; “ Vierteljahrsschrift d. Natur. Gesellschaft in Ziirich,” 
1874, S. 231. 
+ Ast. Mit. xxiv. t De Macularum Solis. Ast, Nach. No. 1193, 1859. 
§ Phil Trans., 1870, p. 397. The word is “maximum” at the place cited; but this is evidently 
‘a clerical error. 
|| Trans. Roy. Soc. Edin. vol. xviil. p. 402, 1848. 
VOL. XXVII. PART IV. é TN 
