Kirkwood.] '^^ [April 2, 



ON THE PERIODICITY OF THE SOLAR SPOTS. 

 By Daniel Kirkwood. 



§ /. — The Results of Observation. 



(1.) The most ancient observations of sun-spots, of which we have any 

 record, are those of the Chinese in the year 321, A. D. The first notice 

 of their detection by Europeans is found in the annals of the Frankish 

 kings. A black spot, according to Adelmus, was seen on the sun's disk, 

 March 17th, 807, and continued visible 8 days. Similar phenomena were 

 again observed from tlie 28th of May to the 26th of August, A. D. 840. 

 The year 1096 was also signalised by the appearance of spots so large as 

 to be visible to the naked eye. The next date, in chronological order, is 

 that of 1161, when a spot was seen by Averroes. Finally, on the 7th, 

 8th, and 16th of December, 1590, "a great blacke spot on the sunne," 

 apparently ' ' about the bignesse of a shilling, ' ' was observed at sea by 

 those on board the ship "Richard of Arundell."* The foregoing are, 

 we believe, the only undoubted instances in which these phenomena were 

 observed previous to the invention of the telescope. 



(2.) From 1610 to 1750 the sun was frequently observed through instru- 

 ments of various optical power, and the sparseness, or even the entire 

 absence of spots, during considerable intervals of time, as well as their 

 great number and magnitude at other epochs, were noticed by different 

 astronomers. From the latter date till the close of the first quarter of the 

 present century the solar observations were more frequent and regular ; 

 still, no idea of the prevalence of law in the varying numbers and magni- 

 tudes of these mysterious objects had been even conjectured. We come 

 now, however, to a most interesting and remarkable epoch in the history 

 of solar physics. e 



(3.) The 11- Year Period of Schicahe.— In 1826, Hofrath Schwabe, of 

 Dessau, commenced a series of sun-spot observations, which have been 

 continued without interruption to the present time (1869). On each 

 clear day he notes the number of visible groups, giving to each a special 

 designation, to guard against counting it twice in a single rotation of the 

 sun. In the first year, 1826, 118 spots were observed; the number was 

 considerably greater in 1827 ; and in 1828 it had increased to 225. During 

 the next five years there was a gradual decrease; the minimum being 

 reached in 1833. The results of 43 years' observations are presented at 

 one view in the following table : 



* For authorities see Humboldt's Cosmos, Vol. IV., and Chambers' Descrip. Astronomy, p. 21. 



