Scientific Lectures. 



211 



which often cross a spot, dividing it like partitions. Sometimes these 

 are intensely brilliant, crossing pennmbra as well as nucleus, while 

 in other cases they are no brighter than the penumbra, and only 

 perceptible when in contrast with the dark nucleus. Both their 

 formation and disappearance have been frequently observed ; tongues 

 of light are seen darting across from one side, or from both sides 

 toward each other, until the bridge is established, and in a way not 

 unlike that which naturalists describe when microscopic organic' 

 forms are in process of development ; and again they are seen to fade 

 gradually awa}^, growing fainter and fainter, until from an intense 

 brightness they have seemed to dissolve and disappear. These also 

 are depicted in the diagrams. 



Frequency of Spots on the Sun. 

 In 1813, after Schwabe had carried on his spot observation for six- 

 teen years, he discovered that their frequency was periodic, and he 

 estimated the length of the period at about ten years. Subsequent 

 observations have most fully corroborated this important discovery,- 

 and the accompanj-ing tabular view presents a summary of the results 

 obtained by Schwabe since the beginning of his series of observa- 

 tions. 



Years. 



1826 . 



1827 . 



1828 . 



1829 . 



1830 . 



1831 . 



1832 . 



1833 . 

 1831 . 



1835 . 



1836 . 



1837 . 



1838 . 



1839 . 



1840 . 

 1811 . 



1842 . 



1843 . 



1844 . 



1845 . 



1846 . 

 1S47 . 



No of 

 groups. 



. . 277 

 . . 273 

 282 

 1^244 

 ,.217 

 . . 239 

 ,.270 

 . . 267 

 ..273 

 ,.244 

 , . 200 

 , . 168 

 , . 202 

 , . 205 

 , . 203 

 , . 283 

 ..307 

 ..312 

 ..321 

 ..332 

 ..314 

 ..276 



Years. 



1848 

 1849 

 1850 

 1851 

 1852 

 1853 

 1854 

 1855 

 1856 

 1857 

 1858 

 1859 

 1860 

 1861 

 1802 

 1803 

 1864 

 1865 

 1806 

 1867 

 1808 



No. of 



days. 



.278 

 .285 

 .308 

 .308 

 .337 

 .279 

 .334 

 .313 

 .321 

 .324 

 .335 

 .343 

 .332 

 ,222 

 !317 

 .330 

 .325 

 .307 

 .349 

 .312 

 ,301 



