128 Sioux- City Academy of Science and Letters. 



nomena may be given here: Sunspots, unless very small, 

 usually consist of two or three parts, the penumbra or 

 outer fringe of a grayish color enclosing a darker central 

 portion, known as the umbra; within the umbra are 

 occasionally seen still darker, almost black "holes" 

 known as "nucleoli;" surrounding the entire spot or 

 group, and at times extending over enormous portions 

 of the sun's suface in the vicinity of the spots, are masses 

 of faculae which appear much more brilliant than the 

 dazzling photosphere. Sunspots vary much in size and 

 number, and are subject to some as yet unexplained 

 periodicity, being more numerous and larger at times, 

 and then gradually diminishing until the disc may be 

 entirely free for many months or perhaps a year or 

 more; these fluctuations occur on the average every 

 eleven and one-tenth years from a maximum to a maxi- 

 mum again, although many exceptions to this period are 

 recorded; from a careful record kept by the writer from 

 1890 to the present time, the last maximum was passed 

 in 1893, and the following minimum was not certainly 

 reached until September, 1901,^®^ the interval from one 

 minimum to the other extending over a period of twelve 

 years. The period of increase from minimum to maxi- 

 mum is more rapid than the decline, and the next 

 maximum is due during the first half of the year 1905. 

 Sunspots are supposed to be cavities in the photo- 

 sphere which may be filled w^ith cooler vapors and gases, 

 rendering the spot darker in color, but this is only a 

 theory, and of late it is seriously questioned, some spots 

 appearing to be at much higher levels than others, and 

 possessing a convex rather than a concave shape. 

 Authorities among solar physicists differ even from geo- 

 metrical considerations of the formation of sunspots as 

 they cross the disc, and the whole question is yet one for 

 earnest and persistent thought and study. A single 

 normal sunspot is the exception; usually they appear 

 in clusters, groups, trains, streams and small spots, dots 

 or veiled markings; they present an endless variety of 

 forms, some with numerous umbrae, others spiral- 

 shaped, oval, circular, carrot-shaiDed, pear-shaped, scor- 

 pion-like and many other curious formations; many 

 large spots present evidence of violent cyclonic action. 



