THE SPOTS ON THE SUN's DISC. 287 



hurtful to the eyes, large dark spots are often seen surrounded 

 by edges which are not quite so dark as the spots themselves, 

 and which are called penumbras. These spots, however, are 

 neither permanent nor unchangeable. When observed from 

 day to day, or even from hour to hour, their form is seen to 

 change ; they expand or contract, and finally disappear ; on 

 other parts of the solar surface new spots spring into exist- 

 ence where none could be discovered before. When they dis- 

 appear, the darker part in the middle of the spot contracts to 

 a point and vanishes sooner than the edge. Sometimes they 

 break up into two or more parts that show all the signs of 

 mobility characteristic of a liquid, and the extraordinary 

 commotion which it seems only possible for gaseous matter to 

 possess. The magnitude of their motion is very great. An 

 arc of 1 second, as seen from our globe, corresponds to 465 

 English miles on the sun's disc ; a circle of this diameter, 

 which measures nearly 220,000 English square miles, is the 

 smallest area that can be seen on the solar surface. Spots, 

 however, more than 45,000 English miles in diameter, and, 

 if we may trust some statements, of even greater dimensions, 

 have been observed. For such a spot to disappear in the 

 course of six weeks (and they rarely last longer), the edges, 

 whilst approaching each other, must move through a space of 

 more than 1000 miles per diem. 



" That portion of the solar disc which is free from spots 

 is by no means uniformly bright. Over it are scattered small 

 dark spots or pores, which are found by careful observation 

 to be in a state of continual change. The slow sinking of 

 some chemical precipitates in a transparent liquid, when 

 viewed from the upper surface and in a direction perpendicu- 

 lar thereto, resembles more accurately than any other phe- 

 nomenon the changes which the pores undergo. The similar- 

 ity is so striking, in fact, that one can scarcely resist the idea 

 that the appearances above described are owing to a luminous 

 medium moving about in a non-luminous atmosphere, either 



