60 



E. WALTER MAUNDER, F.K.A.S., ON 



in the middle of the group, soon concealing thena from sight. 

 Then the rear spot begins to be engulfed, and many streams of 

 bright matter cross it, forming "bridges,'' or flow down into it. 

 Finally, the whole stream disappears, with the exception of the 

 leader, which has now become large and circular. In many cases 

 the leader now begins to move backwards towards the region 

 from which it originally sprang ; sometimes it breaks up into 

 small fragments, but more often it simply contracts on itself, 

 and so gradually disappears. 



Groups of spots differ largely as to their size and duration. 

 The largest group in my experience covered an area of 4000 

 millions of square miles ; a thousand times the area of Australia 

 or of Europe. The smallest spots which w^ould be counted worthy 

 of the name, would cover about a million square miles. Fully- 

 developed circular spots very seldom exceed 600 millions of 

 square miles in area, and more generally run to a quarter or a fifth 

 of that extent. Whatever theory of the constitution of a sunspot 

 is accepted, it remains marvellous that a formation covering 

 hundreds of millions of square miles should be able to plough 

 its way through the solar photosphere at such tremendous speed, 

 wnthout breaking up or suffering deformation of its outline. The 

 differences in the duration of spots are equally marked ; many 

 spots only last a few minutes ; groups of a hundred million miles 

 or more in area frequently last a fortnight or more ; and occa- 

 sionally a group has been known to persist for half a year. 



The nature of sunspots may be briefly explained in the follow- 

 ing manner. The general surface of the sun is intensely hot and 

 Ijright, its temperature being about 6000° Centigrade. It 

 appears mottled in character, minute granules of intensest 

 brilliance being thickly clustered on a slightly less luminous back- 

 ground. The bright granules are sui)posed by many to be luminous 

 clouds, produced by the condensation of carbon at the sun's 

 surface. Sunspots, then, are areas where these clouds have not 

 been able to form or have been torn asunder ; areas which offer 

 a very distant analogy to the storms of our own atmosphere. 

 Thus, in terrestrial storms, we have regions where great differences 

 of atmospheric pressure are striving to adjust themselves ; in 

 sunspots, probably, we have regions where great differences of 

 temperature are similarly in action. Consequently, sunspots 

 are areas both of lowered temperature — say, 3500° Centigrade, 

 instead of 6000° — and of unusual heat. The cooling is evidenced 

 by the formation of compounds, the characteristic spectra of 



