THE PHOTOSPHERE AND SUN-SPOTS. 539 



ranged exactly radially (in some spots not at all so). They are often 

 contorted and thrown over each other, and evince a tendency to curl 

 into sickle-shaped curves as they approach their inner extremities; 

 while, if we examine them at the penumbral circumference, we find 

 them to be apparent prolongations of those minute w^hite objects to 

 w^hich the light of the sun has just been referred. 



These " threads " or " filaments " are difiicult of observation, for 

 their average thickness is probably not over 200 miles, a wadth 

 quite invisible at the sun's distance, in any thing but a very good 

 telescope. Some appearances make it probable, liowever, that they 

 are composed of filaments still finer, just as the finest silk thread 

 is made up of numerous fibres, and they have a certain disposition to 

 unite in fascicles, which are often mistaken for them. The dimension 

 of 200 miles, then, is somewhat an arbitrary one, marking perhaps 

 rather the present limit of vision of our telescopes than any real limit 

 of the actual size ; but, however this may be, the extraordinary length 

 of these filaments is not open to question ; they are quite commonly 

 met w^th three or four thousand miles long, and the writer has occasion- 

 ally distinctly traced one of these attenuated forms uninterruptedly 

 through a much greater distance. What they are is still unknown. 



What are the forces which cause the spot to move as a w^hole upon 

 the solar surface, and what are the nature and direction of those which 

 modify its form, and so completely change in a few days, or even 

 hours, the disposition of its parts over its so vast area ? To the first 

 question there is, as yet, no satisfactory answer, though our knowl- 

 edge, such as it is, seems to point to a constant interchange of matter 

 between the surface of the sun and its interior, far within w^hich seem 

 to be impressed on the ascending currents velocities of rotation which 

 so modify those which obtain at the surface. As to the second, the 

 spectroscope, if appealed to, offers but very partial help, and we here 

 restrict ourselves to a description of methods which do not involve its 

 use. How may we determine the directions of the currents which we 

 cannot doubt exist within the spot ? 



It has happened to the writer to be lost in one of the shallow, laby- 

 rinthine lakes, in the interior of our Northern wilderness, on wiiose 

 still waters the canoe was left to drift aimlessly with the wind, while 

 the guide sought, at first vainly, the traces of some current which 

 would indicate the direction of the outlet ; till, looking below the sur- 

 fa-ce, the common direction of the extremities of the water-grasses, 

 rooted at the bottom, showed the existence and direction of a current 

 otherwise unperceived, and gave the question its solution. 



The long filaments of the penumbra may be used in a similar way, 

 flexible as they are, and rooted, as it were, at one end, while the other 

 sways in the currents of the solar atmosphere, yielding to it as freely 

 as the grasses to the w^ater, or a streamer to the air ; and the analogy 

 is noteworthy in this ; that one end of th« filament is commonly made 



