Page:Popular Science Monthly Volume 5.djvu/557

Rh 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 white objects to which the light of the sun has just been referred.

These "threads" or "filaments" are difficult of observation, for their average thickness is probably not over 200 miles, a width quite invisible at the sun's distance, in any thing but a very good telescope. Some appearances make it probable, however, 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 with three or four thousand miles long, and the writer has occasionally 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 whole 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 knowledge, such as it is, seems to point to a constant interchange of matter between the surface of the sun and its interior, far within which 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, labyrinthine lakes, in the interior of our Northern wilderness, on whose 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 surface, 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 water, or a streamer to the air; and the analogy is noteworthy in this: that one end of the filament is commonly made