Page:Popular Science Monthly Volume 23.djvu/660

642 , whose skeletons now lie white like other skeletons on the hills, lift their tufted branches. The forest-trees fall in natural decay, the mirror waters sleep in peace, while the centuries of the early Tertiary come and go.



HE sun when examined with a suitable telescope, properly adjusted as to the power used, exhibits, scattered over its disk, great facular waves, which are elevated portions of its surface, and are composed of luminous matter which has extended through its denser atmosphere. In order that these waves may be seen, it is requisite that they attain a height of at least forty-five times that of the Himalayas. Their appearance is very rare in polar regions, but very abundant in the close neighborhood of spots; in fact, they generally precede the formation of a spot. The faculæ, at a distance from the spots, change somewhat slowly, remaining for several days without much variation in their appearance. But it is quite otherwise near a spot, for here these waves change with a rapidity which renders it exceedingly difficult to make a draft of them. Movements not less than one thousand miles in an hour are not uncommon. The faculæ are generally round, though sometimes they appear in long strips of light. When they take the shape of a wreath, a group of spots quite soon appears, as a rule. With the discovery of solar spots may be said to have commenced our knowledge of the physical condition of the sun. Kepler was of the opinion that, in lines 441 and 454 of Virgil's first "Georgic," the solar spots were referred to. We also find in the annals of the Chinese, made many centuries ago, that spots were observed by the unaided eye, and in the year 807 an exceedingly large spot was seen for eight days. A solar spot consists, in the main, of two parts—the central part, called the umbra, surrounded by a less dark portion called the penumbra; and, as Professor Young has said, "The appearance is as if the umbra were a hole, and the penumbra filaments overhung and partly shaded it from our view, like bushes at the mouth of a cavern"; the umbra being a depression below the photosphere, filled with less luminous matter, while the penumbra may be seen around the edges.

It is observed that the spot, when half through its existence, is circular in shape, but, as it approaches disruption, it is subject to great change, sudden and violent. Respecting the average life of a spot, we may say it is from two to three months. The spot, however, observed in 1840 and 1841 lasted eighteen months, the longest time on record. Again, some may last but a few hours, being suddenly formed and 