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to view from the earth is uniformly greatest when Venus is on the opposite side of the sun from us, and least when on the same side. When both "Venus and Jupiter are on the side of the sun opposite to us, the spots are much larger than when Venus alone is in that position. In part explanation of this influence of the planets, we may suppose that they, in some manner, modify reflection on the disk of the sun exposed to their action, and thus cause a condensation of gases.
The spots do not influence the fruitfulness of the season.—Sir W. Herschel first advanced the idea that years of abundant spots would be years also of plentiful harvest. This is not now generally received. What two years could be more dissimilar than 1859 and 1860? Both abounded in solar spots, yet one was a fruitful year and the other almost one of famine in Europe.
The spots are cooler than the surrounding surface.— It seems that the breaking out of a spot sensibly diminishes the temperature of that portion of the sun's disk. The faculae, on the other hand, do not increase the temperature. (Secchi.)
The spots are depressions below the luminous surface. —This was thought probable before, but is conclusively proved by the photographs of the sun, which have been taken in large numbers of late at Kew Observatory.
Comparative brightness of spots and sun.—If we represent the ordinary brightness of the sun by
1,000, then that of the penumbra would be 469, and that of the nucleus 7. There may be much light and heat radiated by a spot, which seems totally black as compared with the sun: we remember that when we look through even a Drummond light at the sun, it appears as a black spot on the disk of that luminary.
Faculce, mlhw-leaf, and mottled appearance.—Besides the variety of spots already described, there are other curious appearances worthy of note. Bright ridges or streaks appear, which constitute the most brilliant portions of the sun.— These are called /acuke. They vary from barely discernriCCLJ5- ible, softly-gleaming
tracts 1,000 miles long, to lofty, piled-up, mountainous regions 40,000 miles long and 4,000 broad. Outside of the spots, the entire disk of the sun is covered with minute shady dots, giving it a mottled appearance not unlike that of the skin of an orange, though less coarse. Under a large telescope the surface seems to be entirely made up of luminous masses, imperfectly separated by dark dots called pores. These masses are said by Mr. Nasmyth to have a "willow-leaf" shape; many observers apply other descriptive terms, such as "rice grains," "untidy circular masses," "things twice as long as broad," "granules," etc. The accompanying cut represents the willow-leafed structure of the luminous surface, and also the "bridges"
spanning the solar spot. Indeed, it is said that the spots themselves always have their origin in a "pore," which appears to slowly increase and assume the blackness of an umbra, after which the penumbra begins to appear. Physical Constitution Of The Sun.—Of the constitution of the sun, and consequent cause of the solar spots, very little is definitely known. We shall notice the various theories now adopted by different astronomers.
Wilson's Theory.—This theory supposes that the sun is composed of a solid, dark globe, surrounded by three atmospheres. The first, nearest the black body of the sun, is a dense, cloudy covering, possessing high reflecting power. The second is called the photosphere. It consists of an incandescent gas, and is the seat of the light and heat of the sun. The third, or outer one, is transparent, very like our atmosphere. According to this theory, the spots are to be explained in the following manner. They are simply openings in these atmospheres made by powerful upward currents. At the bottom of these chasms we see the dark sun as a nucleus at the centre, and around this the cloudy atmosphere—the penumbra. This explains a black spot with its penumbra. Sometimes the opening in the photosphere may be smaller than that in the inner or cloudy atmosphere; in that case there will be a black spot without a penumbra. It will be natural to suppose that when the heated gas of the photosphere or second atmosphere is thus violently rent asunder by an eruption or current from below, luminous ridges will be formed on every side of the opening by the heaped-up gas. This will account for the faculce surrounding the sun-spots. It will be natural, also, to suppose that sometimes the cloudy atmosphere below will close up first over the dark surface of the sun, leaving only an opening through the photosphere, disclosing at the bottom a grayish surface of penumbra. We can readily
see, also, how, as the sun revolving on its axis brings a spot nearer and nearer to the centre, thus giving us a more direct view of the opening, we can see more and more of the dark body. Then as it passes by the centre the nucleus will disappear, until finally we can see only the side of the fissure, the